Photonic integration and photonics-electronics convergence on silicon platform

Yamada, Koji

Liu, Jifeng

Baba, Toshihiko

Vivien, Laurent

2015

Frontiers Media SA

Silicon photonics technology, which has the DNA of silicon electronics technology, promises to provide a compact photonic integration platform with high integration density, mass-producibility, and excellent cost performance. This technology has been used to develop and to integrate various photonic functions on silicon substrate. Moreover, photonics-electronics convergence based on silicon substrate is now being pursued. Thanks to these features, silicon photonics will have the potential to be a superior technology used in the construction of energy-efficient cost-effective apparatuses for various applications, such as communications, information processing, and sensing. Considering the material characteristics of silicon and difficulties in microfabrication technology, however, silicon by itself is not necessarily an ideal material. For example, silicon is not suitable for light emitting devices because it is an indirect transition material. The resolution and dynamic range of silicon-based interference devices, such as wavelength filters, are significantly limited by fabrication errors in microfabrication processes. For further performance improvement, therefore, various assisting materials, such as indium-phosphide, silicon-nitride, germanium-tin, are now being imported into silicon photonics by using various heterogeneous integration technologies, such as low-temperature film deposition and wafer/die bonding. These assisting materials and heterogeneous integration technologies would also expand the application field of silicon photonics technology. Fortunately, silicon photonics technology has superior flexibility and robustness for heterogeneous integration. Moreover, along with photonic functions, silicon photonics technology has an ability of integration of electronic functions. In other words, we are on the verge of obtaining an ultimate technology that can integrate all photonic and electronic functions on a single Si chip. This e-Book aims at covering recent developments of the silicon photonic platform and novel functionalities with heterogeneous material integrations on this platform.

General and Civil Engineering

Materials

Science (General)

Physics (General)

English

9782889196937

PHOTONIC INTEGRATION AND
PHOTONICS-ELECTRONICS
CONVERGENCE ON SILICON
PLATFORM
EDITED BY : Koji Yamada, Jifeng Liu, Toshihiko Baba, Laurent Vivien
and Dan-Xia Xu
PUBLISHED IN : Frontiers in Materials and Frontiers in Physics

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ISSN 1664-8714
ISBN 978-2-88919-693-7
DOI 10.3389/978-2-88919-693-7

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1

October 2015  |  Photonic integration and photonics-electronics convergence

PHOTONIC INTEGRATION AND
PHOTONICS-ELECTRONICS
CONVERGENCE ON SILICON PLATFORM
Topic Editors:
Koji Yamada, National Institute of Advanced Industrial Science and Technology, Japan
Jifeng Liu, Thayer School of Engineering, USA
Toshihiko Baba, Yokohama National University, Japan
Laurent Vivien, Institute of Fundamental Electronics, France
Dan-Xia Xu, National Research Council, Canada

Multifunctional integration on a silicon photonic
platform. Photograph by Dr. Patrick Lo Guo-Qiang.
Taken from: Luo X, Cao Y, Song J, Hu X, Cheng
Y, Li C, Liu C, Liow T-Y, Yu M, Wang H, Wang QJ
and Lo PG-Q (2015) High-throughput multiple
dies-to-wafer bonding technology and III/V-onSi hybrid lasers for heterogeneous integration of
optoelectronic integrated circuits. Front. Mater. 2:28.
doi: 10.3389/fmats.2015.00028

Silicon photonics technology, which has
the DNA of silicon electronics technology,
promises to provide a compact photonic
integration platform with high integration
density, mass-producibility, and excellent
cost performance. This technology has
been used to develop and to integrate
various photonic functions on silicon
substrate. Moreover, photonics-electronics
convergence based on silicon substrate is
now being pursued. Thanks to these features,
silicon photonics will have the potential
to be a superior technology used in the
construction of energy-efficient cost-effective
apparatuses for various applications, such as
communications, information processing,
and sensing.

Considering the material characteristics of
silicon and difficulties in microfabrication
technology, however, silicon by itself is not
necessarily an ideal material. For example,
silicon is not suitable for light emitting devices because it is an indirect transition material. The
resolution and dynamic range of silicon-based interference devices, such as wavelength filters,
are significantly limited by fabrication errors in microfabrication processes.

Frontiers in Materials and Frontiers in Physics

2

October 2015  |  Photonic integration and photonics-electronics convergence

For further performance improvement, therefore, various assisting materials, such as indiumphosphide, silicon-nitride, germanium-tin, are now being imported into silicon photonics by
using various heterogeneous integration technologies, such as low-temperature film deposition
and wafer/die bonding. These assisting materials and heterogeneous integration technologies
would also expand the application field of silicon photonics technology. Fortunately, silicon
photonics technology has superior flexibility and robustness for heterogeneous integration.
Moreover, along with photonic functions, silicon photonics technology has an ability of
integration of electronic functions. In other words, we are on the verge of obtaining an ultimate
technology that can integrate all photonic and electronic functions on a single Si chip.
This e-Book aims at covering recent developments of the silicon photonic platform and novel
functionalities with heterogeneous material integrations on this platform.
Citation: Yamada, K., Liu, J., Baba, T., Vivien, L., Xu, D.-X., eds. (2015). Photonic integration and
photonics-electronics convergence on silicon platform. Lausanne: Frontiers Media. doi: 10.3389/978-288919-693-7

Cover image: Large-scale photonic integration on silicon wafer.
Photograph by Koji Yamada

Frontiers in Materials and Frontiers in Physics

3

October 2015 | Photonic integration and photonics-electronics convergence

Table of Contents

05

Editorial: Photonic integration and photonics–electronics convergence
on silicon platform

Koji Yamada
07 Silicon photonic integration in telecommunications

Christopher R. Doerr
23 Small sensitivity to temperature variations of Si-photonic Mach–Zehnder
interferometer using Si and SiN waveguides

Tatsurou Hiraki, Hiroshi Fukuda, Koji Yamada and Tsuyoshi Yamamoto
28 Ultrahigh temperature-sensitive silicon MZI with titania cladding

Jong-Moo Lee
32 Silicon-nitride-based integrated optofluidic biochemical sensors using a
coupled-resonator optical waveguide

Jiawei Wang, Zhanshi Yao and Andrew W. Poon
45 High-throughput multiple dies-to-wafer bonding technology and
III/V-on-Si hybrid lasers for heterogeneous integration of optoelectronic
integrated circuits

Xianshu Luo, Yulian Cao, Junfeng Song, Xiaonan Hu, Yuanbing Cheng,
Chengming Li, Chongyang Liu, Tsung-Yang Liow, Mingbin Yu, Hong Wang,
Qi Jie Wang and Patrick Guo-Qiang Lo
66 Group IV light sources to enable the convergence of photonics and electronics

Shinichi Saito, Frederic Yannick Gardes, Abdelrahman Zaher Al-Attili, Kazuki Tani,
Katsuya Oda, Yuji Suwa, Tatemi Ido, Yasuhiko Ishikawa, Satoshi Kako,
Satoshi Iwamoto and Yasuhiko Arakawa
81 Group IV direct band gap photonics: methods, challenges, and opportunities

Richard Geiger, Thomas Zabel and Hans Sigg
99 Direct growth of Ge1-XSnx films on Si using a cold-wall ultra-high vacuum
chemical-vapor-deposition system

Aboozar Mosleh, Murtadha A. Alher, Larry C. Cousar, Wei Du, Seyed Amir Ghetmiri,
Thach Pham, Joshua M. Grant, Greg Sun, Richard A. Soref, Baohua Li,
Hameed A. Naseem and Shui-Qing Yu
106 Room-temperature near-infrared electroluminescence from boron-diffused
silicon pn-junction diodes

Si Li, Yuhan Gao, Ruixin Fan, Dongsheng Li and Deren Yang

Frontiers in Materials and Frontiers in Physics

4

October 2015  |  Photonic integration and photonics-electronics convergence

Editorial
published: 14 October 2015
doi: 10.3389/fmats.2015.00065

Editorial: Photonic integration and
photonics–electronics convergence
on silicon platform
Koji Yamada*
National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan
Keywords: silicon photonics, photonic integration, additional waveguide system, III–V semiconductors,
germanium-based emitter, wafer bonding, telecommunications applications, bio-chemical applications

Edited and reviewed by:
Lorenzo Pavesi,
University of Trento, Italy
*Correspondence:
Koji Yamada
yamada.koji@aist.go.jp
Specialty section:
This article was submitted to
Optics and Photonics,
a section of the
journal Frontiers in Materials
Received: 24 September 2015
Accepted: 29 September 2015
Published: 14 October 2015
Citation:
Yamada K (2015) Editorial: Photonic
integration and photonics–electronics
convergence on silicon platform.
Front. Mater. 2:65.
doi: 10.3389/fmats.2015.00065

Frontiers in Materials | www.frontiersin.org

Silicon-based photonics technology, which is based on the same paradigm of silicon (Si) electronics
technology, promises to provide us with a compact photonic integration platform with high integration density, mass manufacturing, and excellent cost performance. This technology has been used
to develop various photonic devices based on silicon, such as waveguides, filters, and modulators.
In addition, germanium (Ge) photodetectors have been built on a silicon-based photonic platform.
These photonic devices have already been monolithically integrated on silicon chips. Moreover, photonics–electronics convergence based on silicon photonics is now being pursued. These emerging
compact photonics–electronics convergent modules have the potential to be used in the fabrication
of energy-efficient cost-effective systems for various applications, such as communications, information processing, and sensing.
The last decade first saw the development of Si-based photonic technologies for communication
applications, and commercial products are now available for short-range data communications.
For medium-/long-range telecommunication applications, in which stringent technical standards
are applied to guarantee long-distance data transmission, intensive R&D is now providing us with
technologies for high-performance Si-based photonic modules with complex device integrations
(Doerr, 2015). In such high-performance applications, various assisting technologies should be
implemented on the silicon photonic platform. For example, the resolution and dynamic range of
silicon-based interference devices, such as wavelength filters, are considerably limited by fabrication
errors in microfabrication processes. To overcome such limitations, additional waveguide systems,
based on silicon nitride and silicon-rich silica, have been implemented (Yamada et al., 2014; Doerr,
2015).
Additional waveguide systems can also provide novel functionalities for further performance
improvements. For example, the thermo-optic response of photonic devices can be controlled by
combining silicon nitride and silicon waveguides, which could guarantee temperature-insensitive
operation of data transmission systems (Hiraki et al., 2015). Thermo-optic responses can also be
widely controlled by using titania as a cladding material in a Si waveguide (Lee, 2015). Moreover,
an additional waveguide system can expand the application field of the Si photonic platform. For
instance, silicon nitride waveguides, which are transparent to visible light, can be used to construct
compact bio-sensing systems on a small Si chip (Wang et al., 2015).
Light-source integration, which is the most important open issue for the Si photonic platform,
requires the help of other materials. For this purpose, III–V semiconductor materials have been
bonded on silicon by using various hybrid integration techniques, such as direct die-to-wafer
bonding (Fang et al., 2006; Luo et al., 2015). For monolithic integration, Ge-based light sources are
now being studied intensively. The most important technology for Ge-based light sources is bandgap engineering, which aims to achieve a direct transition in Ge, which is originally an indirecttransition material. The recent status of Ge-based light sources on Si is reviewed in this special issue
(Saito et  al., 2014; Geiger et  al., 2015). Mechanical stress and heavily doped n-type carriers

5

October 2015 | Volume 2 | Article 65

Yamada

Photonic integration on silicon platform

would significantly contribute to making Ge a direct-transition material. GeSn alloy is also a very attractive material for
light sources on a Si platform because Sn, which can easily
be dissolved in Ge, offers an important degree of freedom in
band-gap engineering (Mosleh et al., 2015). Another approach
now being investigated for monolithic integration of light
sources is Si-based electro-luminescence at room temperature, although its physical origin has not fully understood (Li
et al., 2015).
The Si-based photonic platform requires various assisting
materials for accomplishing practical photonic functions.

Fortunately, it has superior flexibility and robustness for integrating these materials. Along with photonic functions, the Si-based
photonic platform can integrate electronic functions monolithically. In other words, we are on the verge of obtaining an ultimate
technology that can integrate all photonic and electronic
functions on a single Si chip.

ACKNOWLEDGMENTS
I would like to acknowledge all the authors, reviewers, editors, and publishers, who have supported this Research Topic.

REFERENCES

vacuum chemical-vapor-deposition system. Front. Mater. 2:30. doi:10.3389/
fmats.2015.00030
Saito, S., Gardes, F. Y., Al-Attili, A. Z., Tani, K., Oda, K., Suwa, Y., et al. (2014).
Group IV light sources to enable the convergence of photonics and electronics.
Front. Mater. 1:15. doi:10.3389/fmats.2014.00015
Wang, J., Yao, Z., and Poon, A. W. (2015). Silicon-nitride-based integrated optofluidic biochemical sensors using a coupled-resonator optical waveguide. Front.
Mater. 2:34. doi:10.3389/fmats.2015.00034
Yamada, K., Tsuchizawa, T., Nishi, H., Kou, R., Hiraki, T., Takeda, K.,
et  al. (2014). High-performance silicon photonics technology for telecommunications applications. Sci. Technol. Adv. Mater. 15, 024603.
doi:10.1088/1468-6996/15/2/024603

Doerr, C. R. (2015). Silicon photonic integration in telecommunications. Front.
Phys. 3:37. doi:10.3389/fphy.2015.00037
Fang, A. W., Park, H., Cohen, O., Jones, R., Paniccia, M. J., and Bowers, J. E. (2006).
Electrically pumpled hybrid AlGaInAs-silicon evanescent laser. Opt. Express
14, 9203–9216. doi:10.1364/OE.14.009203
Geiger, R., Zabel, T., and Sigg, H. (2015). Group IV direct band gap photonics:
methods, challenges, and opportunities. Front. Mater. 2:52. doi:10.3389/
fmats.2015.00052
Hiraki, T., Fukuda, H., Yamada, K., and Yamamoto, T. (2015). Small sensitivity to
temperature variations of Si-photonic Mach-Zhender interferometer using Si
and SiN waveguides. Front. Mater. 2:26. doi:10.3389/fmats.2015.00026
Lee, J.-M. (2015). Ultrahigh temperature-sensitive silicon MZI with titania cladding. Front. Mater. 2:36. doi:10.3389/fmats.2015.00036
Li, S., Gao, Y., Fan, R., Li, D., and Yang, D. (2015). Room-temperature near-infrared
electroluminescence from boron-diffused silicon pn-junction diodes. Front.
Mater. 2:8. doi:10.3389/fmats.2015.00008
Luo, X., Cao, Y., Song, J., Hu, X., Cheng, Y., Li, C., et al. (2015). High-throughput
multiple dies-to-wafer bonding technology and III/V-on-Si hybrid lasers for
heterogeneous integration of optoelectronic integrated circuits. Front. Mater.
2:28. doi:10.3389/fmats.2015.00028
Mosleh, A., Alher, M. A., Cousar, L. C., Du, W., Ghetmiri, S. A., Pham, T., et al.
(2015). Direct growth of Ge1-xSnx films on Si using a cold-wall ultra-high

Frontiers in Materials | www.frontiersin.org

Conflict of Interest Statement: The author declares that the research was conducted in the absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Copyright © 2015 Yamada. This is an open-access article distributed under the terms
of the Creative Commons Attribution License (CC BY). The use, distribution or
reproduction in other forums is permitted, provided the original author(s) or licensor
are credited and that the original publication in this journal is cited, in accordance
with accepted academic practice. No use, distribution or reproduction is permitted
which does not comply with these terms.

6

October 2015 | Volume 2 | Article 65

REVIEW
published: 05 August 2015
doi: 10.3389/fphy.2015.00037

Silicon photonic integration in
telecommunications
Christopher R. Doerr *
Acacia Communications, Hazlet, NJ, USA

Silicon photonics is the guiding of light in a planar arrangement of silicon-based materials
to perform various functions. We focus here on the use of silicon photonics to create
transmitters and receivers for fiber-optic telecommunications. As the need to squeeze
more transmission into a given bandwidth, a given footprint, at a given cost increases,
silicon photonics makes more and more economic sense.
Keywords: integrated optics, silicon photonics, optical fiber, optical communications, coherent, gratings,
waveguides

1. Introduction
Edited by:
Qiaoliang Bao,
Soochow University, China
Reviewed by:
Lukas Novotny,
ETH Zurich, Switzerland
Satoshi Iwamoto,
The University of Tokyo, Japan
Xiangping Li,
Swinburne University
of Technology, Australia
*Correspondence:
Christopher R. Doerr,
Acacia Communications, 1301 Route
36, Hazlet, NJ 07730 USA
chris.doerr@acacia-inc.com
Specialty section:
This article was submitted to
Optics and Photonics,
a section of the journal
Frontiers in Physics
Received: 11 February 2015
Paper pending published:
23 March 2015
Accepted: 13 May 2015
Published: 05 August 2015
Citation:
Doerr CR (2015) Silicon photonic
integration in telecommunications.
Front. Phys. 3:37.
doi: 10.3389/fphy.2015.00037

Frontiers in Physics | www.frontiersin.org

Until circa 2002, fiber-optic communication for metropolitan distances (80—600 km) and longhaul distances (600–15,000 km) employed mostly simple on-off keying (OOK) transmission. On-off
keying is simply turning on and off the light to transmit “1” s and “0” s. Higher performance, i.e., a
lower bit-error rate (BER) for the same received optical power and/or for the same optical signal-tonoise ratio (OSNR), can be obtained by using phase-modulated formats, such as binary phase-shift
keying (BPSK) or quadrature phase-shift keying (QPSK). They maximize the distance between
constellation points for the same average signal power. In these “advanced” modulation formats
[1], the term “symbol” is used to represent each data portion in time, because each symbol can
carry multiple bits of information. Early BSPK and QPSK were detected by differential detection,
i.e., by interfering one symbol with the previous symbol in an interferometer in the receiver.
However, bandwidth needs have been constantly growing exponentially. It is expensive to install
new optical fibers, ∼ $30 k per mile [2], so carriers and data-center operators needed to send
more bits per second in the same fiber in the same optical bandwidth. One key way is to use both
optical polarizations, because this doubles the available bandwidth. Although signal orthogonality
is maintained, their polarizations are essentially randomly changed during propagation through
fiber. To unscramble them requires significant signal processing. Optical coherent detection allows
this to be done by digital electronics.
Optical coherent detection was a hot topic in the 1980s, because it is a form of optical
amplification. However, the invention of the erbium-doped fiber amplifier (EDFA) eliminated
that advantage and coherent interest died away. Another advantage of coherent detection is the
ability to receive the full optical field, both the real and imaginary parts of both polarizations. With
improvements in complementary metal-oxide-semiconductor (CMOS) electronics, digital signal
processing (DSP) became available circa 2002 to handle coherent detection even up to 100-Gb/s,
causing a revival of coherent detection. In the past, coherent detection was simply single quadrature
and single polarization. Now it is dual quadrature and dual polarization.
100-Gb/s coherent systems have proven to be extremely compelling. They allow an upgrade of
a 10-Gb/s channel to a 100-Gb/s channel with actually improved reach. Industry analyses show
the number of metro and long-haul 100-Gb/s coherent transceivers sold per year to be on a steep
upwards ramp as 10-Gb/s OOK transceivers are replaced by 100-Gb/s coherent transceivers.

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Silicon photonic integration in telecommunications

FIGURE 1 | 100-Gb/s coherent transceiver form-factor evolution. It went from a full line card, to a multi-source agreement (MSA) module, to a 100-Gb/s
form-factor pluggable (CFP) module. The D in D-CFP means that it is a digital module and the DSP is included inside, as opposed to a module that contains only the
optics.

However, the price of 100-Gb/s coherent transceivers is
expected to drop significantly. This is because users want to
pay the same price per connection even though the bit rate
keeps increasing. However, not only must the price drop, but the
footprint and power consumption as well. As seen in Figure 1,
100-Gb/s modules have gone from full line cards to 5 × 7
in2 screwed-on modules to 3.2 × 5.7 in2 pluggable modules
today. Today’s 100-Gb/s pluggable form factor is called a CFP.
Tomorrow’s will be a CFP2, which is half the size, and eventually a
CFP4, which is a quarter the size. Power consumptions have gone
from more than 100 W on a line card, to 70 W for the screwedon module, to 28 W for the CFP. The next step, the CFP2, allows
only 12W.
There are two main components in a coherent transceiver—
the DSP chip and the optics. Today’s coherent CFP contains both.
There is a, possibly temporary, trend to take the DSP out of the
module and put it on the line card. Such modules are called
“analog” modules, rather than digital. With today’s technology, it
is not possible to have both the optics and DSP be under 12 W, the
maximum power in a CFP2. However, in 1–2 years, technology
will likely be ready for a “digital” CFP2.
To meet these requirements of lower price, lower power and
smaller footprint, one must make advancements in technology.
For the DSP, one can take advantage of the steady reduction in
transistor size in industry, which reduces power and footprint.
Node size and introduction year are shown in Table 1 [3]. Today’s
coherent DSPs use 20–28 nm. Tomorrow’s will use 14 nm.
For the optics, one must use photonic integration, the focus of
this article. Most of today’s coherent transceivers are built using
separate LiNbO3 /planar lightwave circuit (PLC) modulators and
InP/PLC receivers, as shown in Figure 2. More and more, smaller
InP modulators and InP receivers are being used. In today’s
coherent CFP, there is a single silicon photonic (SiPh) integrated
circuit (PIC) containing both the transmitter and receiver [4].
Not shown is a separate tunable laser.
Finally, a dominant cost for the DSP and optics is the
packaging; one can further reduce cost, power, and footprint by
co-packaging the DSP and optics. Such transceivers are expected
in 2–3 years.
Figure 3 shows many of the elements that may be integrated
in a PIC. The blue are passive, the red are active (have an intended
dynamic interaction between light and matter), and the green
are electronic components. PICs have been around more than 20

Frontiers in Physics | www.frontiersin.org

TABLE 1 | Node size and first year of commercial introduction for CMOS
electronics.
Node size

Year

10 µm

1971

6 µm

1974

3 µm

1977

1.5 µm

1982

1 µm

1985

800 nm

1989

600 nm

1994

350 nm

1995

250 nm

1997

180 nm

1999

130 nm

2001

90 nm

2004

65 nm

2006

45 nm

2008

32 nm

2010

22 nm

2012

14 nm

2014

10 nm

2016

7 nm

2018

5 nm

2020

years. The main advantages of photonic integration are a small
footprint, due to strongly confining waveguides and lens-free
connections between parts; low power, due to an obviation of
50- RF lines; higher bandwidth RF connections; and low price,
due to fewer touch points, no mechanical adjustments, less test
equipment, and less material. The main disadvantages of PICs
are typically a higher insertion loss and the inability to optimize
components independently.

2. PIC Material Systems
Figure 4 shows the most popular PIC material systems. From left
to right there are silica-on-silicon PICs, also called PLCs; siliconon-insulator PICs, also called silicon photonics; lithium niobite
(LiNbO3 ); and III–V PICs, such as InP and GaAs. This article
focuses on silicon photonics. In silicon photonics, the light is
mostly guided in silicon, which has an indirect bandgap of 1.12

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August 2015 | Volume 3 | Article 37

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Silicon photonic integration in telecommunications

eV (1.1 µm). The silicon is a pure crystal grown in a boule and
then sliced into wafers, today typically 300 mm in diameter, as
shown in Figure 5. The surfaces are oxidized to form SiO2 layers.
One wafer is bombarded with hydrogen atoms to a specified
depth. Then the two wafers are placed together in a vacuum,
and the oxide layers bond to each other. The assembly is cracked
at the hydrogen implantation line. Then the silicon layer where
the crack was is polished, and one is left with a thin layer of
crystalline silicon on a layer of oxide on a full silicon “handle”
wafer. The waveguides are formed from this thin crystalline layer.
While these silicon-on-insulator (SOI) wafers are what makes
low-loss silicon photonic waveguides possible, they are actually
used mostly for low-power CMOS circuits, because of the low
leakage currents they offer.
There is a wide family of possible silicon-based optical
waveguides, shown in Figure 6. They range from micro-scale
Ge-doped SiO2 waveguides to nano-scale Si wire waveguides. By
adding Ge, one can make photodetectors and electro-absorption
modulators. Potentially even optical amplifiers. By doping the
silicon one can make optical modulators. From left to right at the
bottom are silicon wire waveguides, silicon nitride waveguides,

FIGURE 2 | 100-Gb/s coherent optics evolution, going from LiNbO3
modulators and planar lightwave circuit (PLC)-based receivers to
InP-based modulators and receivers to silicon photonic modulators
and receivers.

FIGURE 3 | Some of the possible PIC integration elements.

FIGURE 4 | Popular PIC material systems. The second from the left is a silicon wire waveguide and is the focus of this article.

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silicon oxynitride waveguides, thick silicon rib waveguides, thin
silicon nitride waveguides, and doped silica waveguides. From left
to right at the top are a depletion modulator, a Ge photodetector,
and a Ge optical amplifier.

Another key element is a spot-size converter, which converts
the ∼ 0.5 × 1 µm2 mode of a Si wire waveguide to the ∼ 10 ×
10 µm2 mode of an optical fiber. A typical method is to use
an inverse taper, in which the waveguide is narrowed down to
a small tip, causing the optical mode to expand very large [8].
The mode can be captured by a suspended glass waveguide, such
as in Figure 8 [9]. Coupling losses less than 1.5 dB are readily
achievable with such spot-size converters.
Another key passive element is a polarization splitter. Some
polarization splitter examples are shown in Figure 9. The first
is a Mach-Zender interferometer with a different birefringence
in each arm [10]. The second is a simple directional coupler
[11]. The shape birefringence is so high in typical silicon wire
waveguides, that the transverse-magnetic (TM) polarization can
couple fully while the transverse-electric (TE) polarization has
barely begun to couple. The third is a grating coupler in which the
fiber is placed at an angle such that TE couples in one direction

3. Si Photonic Passive Elements
There are several key silicon photonic passive elements. One
is the surface-emitting grating coupler, as shown in Figure 7A
[5, 6]. It consists of a strong grating in the waveguide with a pitch
approximately equal to the wavelength in the waveguide. This
causes light to emit or be received vertical to the surface, which is
well-suited for wafer level measurements and/or coupling to an
optical fiber. The grating coupler is somewhat unique to silicon
photonics because it requires a high vertical index contrast. For
example, if one tried to do a grating coupler in traditional InP
waveguides, the light would simply leak away into the substrate
rather than be emitted vertically, because the average index of the
grating waveguide would be below that of the substrate. To make
it work in InP, one must undercut the material under the grating,
suspending it, as shown in Figure 7B [7].

FIGURE 7 | Surface-emitting 1-D grating couplers in silicon (A) and
InP (B). In (A), the gray and light blue represent silicon and silicon dioxide,
respectively. In (B), the red and orange represent InGaAsP and InP,
respectively. (C,D) SEM pictures of the InP suspended cantilever grating
coupler.

FIGURE 5 | How a silicon-on-insulator (SOI) wafer is made. Each wafer
is made from two silicon wafers. The wafers are oxidized, bonded, and one is
cut and polished to a thin layer.

FIGURE 6 | Cross sections of the family of Si-based optical waveguides. Also shown are typical propagation losses and refractive indices.

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and TM the other [12]. The fourth is a 2D grating coupler
[13]. The fiber mode with its electric field perpendicular to the
waveguide propagation direction will couple to that waveguide.
The fiber can be either tilted and couple to two waveguides
or be normal to the surface and couple to four waveguides.
The 2D grating coupler has the added advantage of acting as a
polarization rotator, in that all the light on the chip has the same
polarization yet was two orthogonal polarizations in the fiber.

number of free electrons and holes, either by doping, electrical
means, or optical means, as shown in Equations (1, 2), obtained
by fitting to data in Soref and Bennett at 1550-nm wavelength
[14]. The holes have a larger ratio of real to imaginary index
change, i.e., more phase change for a given loss change, and
thus are usually favored for making the phase modulators in
Mach-Zehnder and ring modulators.
1nr = −8.8 × 10−22 Ne − 8.5 × 10−18 Nh0.8
ni = 1.0 × 10

4. Si Photonic Active Elements

−22

Ne + 7.4 × 10

−23

Nh

(1)
(2)

Various Si modulator types are shown in Figure 10A. In the
carrier injection modulator, the light is in intrinsic silicon
inside a very wide p-i-n junction, and electrons and holes are
injected. Such a modulator is slow, however, typically 500-MHz
bandwidth, because it takes a long time for the free electrons
and holes to recombine after injection. Thus, such structures
are usually used as variable optical attenuators (VOAs) rather
than modulators [15, 16]. In the carrier depletion modulator,
the light is partly in a narrow p-n junction, and the depletion
width of the p-n junction is varied by an applied electric field.
Such a modulator can operate at over 50 Gb/s [17], but has
a high background insertion loss. A typical Vπ L is 2 V-cm.
The metal-oxide-semiconductor (MOS) (really semiconductoroxide-semiconductor) modulator contains a thin oxide layer in
the p-n junction [18]. It allows for some carrier accumulation
as well as carrier depletion, allowing for a smaller Vπ L of ∼0.2
V-cm, but with the drawbacks of higher optical loss and higher
capacitance per unit length. There are also SiGe electroabsorption
modulators [19] that rely on band-edge movement in SiGe. There
are also graphene modulators that rely on switching the graphene
between an absorbing metal and a transparent insulator [20].
Various Si-based photodetectors are shown in Figure 10B.
The absorption material is Ge. Ge absorbs light with wavelengths
up to about 1.6 µm. Shown on the left is a p-i-n configuration

As mentioned above, a photonic active element has an intentional
dynamic interaction between light and matter. A typical photonic
active element is an optical modulator. All the Si optical
modulators today are based on the plasma free carrier effect. The
complex refractive index of the silicon changes by changing the

FIGURE 8 | Spot-size converter for silicon wire waveguides. The silicon
is inverse tapered inside a suspended glass waveguide. The silicon substrate
has been etched away under the suspended glass waveguide.

FIGURE 9 | Various polarization splitters.

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FIGURE 10 | (A) Cross sections of various silicon-based optical modulator designs and (B) photodetector designs.

FIGURE 11 | Configurations for integrating optical gain into silicon photonics. Fabrication insertion point becoming later in the process as one moves from left
to right.

[21], the most successful commercially today. It consists of pdoped silicon on which Ge is grown. Ge and Si have a 4% lattice
mismatch, so to minimize dislocations, a thin layer of SiGe is
grown first. The top of the Ge is n doped. Shown in the middle is
a metal-semiconductor-metal (MSM) photodiode [22] and at the
right avalanche photodiodes (APDs) [23]. The APD avalanche
region is in Si, which has a lower noise than avalanche regions
in III–V materials.
There is still no clear-winning solution for integrating optical
gain with silicon photonics. Some of the various option are shown
in Figure 11, organized by assembly level. On the far left is

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monolithic integration, including using epitaxially grown Ge as
an optical gain material [24], Er-doped glass waveguides, such
as Al2 O3 , (which require optical pumping) [25], and epitaxially
grown GaAs quantum dots [26]. The next column is waferto-wafer assembly, including oxide bonding [27] and organic
bonding [28] of III–V gain regions. The next column is die-towafer assembly, including inserting III–V die into cavities in the
Si wafer and then patterning the waveguides [29]. The advantages
of all the left three columns is that the full device can be tested
on the wafer level, before it is diced out. The far right column
is die-to-die assembly, including butt coupling of a Si die and a

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III–V die and coupling with a lens and a grating coupler [30].
Commercial deployment is tending to move from the right to left
of this figure.
An element that is partway between an active and passive
element is an optical isolator. Optical isolators are required to
stop back reflections from causing noise and oscillations in lasers
and optical amplifiers. An isolator requires a non-reciprocal
element [31]. In silicon photonics, two main types of isolators
have been reported: magneto-optic and modulation-based.
In magneto-optic isolators, garnets are placed on the side or
top of the waveguide [32, 33]. In a modulation-based isolator,
the optical field is modulated with either a traveling wave or
a time delay between multiple modulators [34]. Figure 12
shows a modulation-based isolator design based on a parallel
arrangement of phase modulators in series [35]. Each modulator
is driven by a sine wave. In the forward direction, the second
modulator in each arm undoes the modulation of the first
modulator; but in the backward direction, the two modulators
add constructively. Thus, there is no effect at all on the signal in
the forward direction but in the backward direction it is strongly
phase modulated. If the phase modulation amplitude is just right,
then a continuous-wave signal passing backwards is completely
attenuated at its original frequency. This gives narrow-band
isolation. By having multiple such narrow-band isolators in
parallel, driven by the same frequency but appropriate different
RF drive phases in each arm, one can achieve broadband
isolation. A two-arm version was demonstrated in silicon
photonics, achieving ∼3 dB of isolation. The modulation
was done by carrier injection in the silicon waveguide. The
isolation can be improved by reducing the residual amplitude
modulation in the phase modulators, by increasing the speed of
the modulators, and/or by increasing the number of arms in the
interferometer.

Silicon photonics is usually considered only for low-cost,
short-reach, high-volume (>1M/year) products. This is because
it is assumed that a large number of wafer starts is required
to pay for mask and development costs and that silicon
photonics has a significant performance penalty for metro and
long-haul products. However, the real situation is actually the
opposite. This is because in low-cost, short-reach, high-volume
applications, there is tremendous competition from vertical
cavity surface-emitting lasers (VCSELs) and directly modulated
lasers (DMLs), and silicon photonics’ weakness of not having
an easy way to integrate lasers is a significant disadvantage.
On the other hand, in metro and long-haul applications, it is
better to keep the laser separate anyway as it is preferable to
integrate the silicon photonics and DSP together, which is a hot
environment. Also, coherent detection can make up for many

FIGURE 12 | (A) configuration of an optical isolator that uses a tandem
arrangement of phase shifters. There are N arms in the interferometer. The
more the arms, the higher the broadband isolation. (B,C) 2-arm version built in
silicon photonics.

5. PIC Material System Comparison
Table 2 shows a comparison between InP and Si. InP is a much
more expensive material than Si because of the rarity of In. Si
circuits tend to have a higher yield than InP circuits because there
is much less epitaxy involved in Si circuits. In Si circuits, usually
the only epitaxy is Ge, used in the photodetectors, whereas in
InP all of the waveguides, even the passive ones, must be grown
by epitaxy. Epitaxy tends to have a higher defect density than
crystal growth from a boule. InP waveguides have high index
contrast only laterally, whereas Si waveguides have high index
contrast laterally and vertically. This allows much smaller bend
radii and other more compact structures in Si. InGaAsP has a
direct bandgap, whereas Si and Ge do not. Thus, the InP material
system has a much more efficient laser. The native oxide of the
InP system is much less robust than the native oxide of Si, which
is SiO2 . Silicon is a stronger material than InP, allowing for much
larger wafers, 75 mm compared to 300 mm (going to 450 mm
soon). InP modulators usually depend on the quantum-confined
Stark effect, which is temperature sensitive because of the band
edge movement with temperature. Silicon modulators have very
minimal temperature dependence.

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TABLE 2 | Pros and cons of InP and Si for photonic integrated circuits.
InP

Si

Expensive material

Cheap material

• In is scarce

Medium yield
• W.g. material from epitaxy

Small footprint
• High index contrast in 1D

• W.g. material from original boule

Extremely small footprint
• High index contrast in 2D

Native laser

No native laser

Poor native oxide

Excellent native oxide

Low dark current

Medium dark current

Small wafers (75 mm typ.)

Large wafers (300 mm typ.)

• 75 mm typical
• Brittle material

Modulator temperature sensitive
• Band edge moves with temperature

13

• 27% mass Earth’s crust is Si

High yield

• 300 mm typical
• Strong material

Modulator temperature insensitive
• Carrier density not v. temp. dep.

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FIGURE 13 | Simulation results from 3D sparse FDTD. (A) is the top
view of the structure being simulated, which is a directional coupler. (B)
Shows a screen shot from a simulation using a quasi-TE launch. The top two

figures show the top views of the quasi-TE and quasi-TM signals, and the
lower two figures show the corresponding cross-section views. (C) Shows a
screen shot from a simulation using a quasi-TM launch.

FIGURE 14 | Silicon photonics 8-PSM transceiver. Courtesy of Luxtera.

FIGURE 15 | Silicon photonics 8-WDM receiver. The upper figure shows a photograph of the chip, the lower left figure shows the measured responsivities to the 8
detectors vs. wavelength, and the lower right figure shows the measured bit-error rate at 1.25 Gb/s for one of the channels using a polarization scrambler.

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of silicon photonics’ imperfections, such as the dark current
is much smaller than the local oscillator photocurrent. Also,
the argument that one needs a large number of wafer starts
to pay for mask and development costs is fallacious, because
silicon photonics is done in a very large node size compared to

state-of-the-art CMOS, and thus the masks and runs are relatively
inexpensive.

6. PIC Design
PICs are usually laid out in using mathematical scripts. This
is because usually in PICs, path lengths matter, when in
interferometers or because of skew. The PIC is made by
patterning multiple layers, typically 10 to 30, on a wafer. These
layers consist of many polygon shapes, typically in a GDSII
format. Before sending the files to the photomask shop, there
is a strong desire to be able to simulate the PIC to verify the
design. There are multiple levels of simulation. The lowest level
is 3D electromagnetic (EM) simulation, in which simulation
is done at the sub-wavelength level. Interaction with atoms
in the materials is done on the macroscopic scale. Typical
methods are the 3D finite-difference time domain (3D FDTD)
[36] and eigenmode expansion (EME) methods [37]. These
methods are the most accurate but simulation times for an
entire PIC are prohibitive. The next level is 2.5D EM simulation,
such as the finite-difference beam propagation method (FDBPM). These methods are significantly faster, with a tradeoff
of accuracy. Also, BPMs can handle only paraxial propagation,
e.g., they cannot be used to simulate a resonator. The next
level is 2D EM simulation, such as 2D FDTD and 2D BPM.
Again, these are faster, but limited. These cannot simulate
e.g., a polarization rotator. The next level up is transmission
and/or scattering matrix simulation. Each main component is
reduced to an element with inputs and outputs, and connecting
waveguides are reduced to phase shift and attenuation elements.
These simulations are extremely fast. A transmission matrix is

FIGURE 16 | Measured PAM-2, -4, and -8 optical eye diagrams at 28
Gbaud using a silicon photonics modulator.

FIGURE 17 | 80-Gb/s dual-polarization transmitter in InP. It consists of
two electro-absorption InGaAsP modulators, a polarization splitter, and a
polarization combiner. The incoming laser has its polarization oriented at 45◦ .

FIGURE 18 | DP-DQPSK receiver in silicon photonics. Uses optical
polarization tracking. The upper figure shows a schematic, and the lower
figure a device photograph. The incoming signal is separated into two

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polarizations by a 2D grating coupler, a series of couplers and phase shifters
demultiplex the two signals, which had been mixed during fiber transmission,
and two Mach-Zehnder delay interferometers demodulate the signals.

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FIGURE 19 | 7-core-fiber silicon photonics receiver. The upper right
figure shows a photograph of the fiber cross section, showing the seven
cores. The top shows the schematic for each channel in the silicon photonic

circuit shown in the bottom figure. The incoming fiber is tilted at the proper
angle to the 1D grating couplers such that TE polarization couples to the left
and TM polarization couples to the right.

FIGURE 21 | First reported vector modulator. It was in GaAs.
FIGURE 20 | PIC for coupling to the multimodes of a ring-core fiber by
using a circular grating coupler connected to a star coupler. The upper
figure shows a schematic of the silicon photonic circuit photograph shown at
the bottom. The circuit contains a circular grating coupler connected to an
array of waveguides of equal length.

without sufficient high-order mode rejection, or two waveguides
that pass too close to each other and have undesired coupling are
unlikely to be caught.
A technique called sparse FDTD allows one to do 3D and
2D FDTD simulation directly on the entire PIC design to verify
the design [38]. While it is unlikely any EM simulation tool
can simulate a very large PIC, sparse FDTD can simulate quite
large portions. In conventional 3D FDTD, one starts with all six
components of the EM fields in a specified quantized volume.
Time is advanced a step, and the new field components are
calculated in the volume, and so on. So many calculations every
step takes a very long time. In sparse 3D FDTD, rather than do
calculations for every point in the volume every step, a list of
field components is maintained, theoretically in an arbitrarily
large volume, is maintained and calculations are done on these.
At each time step points neighboring the field components are

multiplied by the incoming signals to find the outgoing signals.
A scattering matrix (whose elements are called s-parameters) is
multiplied by the incoming and outgoing signals on one side of
the element to find the incoming and outgoing signals on the
other side of the element. Basically, scattering matrices include
reflections within the element. Scattering matrices are typically
twice as large in each dimension as transmission matrices.
However, relying on EM simulation of some elements and
scattering/transmission matrices to simulate the entire PIC does
not guarantee that the design is error-free before tape out. For
example, a miscalculated path length, a multimode waveguide

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added, and field components with power below a certain level are
discarded. For certain structures this calculation can be orders of
magnitude faster than conventional 3D FDTD. However, sparse
FDTD performs poorly with dispersive structures, because then
the optical field spreads out too much, making the list too long.
Example screen shots from 3D FDTD simulation of a PBS like
that shown in Figure 9B are shown in Figure 13 [39].

for a wide diversity of solutions. These various solutions do not
interoperate, but the users do not care so much, as long as prices
are low.
Today, most of the short reach links are based on verticalcavity surface-emitting lasers (VCSELs) over multimode fiber,
i.e., do not involve PICs at all. VCSELs are very inexpensive and
easy to couple to multimode fiber. It is nearly impossible for PICs
to compete against VCSELs on price. However, the bandwidthdistance product for a VCSEL over multimode fiber is ∼ 2 GHzkm. At 25 Gb/s, this limits distances to ∼100 m. Also, multimode
fiber (MMF) costs more than standard single-mode fiber (SSMF),
because many more km of SSMF have been produced than MMF.
Thus, when new data centers are built, it can be advantageous
to outfit them with SSMF. Single-mode VCSELs are difficult to
make today, so this is a good opportunity for PICs. However,
VCSEL technology is constantly improving, providing a constant
challenge to PICs in short-reach applications.
A successful PIC short-reach commercial solution today is
based on parallel single-mode fibers (PSM). Figure 14 shows an

7. Short Reach PICs
Short reach communications typically means less than 2 km, but
can sometimes include up to 40 km. Short reach is usually for
intra-data center, connecting racks, or client-side optics. There is
an emerging need for very short reach communications in which
boards are connected optically within a rack. Such optics are no
longer considered “transceivers” and for the sake of focus are left
out of this article.
Because of the fast growth and turn-over in data centers there
is usually insufficient time for standards to develop. This allows

FIGURE 22 | Early reported silicon photonic vector modulators.

FIGURE 23 | 2-bit optical DAC in silicon photonics.

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8-fiber PSM solution (4 fibers out and 4 fibers in) based on
silicon photonics from Luxtera [40]. The chip contains a 1.4µm laser in a small hermetic assembly on top of the PIC. This
wavelength was chosen as optimum for the grating couplers that
couple the laser light into the PIC. This laser is split four ways
to four 10-Gb/s on-off-keying (OOK) distributed-driven MachZehnder-interferometer modulators (MZMs). The CMOS drive
electronics are monolithically integrated with the photonics.
Distributed driven means that the modulator is broken into N
sections in series, each with a separate driver timed appropriately.
This saves power consumption over a traveling-wave modulator,
because a traveling-wave modulator has a termination resistor
into which power must be dumped.
Another successful PIC short-reach solution is based on
wavelength-division multiplexing (WDM). Typically four
wavelengths, each modulated with OOK at 25 Gb/s, are
multiplexed in the transmitter and demultiplexed at the receiver.

The advantage over PSM is requiring only two fibers instead
of eight, and the disadvantage is requiring four lasers instead
of one. WDM makes more sense as the cost of transceivers
drops compared to the cost of fiber and installing it, especially
ribbon fibers. Figure 15 shows an 8-channel CWDM receiver in
silicon photonics [41]. It uses a silicon nitride spot-size converter
and arrayed waveguide grating (AWG), which is polarization
independent via variation of waveguide widths, silicon output
multimode waveguides, and Ge photodetectors.
Yet another solution is to use multi-level modulation, called
pulse amplitude modulation (PAM). Figure 16 shows PAM4 and
PAM8 eye diagrams at 28 Gb/s generated by a silicon photonics
MZM.
One can also use polarization-division multiplexing (PDM),
also called dual-polarization (DP) transmission. In this case,
different signals are in each polarization. Figure 17 shows a
dual-polarization 80-Gb/s modulator in InP [42]. Such a design

FIGURE 24 | Early (A) InP and (B) silicon integrated coherent receivers.

FIGURE 25 | Single-chip silicon photonic coherent transceiver.

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could be readily made in silicon photonics. In the fiber, the
two signals will stay predominantly orthogonally polarized,
but the polarization will vary unpredictably with time. At the
receiver, if one does not use coherent detection, one needs to
optically demultiplex the two polarizations. Figure 18 shows
a device in silicon photonics that can optically demultiplex
polarization [43, 44]. It does this by receiving two orthogonal
polarizations from the fiber, these polarizations not necessarily
that of the signals, and then interferes the two with a
controllable phase and coupling ratio to demultiplex them. To
do this in an endless fashion, i.e., without ever needing phaseshifter resets back to zero, one needs multiple interferometer
stages.
In the far future one may find the data center interconnections
so crowded that one must reduce the number of fiber strands
and instead put multiple cores and/or modes in a single fiber.
Figure 19 shows a PIC for receiving from a 7-core fiber, using
polarization diversity [45]. It includes optical filters for WDM.
Figure 20 shows a PIC for receiving from a multi-mode ringcore fiber [46]. A multi-mode ring core fiber is advantageous
because the modes can be accessed without waveguide crossings
and conveniently demultiplexed by a star coupler.

coherent transmission. This is because long fiber routes are
expensive to install/obtain, and thus the user wants to push
as much information over each fiber as possible. Coherent
receivers make it possible to receive WDM, PDM, and high-order
constellations with high-performance, because the complete
optical field is received and acted on by a DSP. In intradyne
coherent communications, the transmitted signal comes from
a dual-polarization vector modulator, and the received signal
is interfered with a continuous-wave (CW) laser signal whose
frequency is close to the carrier of the signal (within ∼2–3 GHz),
but does not need to be exact.
The first reported vector modulator was a GaAs PIC,
shown in Figure 21 [47]. It consists of two MZMs in a larger
interferometer. Figure 22 show some early vector modulators
in silicon photonics. The modulator in Figure 22A contains
two vector modulators, one for each polarization, along with
the polarization splitting optics [48]. The single-polarization
modulator in Figure 22B uses a thin-oxide layer in the p-n
junction to obtain a low Vπ L product and is driven directly
by CMOS inverters [49]. By using multiple segments in the
modulator, one can create an optical digital-to-analog converter
(DAC). The segment lengths are in a geometric sequence.
Figure 23 shows a demonstration that achieved 16-QAM
modulation at 13 Gbaud using a silicon photonic optical DAC
[50].
The first reported coherent receivers were in InP, as
shown in Figure 24A [51–55]. Figure 24B shows an early

8. Metro and Long-reach PICs
Unlike short-reach links, which we saw have many choices of
transmission type, metro and long-reach links demand intradyne

FIGURE 26 | (A) Measurement setup, (B) measured 30-Gbaud eye diagram for DP-QPSK, (C) real-time-processed constellations at 120 Gb/s, and (D–F) bit-error
rate vs. OSNR for various cases for the silicon photonics single-chip coherent transceiver in an optical loop back configuration.

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Figure 26D shown for comparison is the performance of discrete
optics. The performance of the silicon PIC is nearly the same
as the discrete optics. Figure 26E shows the performance at
various temperatures, showing that the silicon photonics can
indeed work without temperature control. Figure 26F shows the
performance up to 3000 km without significant penalty. This
shows that the chirp of the silicon photonics modulator is low.
Figure 27 shows this PIC in a 100-Gb/s CFP module. As one
can see, the module is tightly packed and would be very difficult
to make with discrete optics.
This single-chip coherent transceiver contains all the optics
needed for a coherent transmitter except the tunable laser. As
mentioned earlier, it is probably better to keep the laser separate
anyways because this chip can be co-packaged with the DSP,
which runs very hot.

FIGURE 27 | 100-Gb/s coherent CFP module using silicon photonics.

dual-polarization, dual-quadrature receiver in silicon photonics
[56]. It uses a 2-D grating coupler as a fiber coupler, polarization
splitter, and polarization rotator.
Figure 25 shows a recent silicon photonic PIC that contains
the full vector modulator and full coherent receiver on a single
chip [4]. This is lower cost and smaller footprint than separate
transmitter and receiver chips. There are three fibers connected
to the module: laser input, which is split between transmitter and
receiver; transmitter output; and receiver input. The fibers are
connected in a 3-fiber array, reducing cost and assembly time. It
is co-packaged in a hermetic gold box with four drivers and four
transimpedance amplifiers. It does not require any temperature
control, allowing the total power consumption to be less than
5 W, −5 to 80◦ C. A silicon photonics modulator does have
some imperfections compared to Pockels-effect modulators, like
GaAs and LiNbO3 . It has residual amplitude modulation, diode
nonlinearity, capacitance change with voltage, and bandwidth
limitations. A simulation including these effects shows that the
imperfection performance penalty is only 0.1 dB compared to an
ideal modulator.
Each PIC was tested in a socket in an optical loop-back
configuration using a 100-Gb/s DSP for real-time measurements,
as shown in Figure 26A. Optical loop-back insures that any
potential crosstalk between the transmitter and receiver would
show up as degradation. Figure 26B shows a measured 30Gbaud DP-QPSK eye diagram. There are five levels in such
a signal. Figure 26C shows measured real-time-processed 120Gb/s DP-QPSK constellations. Measured BER vs. OSNR curves
at multiple wavelengths across the C-band are shown in

9. Conclusion
The touted advantage of silicon photonics is the die are lower
cost than any other solution. While this may be true, it is
of limited help in short-reach applications, where the lack
of an integrated laser puts silicon photonics at a significant
disadvantage compared to the incumbents, such as VCSELs and
DMLs. Instead, the less-touted advantages of silicon photonics:
high yield, low modulator temperature sensitivity, high chip
strength, and ability to do polarization handling; make it ideal
for metro and long-haul applications. Spending its adolescence
in metro and long-haul, silicon photonics will have time to
develop mature laser integration methods, more routine foundry
services, and sophisticated packaging solutions so it can later
take on the short-reach incumbents. By that time, coherent
transmission may be cost- and power-effective enough to work
in very short links, bringing its advantages of high sensitivity,
high spectral efficiency, high-order modulation, and wavelength
selection.

Acknowledgments
The author is indebted to Long Chen, Diedrik Vermeulen,
Torben Nielsen, Scott Stulz, Saeid Azemati, Greg McBrien, Benny
Mikkelsen, Christian Rasmussen, Mehrdad Givehchi, Seo Yeon
Park, Jonas Geyer, Xiao-Ming Xu, and many others.

References

6. Mekis A, Gloeckner S, Masini G, Narasimha A, Pinguet T. A grating-couplerenabled CMOS photonics plaform. J Sel Topics Quant Electron (2011) 17:597–
608.
7. Chen L, Zhang L, Doerr CR, Dupuis N, Weimann NG, Kopf RF. Efficient
membrane grating couplers on InP. IEEE Photon Tech Lett. (2010) 22:890–2.
doi: 10.1109/LPT.2010.2047391
8. Shoji T, Tsuchizawa T, Watanabe T, Yamada K, Morita H.
Low
loss mode size converter from 0.3 um square Si wire waveguides
to singlemode fibres.
Electron Lett. (2002) 38:1669. doi: 10.1049/el:
20021185
9. Chen L, Doerr CR, Chen YK, Liow TY.
Low-loss and broadband
cantilever couplers between standard cleaved fibers and high-index-contrast
SiN or Si waveguides. IEEE Photon Tech Lett. (2010) 22:1744–6. doi:
10.1109/LPT.2010.2085040

1. Winzer P. Advanced optical modulation formats. Proc IEEE (2006) 94:952–85.
doi: 10.1109/jproc.2006.873438
2. Wellbrock G. Where do we go from here? In: OIDA Workshop on State of the
Art Integrated Photonics. Washington, DC (2014).
3. Wikipedia. Semiconductor Device Fabrication (2008).
4. Doerr CR, Chen L, Vermeulen D, Nielsen T, Azemati S, Stulz S, et al.
Single-chip silicon photonics 100-Gb/s coherent transceiver. In: Optical Fiber
Communication Conference: Postdeadline Papers. San Jose, CA (2014).
5. Taillaert D, Van Laere F, Ayre M, Bogaerts W, Van Thourhout D, Bienstman P,
et al. Grating couplers for coupling between optical fibers and nanophotonic
waveguides.
Jpn J Appl Phys. (2006) 45:6071–7. doi: 10.1143/JJAP.
45.6071

Frontiers in Physics | www.frontiersin.org

20

August 2015 | Volume 3 | Article 37

Doerr

Silicon photonic integration in telecommunications

31. Jalas D, Petrov A, Eich M, Freude W, Fan S, Yu Z, et al. What is and what is
not an optical isolator. Nat Photonics (2013) 7:579–82.
32. Levy M, Osgood RMJr, Hegde H, Cadieu FJ, Wolfe R, Fratello VJ. Integrated
optical isolators with sputter-deposited thin-film magnets. IEEE Photonics
Technol Lett. (1996) 8:903–5. doi: 10.1109/68.502265
33. Shoji Y, Mizumoto T, Yokoi H, Hseih IW, Osgood R. Magneto-optical isolator
with silicon waveguides fabricated by direct bonding. Appl Phys Lett. (2008)
92:071117. doi: 10.1063/1.2884855
34. Bhandare S, Ibrahim SK, Sandel D, Zhang H, Wust F, Noe R. Novel
nonmagnetic 30-dB traveling-wave single-sideband optical isolator integrated
in III/V material. IEEE J Sel Top Quantum Electron. (2005) 11:417–21. doi:
10.1109/JSTQE.2005.845620
35. Doerr CR, Chen L, Vermeulen D. Silicon photonics broadband modulationbased isolator. Opt Express (2014) 22:4493. doi: 10.1364/OE.22.004493
36. Yee KS. Numerical solution of initial boundary value problems involving
Maxwell’s equations in isotropic media. Antennas Propagation IEEE
Trans Livermore, CA: IEEE (1966). 302–7. doi: 10.1109/TAP.1966.
1138693
37. Bienstman P, Baets R. Optical modelling of photonic crystals and VCSELs
using eigenmode expansion and perfectly matched layers. Opt Quantum
Electron. (2001) 33:327–41. doi: 10.1023/A:1010882531238
38. Doerr CR. Sparse finite difference time domain method. IEEE Photonics
Technol Lett. (2013) 25:2259–62. doi: 10.1109/LPT.2013.2285181
39. Doerr CR. 3D sparse finite-difference time-domain simulation of silicon
photonic integrated circuits. In: Optical Fiber Communication Conference. Los
Angeles, CA (2015).
40. Narasimha A, Analui B, Liang Y, Member S, Sleboda TJ, Abdalla S, et al. A
fully integrated 4 x 10-Gb / s DWDM optoelectronic transceiver implemented
in a standard 1.3 um CMOS SOI technology. IEEE J Solid State Circ. (2007)
42:2736–44. doi: 10.1109/JSSC.2007.908713
41. Doerr CR, Chen L, Buhl LL, Chen YK. Eight-channel SiO2 /Si3 N4 /Si/Ge
CWDM receiver. IEEE Photonics Technol Lett. (2011) 23:1201–03. doi:
10.1109/LPT.2011.2158091
42. Doerr CR, Zhang L. Monolithic 80-Gb / s dual-polarization On-off-keying
modulator in InP. J Lightwave Technol. (2008) 2:19–21.
43. Doerr CR, Chen L. Monolithic PDM-DQPSK receiver in silicon. In: Optical
Communication (ECOC), 2010 36th European Conference and Exhibition on
(IEEE) (2010). p. 1–3. Available online at: http://ieeexplore.ieee.org/xpls/abs_
all.jsp?arnumber=5621418
44. Doerr CR, Fontaine NK, Buhl LL. PDM DQPSK silicon receiver with
integrated monitor and minimum number of controls. IEEE Photonics
Technol Lett. (2012) 24:697–9. doi: 10.1109/LPT.2012.2187048
45. Doerr CR, Taunay TF. Silicon photonics core-, wavelength-, and polarizationdiversity receiver. IEEE Photonics Technol Lett. (2011) 23:597–9. doi:
10.1109/LPT.2011.2118748
46. Doerr CR, Fontaine N, Hirano M, Sasaki T, Buhl L, Winzer P. Silicon photonic
integrated circuit for coupling to a ring-core multimode fiber for spacedivision multiplexing. In: European Conference and Exposition on Optical
Communications (2011). Available online at: http://www.opticsinfobase.org/
abstract.cfm?URI=ECOC-2011-Th.13.A.3
47. Griffin R, Johnstone R, Walker R, Wadsworth S, Carter A, Wale M. Integrated
DQPSK transmitter for dispersion-tolerant and dispersion-managed DWDM
transmission. OFC 2003 Optical Fiber Communications Conference (2003). p.
7–8. doi: 10.1109/OFC.2003.1248559
48. Dong P, Xie C, Chen L, Buhl LL, Chen YK.
112-Gb/s monolithic
PDM-QPSK modulator in silicon. Opt Express (2012) 20:B624–9. doi:
10.1364/OE.20.00B624
49. Milivojevic B, Raabe C, Shastri A, Webster M, Metz P, Sunder S, et al. 112Gb / s
DP-QPSK transmission over 2427km SSMF using small-size silicon photonic
IQ modulator and low-power CMOS driver. In: Optical Fiber Communications
Conference. San Jose, CA (2013). p. 5–7.
50. Shastri A, Webster M, Jeans G, Metz P, Sunder S, Chattin B, et al. Experimental
demonstration of ultra-low-power single polarization 56 Gb / s QAM-16
generation without DAC using CMOS photonics. In: European Conference
and Exposition on Optical Communications. Cannes (2014). p. 16–8.
51. Koch T, Koren U, Gnall R, Choa F, Hernandez-Gil F, Burrus C, et al.
GaInAs/GaInAsP multiple-quantum-well integrated heterodyne receiver.
Electron Lett. (1989) 25:1621. doi: 10.1049/el:19891086

10. Yamazaki H, Yamada T, Goh T, Kaneko A, Sano A. Integrated 100-Gb/s
PDM-QPSK modulator using a hybrid assembly technique with silicabased PLCs and LiNbO3 phase modulators. In: European Conference and
Exposition on Optical Communications Brussels: IEEE (2008). 1–4. doi:
10.1109/ECOC.2008.4729119
11. Fukuda H, Yamada K, Tsuchizawa T, Watanabe T, Shinojima H, Itabashi
SI. Silicon photonic circuit with polarization diversity. Opt Express (2008)
16:4872–80. doi: 10.1364/OE.16.004872
12. Tang Y, Dai D, He S.
Proposal for a grating waveguide serving as
both a polarization splitter and an efficient coupler for silicon-on-insulator
nanophotonic circuits. IEEE Photonics Technol Lett. (2009) 21:242–4. doi:
10.1109/LPT.2008.2010528
13. Taillert D, Chong H, Borel PI, Frandsen LH, Rue RMDL, Baets R. A
compact two-dimensional grating coupler used as a polarization splitter.
IEEE Photonics Technol Lett. (2003) 15:1249–51. doi: 10.1109/LPT.2003.
816671
14. Soref R, Bennett B. Electrooptical effects in silicon. IEEE J Quantum Electron.
(1987) 23:123–9. doi: 10.1109/JQE.1987.1073206
15. Zheng D, Smith B. Improved efficiency Si-photonic attenuator. Opt Express
(2008) 16:16754–65. doi: 10.1364/OE.16.016754
16. Dong P, Chen L. Monolithically integrated VOA-MUX with power monitors
based on submicron silicon photonics platform. Opt Fiber Commun Los
Angeles, CA: IEEE (2012) 9–11. doi: 10.1364/ofc.2012.otu2i.5
17. Thomson DJ, Gardes FY, Fedeli JM, Zlatanovic S, Hu Y, Ping B, et al. 50-Gb/s
silicon optical modulator. IEEE Photonics Technol Lett. (2012) 24:234–6. doi:
10.1109/LPT.2011.2177081
18. Liu A, Jones R, Liao L, Samara-Rubio D, Rubin D. A high-speed silicon optical
modulator based on a metal âĂŞ oxide âĂŞ semiconductor capacitor. Nature
(2004) 427:615–8. doi: 10.1038/nature02310
19. Liu J, Beals M, Pomerene A, Bernardis S, Sun R, Cheng J, et al. Waveguideintegrated, ultralow-energy GeSi electro-absorption modulators.
Nat
Photonics (2008) 2:433–7. doi: 10.1038/nphoton.2008.99
20. Liu M, Yin X, Ulin-Avila E, Geng B, Zentgraf T, Ju L, et al.
A
graphene-based broadband optical modulator. Nature (2011) 474:64–7. doi:
10.1038/nature10067
21. Ahn D, Hong Cy, Liu J, Giziewicz W, Beals M, Kimerling LC, et al. High
performance, waveguide integrated Ge photodetectors. Opt Express (2007)
15:3916–21. doi: 10.1364/OE.15.003916
22. Assefa S, Xia F, Bedell S, Zhang Y. CMOS-integrated 40GHz germanium
waveguide photodetector for on-chip optical interconnects. In: Optical Fiber
Communication Conference (2009). p. 10–2. Available online at: http://www.
opticsinfobase.org/abstract.cfm?id=177916
23. Kang Y, Liu H, Morse M, Paniccia M. Monolithic germanium / silicon
avalanche photodiodes with 340 GHz gain-bandwidth product. Nat Photonics
(2008) 3:59–63. doi: 10.1038/nphoton.2008.247
24. Michel J, Camacho-aguilera RE, Cai Y, Patel N, Bessette JT, Dutt BR, et al.
An electrically pumped Ge-on-Si laser. In: Optical Fiber Communication
Conference. Los Angeles, CA: IEEE (2012). p. 5–7.
25. Purnawirman, Sun J, Adam TN, Leake G, Coolbaugh D, Bradley JDB, et al.
C-and L-band erbium-doped waveguide lasers with wafer-scale silicon nitride
cavities. Opt Lett. (2013) 38:2–5. doi: 10.1364/OL.38.001760
26. Liu AY, Zhang C, Norman J, Snyder A, Lubyshev D, Fastenau JM, et al. High
performance continuous wave 1.3 µ m quantum dot lasers on silicon. Appl
Phys Lett. (2014) 104:3–7. doi: 10.1063/1.4863223
27. Fang A, Park H, Cohen O, Jones R, Paniccia M. Electrically pumped hybrid
AlGaInAs-silicon evanescent laser. Opt Express (2006) 14:9203–10. doi:
10.1364/OE.14.009203
28. Roelkens G, Brouckaert J, Van Thourhout D, Baets R, NoÌĹtzel R, Smit
M. Adhesive bonding of InPâĹŢInGaAsP dies to processed silicon-oninsulator wafers using DVS-bis-benzocyclobutene. J. Electrochem Soc. (2006)
153:G1015. doi: 10.1149/1.2352045
29. Marchena E, Creazzo T, Krasulick SB, Yu PKL, Van Orden D, Spann JY, et al.
Integrated tunable CMOS laser. In: Optical Fiber Communication Conference
(2013). Available online at: http://www.opticsinfobase.org/abstract.cfm?URI=
oe-21-23-28048
30. Snyder B, Corbett B, O’Brien P. Hybrid integration of the wavelength-tunable
laser with a silicon photonic integrated circuit. J Lightwave Technol. (2013)
31:3934–42. doi: 10.1109/JLT.2013.2276740

Frontiers in Physics | www.frontiersin.org

21

August 2015 | Volume 3 | Article 37

Doerr

Silicon photonic integration in telecommunications

52. Takeuchi H, Kasaya K, Kondo Y, Yasaka H, Oe K, Imamura Y. Monolithic
integrated coherent receiver on InP substrate. IEEE Photonics Technol Lett.
(1989) 1:398. doi: 10.1109/68.43392
53. Deri RJ, Pennings E, Scherer A, Gozdz A, Caneau C, Andreadakis N,
et al. Ultracompact monolithic integration of balanced, polarization diversity
photodetectors for coherent lightwave receivers. IEEE Photonics Technol Lett.
(1992) 4:1238. doi: 10.1109/68.166954
54. Bach H, Matiss A, Leonhardt CC, Kunkel R, Schmidt D, Schell M, et al.
Monolithic 90◦ hybrid with balanced PIN photodiodes for 100 Gbit / s PMQPSK receiver applications. Optical Fiber Communication Conference (2009).
p. 4–6.
55. Doerr CR, Zhang L, Winzer PJ. Monolithic InP multiwavelength coherent
receiver using a chirped arrayed waveguide grating. J Lightwave Technol.
(2011) 29:536–41. doi: 10.1109/JLT.2010.2097240

Frontiers in Physics | www.frontiersin.org

56. Doerr CR, Winzer PJ, Chen YK, Chandrasekhar S, Rasras MS, Chen L, et al.
Monolithic polarization and phase diversity coherent receiver in silicon. J
Lightwave Technol. (2010) 28:520–5. doi: 10.1109/JLT.2009.2028656
Conflict of Interest Statement: The author declares that the research was
conducted in the absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Copyright © 2015 Doerr. This is an open-access article distributed under the terms
of the Creative Commons Attribution License (CC BY). The use, distribution or
reproduction in other forums is permitted, provided the original author(s) or licensor
are credited and that the original publication in this journal is cited, in accordance
with accepted academic practice. No use, distribution or reproduction is permitted
which does not comply with these terms.

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ORIGINAL RESEARCH ARTICLE

MATERIALS

published: 30 March 2015
doi: 10.3389/fmats.2015.00026

Small sensitivity to temperature variations of Si-photonic
Mach–Zehnder interferometer using Si and SiN
waveguides
Tatsurou Hiraki 1,2 *, Hiroshi Fukuda 3 , Koji Yamada 1,2 and Tsuyoshi Yamamoto 1
1
2
3

NTT Device Technology Laboratories, NTT Corporation, Kanagawa, Japan
NTT Nanophotonics Center, NTT Corporation, Kanagawa, Japan
NTT Device Innovation Center, NTT Corporation, Kanagawa, Japan

Edited by:
Toshihiko Baba, Yokohama National
University, Japan
Reviewed by:
Junichi Fujikata, Photonics Electronics
Technology Research Association,
Japan
Yosuke Terada, Yokohama National
University, Japan
*Correspondence:
Tatsurou Hiraki , Device Technology
Laboratories, NTT corporation, 3-1,
Morinosato Wakamiya, Atsugi-shi,
Kanagawa 243-0198, Japan
e-mail: hiraki.tatsurou@lab.ntt.co.jp

We demonstrated a small sensitivity to temperature variations of delay-line Mach–Zehnder
interferometer (DL MZI) on a Si photonics platform. The key technique is to balance a
thermo-optic effect in the two arms by using waveguide made of different materials. With
silicon and silicon nitride waveguides, the fabricated DL MZI with a free-spectrum range of
~40 GHz showed a wavelength shift of -2.8 pm/K with temperature variations, which is 24
times smaller than that of the conventional Si-waveguide DL MZI. We also demonstrated
the decoding of the 40-Gbit/s differential phase-shift keying signals to on-off keying signals
with various temperatures. The tolerable temperature variation for the acceptable power
penalty was significantly improved due to the small wavelength shifts.
Keywords: silicon photonics, thermo-optic effect, Mach-Zehnder interferometer, waveguide, silicon nitride

INTRODUCTION
Silicon (Si) photonics is one of the most promising technologies
for overcoming the limitations on integration in commercially
available silica-based planar-lightwave circuits. This is because
it provides ultra-compact waveguides and makes the monolithic
integration of active and passive devices possible (Lockwood and
Pavesi, 2010; Vivien and Pavesi, 2013). Many compact devices,
such as arrayed-waveguide gratings, Mach–Zehnder interferometers (MZIs), and ring resonators, have been reported using Si
(Fukazawa et al., 2004; Xia et al., 2007) and silicon nitride (SiN)
waveguides (Gondarenko et al., 2009; Chen et al., 2011). One of
the issues with these devices is performance degradation with temperature variations due to the thermo-optic (TO) coefficient’s of
Si (~1.86 × 10-4 /K) and SiN (4 ~ 5 × 10-5 /K) being higher than
that of the silica (~1.0 × 10-5 /K). To overcome this issue, athermal
designs of Si-waveguide delay line (DL) MZIs have used different
effective-index changes with temperature (dneff /dT) in the two
arms to balance the TO effects in them (Uenuma and Motooka,
2009; Guha et al., 2010; Hai and Liboiron-Ladouceur, 2011). In
the previous studies, dneff /dT was controlled by means of the different optical confinement in the Si cores of narrow and wide
Si waveguides. However, the dneff /dT of the narrow waveguides
significantly depended on the core width; therefore, inevitable
fabrication errors made it difficult to minimize the TO effect.
To prevent the problem, the dneff /dT should be controlled by
changing the TO coefficients of the materials, without using a narrow waveguide. In our previous work, we reported control of the
refractive indices and TO coefficients of complementary metaloxide semiconductor (CMOS) compatible materials by changing
the atomic composition of SiOx, SiOxNy, and SiN (Tsuchizawa
et al., 2011; Nishi et al., 2012; Hiraki et al., 2013). Using these

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materials, in this work, we minimized the temperature sensitivity
of the DL MZI. In the following sections, we show the details of
the design and fabrication of the DL MZI and present experimental results. In addition, as a feasibility demonstration, we show the
thermal stability of the decoding of differential phase-shift eying
(DPSK) signals to on-off keying (OOK) signals at 40 Gbit/s.

DESIGN AND FABRICATION
Figure 1 shows a schematic of the DL MZI. The temperature sensitivity could be minimized by balancing the TO effect between the
two arms, while keeping the differential delay between them. The
interference condition is expressed as following equation (Guha
et al., 2010)
mλ = neff ,2 L2 − neff ,1 L1
Here, m is an integer for constructive interference or a halfinteger destructive interference, n eff, 1 and n eff, 2 are the effective
indices, and L 1 and L 2 are the physical lengths of arm 1 and 2.
Then, the temperature sensitivity of the interference spectrum
could be obtained by differentiating above equation with respect
to temperature, as expressed by following

 


dλ
dneff ,2
dneff ,1
dneff ,2
dneff ,1
=
L2 −
L1 / m −
L2 −
L1
dT
dT
dT
dλ
dλ
Athermal condition is given by the numerator of this equation
to be 0. Since we have two design parameters L 1 and L 2 , we can
make dλ/dT to be 0 while keeping the differential delay. The key
technique is to control dneff /dT by changing the core materials of
the two arms. In this work, we used Si and SiN waveguides in the

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Hiraki et al.

Temperature-insensitive Si-SiN-waveguide MZI

FIGURE 3 | Microscope image of fabricated Si–SiN-waveguide DL MZI.
FIGURE 1 | Schematic of Si–SiN-waveguide DL MZI.

FIGURE 2 | Relationships between dneff /dT and core width of Si and
SiN waveguides.

CMOS compatible materials. In the design, the refractive index
and the TO coefficient of the SiN core are 2.0 and 4.0 × 10-5 /K,
respectively. The core thicknesses of the waveguides were fixed at
220 and 400 nm, respectively. Figure 2 shows the calculated results
of relationships between dneff /dT and the core widths of the Si
and SiN waveguides. For little change of the dneff /dT with width
variations, we used a 440-nm-wide Si waveguide as arm 1, and an
800-nm-wide SiN-waveguide as arm 2, respectively. We designed
the DL MZI with a free spectral range (FSR) of 40 GHz. The FSR
is given by the inverse of the differential delay, or 1-bit delay time
∆t = (n g, 2 L 2 - n g, 1 L 1 )/c, where n g, 1 and n g, 2 are group indices of
the arm 1 and 2, and c is the speed of light in vacuum. Under
this differential delay condition, the dλ/dT can be 0 by choosing
the L 1 and L 2 as 0.95 mm and 5.77 mm, respectively. It is notable
that if we could use the state-of-the-art fabrication process with
width variations of 3 nm (Shimura et al., 2014), the dλ/dT could
be less than 0.1 pm/K, which is over 10 times smaller than that
using a 280-nm-wide (narrow) Si-waveguide as arm 2 (Hai and
Liboiron-Ladouceur, 2011) with the same width variations.
As other features to construct the DL MZI structure, we used
the inverse taper of the Si waveguide for the fiber-chip interface,
and 2 × 2 Si-waveguide multimode interference (MMI) couplers.

Frontiers in Materials | Optics and Photonics

The taper-tip width and the taper length of the fiber-chip interface were 200 nm and 300 µm, respectively. Since the Si and
SiN waveguides were formed in different layers, the interlayer
coupler (ILC) between them was designed using adiabatically
tapers (Huang et al., 2014). We introduced the ILCs into both
arms to cancel out their phase delays. In addition, as reference samples, we designed a conventional Si-waveguide DL MZI
and a SiN-waveguide DL MZI without any compensation for
thermal sensitivity (dneff, 1 /dT = dneff,2 /dT). In the conventional
DL MZIs, both arms comprised of the same structures, which
were the 440-nm-wide Si waveguide and the 800-nm-wide SiN
waveguide.
The DL MZI was fabricated on an 8-inch silicon-on-insulator
wafer, whose buried-oxide thickness was 3 µm. The Si waveguides
were first patterned; then, a clad film was deposited. After that, the
clad film was flattened, and SiN-waveguide cores were formed. The
interlayer clad thickness between the Si and SiN waveguides was
controlled to be 100 nm. Finally, an overclad film was deposited.
A microscope image of the fabricated Si–SiN-waveguide DL MZI
is shown in Figure 3. The total size of the fabricated DL MZI
is ~0.56 mm2 /ch, which is comparable to that of the conventional SiN-waveguide DL MZI. It is still larger than that of the
Si-waveguide DL MZI; however, it is several-hundred times smaller
than one made of the commercially-used silica.

RESULTS AND DISCUSSIONS
We measured transmission spectra of the fabricated DL MZI. We
used a tunable laser diode (TLD) as a light source and swept the
wavelength of the input light, and measured output light power
from the bar port. The input and output fibers were lensed fibers
with mode-field diameters of ~3.5 µm, and the polarization of
the input light was adjusted to the transverse electric (TE) mode.
The chip was set on a temperature-controlled stage by using a
heat-dissipation tape. We measured the transmission spectra of
the DL MZIs, while varying the chip-stage temperature range
from 298–302 K so that the wavelength shift should not exceed the
FSR. Figures 4A–C show the transmission spectra of the Si–SiNwaveguide DL MZI, the conventional SiN-waveguide DL MZI,
and the conventional Si-waveguide DL MZI at 298 and 300 K.
The output powers were normalized by the fiber-to-fiber transmission spectra. It is clear that the Si–SiN-waveguide DL MZI
highly suppresses the wavelength shift with temperature variations. The measured FSRs and the dλ/dT are listed in Table 1.
The TO effects of the fabricated SiN- and Si waveguides are
almost consistent with their designs. The dλ/dT of the Si-SiN
DL MZI is over six times smaller than that of the conventional

March 2015 | Volume 2 | Article 26 | 24

Hiraki et al.

Temperature-insensitive Si-SiN-waveguide MZI

FIGURE 4 | Transmission spectra of (A) Si–SiN-waveguide DL MZI, (B) SiN-waveguide DL MZI, and (C) Si-waveguide DL MZI.

Table 1 | Measured FSRs and dλ/dT.
Sample

Arm 1

Arm 2

FSR (GHz)

dλ/dT (pm/K)

Si–SiN DL MZI

Si

SiN

40.5

−2.8

SiN DL MZI (ref.)

SiN

SiN

40.8

+17.0

Si DL MZI (ref.)

Si

Si

38.3

+68.5

SiN waveguide, and 24 times smaller than that of the conventional Si-waveguide DL MZI. Although the dλ/dT of the Si–SiN
DL MZI is larger than the expected value (<0.1 pm/K) because
of the large fabrication error over 3 nm, the measured result is
still better than those for DLI MZIs with narrow Si waveguides
(Uenuma and Motooka, 2009; Guha et al., 2010). The FSR of the
Si–SiN DL MZI is 40.5 GHz, which is only about 1% different
from the target value. The insertion loss of the Si–SiN-waveguide
DL MZI is ~13 dB, which includes the fiber-chip coupling loss
of ~2 dB/facet and the interlayer coupling loss of ~0.5 dB/couple.
The extinction ratio is ~17 dB, which is mainly determined by an
imbalance of propagation loss between two arms of the MZI and
an unintentional imbalance of the MMI branch. The propagation loss of SiN and Si waveguides are ~12 dB/cm and ~4 dB/cm,
respectively. The unintentional imbalance of the MMI branch is
~1.3 dB. Although the imbalance between two arms is large, the
imbalance of the MMI branch improves the extinction ratio. The
insertion loss and extinction ratio would be further improved
because a well-controlled fabrication environment would reduce
the SiN-waveguide loss (Huang et al., 2014). We must mention that the device was designed for only TE mode light. For
polarization diversity circuits (Fukuda et al., 2008), a polarization
rotator using parallel cores has already been demonstrated using
the same Si and SiN layers as in this work (Fukuda and Wada,
2014).
As a feasibility demonstration, we applied the Si–SiNwaveguide DL MZI to decode 40-Gbit/s DPSK signals. Figure 5
shows the experimental setup. We input the DPSK signals
with a non-return-to-zero (NRZ) 40-Gbit/s pseudo-random bit
sequence (PRBS) of length 231 - 1. The eye diagram of the input

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FIGURE 5 | Experimental setup for decoding of 40-Gbit/s DPSK signals.

signals is shown in the inset of Figure 5. The polarization state
of the input light was adjusted to the TE mode. The output
light was coupled by the lensed fiber and then switched to
the optical spectrum analyzer or the photodiode. The electrical signals from the photodiode were amplified then fed into
the sampling oscilloscope. Figures 6A,B show the constructiveand destructive-interference spectra of the bar port at frequency
of 193.611 and 193.592 THz, respectively. They were measured
at 298 K. The decoded 40-Gbit/s signals were observed as 40GHz-span dips in the destructive-interference spectra. Their
eye diagrams are shown in the insets of Figures 6A,B, respectively. Here, the vertical axis is 20 mV/div and the horizontal axis is 10 ps/div. The two output signals carried logically
inverted data streams in the DPSK format. The constructive interference carried duobinary, whereas the destructive-interference
carries alternate-mark inversion (Gnauck and Winzer, 2005).
Using the destructive interference, we demonstrated a thermal tolerance to decode the DPSK format to the OOK format
(Winzer and Leuthold, 2001; Lazzeri et al., 2010). Figures 7A,B
show the eye diagrams of the destructive-interference signals at
299 and 302 K, respectively. The eye diagrams clearly open at
302 K, corresponding to a temperature variation of 4 K from
the initial temperature (298 K). From the measured dλ/dT of
-2.8 pm/K, the estimated frequency shift by the temperature

March 2015 | Volume 2 | Article 26 | 25

Hiraki et al.

Temperature-insensitive Si-SiN-waveguide MZI

FIGURE 6 | Decoded-signal spectra and eye diagrams (insets) of (A) constructive- and (B) destructive interference at 298 K.

FIGURE 7 | Eye diagrams of Si–SiN-waveguide DL MZI at (A) 299 and
(B) 302 K.

variation of +4 K is -1.4 GHz, which could cause only about
1-dB penalty for a 40-Gbit/s system (Hoon and Winzer, 2003).
By using the state-of-the-art fabrication process, as discussed
in the above section, the tolerable temperature variations could
be over 90 K. As references, the eye diagrams of the conventional Si-waveguide DL MZI at 298 and 299 K are shown in
Figures 8A,B, respectively. Here, the vertical axis is 69.7 mV/div
and the horizontal axis is 10.0 ps/div. The eye diagram was completely closed with a temperature variation of only +1 K. These
results clearly show that the Si–SiN-waveguide DL MZI actually
improves the thermal-insensitivity of the Si-photonic DL MZI
and that it has a potential to be used in the telecommunications devices. A well-controlled fabrication environment would
reduce the insertion loss of the DL MZI and also improves the
signal-to-noise ratio.

CONCLUSION
We demonstrated a small sensitivity to temperature variations
of DL MZI on a Si-photonics platform. The key technique
is to balance the TO effect in two arms by using waveguides
made of different materials, which are Si and SiN. The Si–SiNwaveguide DL MZI with an FSR of 40 GHz showed dλ/dT of
-2.8 pm/K, which is about 24 times smaller than that of the conventional Si-waveguide DL MZI. The technology has the potential to reduce temperature sensitivities of various Si-photonic
devices, such as wavelength filters, phase demodulators, and ring
resonators.

Frontiers in Materials | Optics and Photonics

FIGURE 8 | Eye diagrams of Si-waveguide DL MZI at (A) 298 and (B)
299 K.

REFERENCES
Chen, L., Doerr, C. R., Dong, P., and Chen, Y. K. (2011). Monolithic silicon chip
with 10 modulator channels at 25 Gbps and 100-GHz spacing. Opt. Express 19,
B946–B951. doi:10.1364/OE.19.00B946
Fukazawa, T., Ohno, F., and Baba, T. (2004). Very compact arrayed-waveguidegrating demultiplexer using Si photonic wire waveguides. Jpn. J. Appl. Phys. 43,
L673–L675. doi:10.1143/JJAP.43.L673
Fukuda, H., and Wada, K. (2014). Parallel-core-type polarization rotator for silicon
wire waveguide platform. Photonics Res. 2, A14–A18. doi:10.1364/PRJ.2.000A14
Fukuda, H., Yamada, K., Tsuchizawa, T., Watanabe, T., Shinojima, H., and Itabashi,
S. (2008). Silicon photonic circuit with polarization diversity. Opt. Express 16,
4872–4880. doi:10.1364/OE.16.004872
Gnauck, A. H., and Winzer, P. J. (2005). Optical phase-shift-keyed transmission. J.
Lightw. Technol. 23, 115–130. doi:10.1109/JLT.2004.840357
Gondarenko, A., Levy, J. S., and Lipson, M. (2009). High confinement micronscale silicon nitride high Q ring resonator. Opt. Express 17, 11366–11370.
doi:10.1364/OE.17.011366
Guha, B., Gonarenko, A., and Lipson, M. (2010). Minimizing temperature sensitivity of silicon Mach-Zehnder interferometers. Opt. Express 18, 1879–1887.
doi:10.1364/OE.18.001879
Hai, M. S., and Liboiron-Ladouceur, O. (2011). “Temperature compensated 50
Gb/s DPSK demodulator,” in Conference on Lasers and Electro-Optics (Baltimore:
OSA), JTuI76.
Hiraki, T., Nishi, H., Tsuchizawa, T., Kou, R., Fukuda, H., Takeda, K., et al. (2013). SiGe-Silica monolithic integration platform and its application to a 22-Gb/s x 16ch WDM receiver. Photonics J. 5, 4500407. doi:10.1109/JPHOT.2013.2269676
Hoon, K., and Winzer, P. J. (2003). Robustness to laser frequency offset in
direct-detection DPSK and DQPSK systems. J. Lightw. Technol. 21, 1887–1891.
doi:10.1109/JLT.2003.816816
Huang, Y., Song, J., Luo, X., Liow, T. Y., and Lo, G. Q. (2014). CMOS compatible
monolithic multi-layer Si3 N4 -on-SOI platform for low-loss high performance

March 2015 | Volume 2 | Article 26 | 26

Hiraki et al.

silicon photonics dense integration. Opt. Express 22, 21859–21865. doi:10.1364/
OE.22.021859
Lazzeri, E., Nguyen, A. T., Serafino, G., Kataoka, N., Wada, N., Bogoni, A., et al.
(2010). “All-optical NRZ-DPSK to RZ-OOK format conversion using optical
delay line interferometer and semiconductor optical amplifier,” in Photonics in
Switching Conference (Monterey), JTuB45.
Lockwood, D. J., and Pavesi, L. (eds) (2010). Silicon Photonics. Berlin: Springer.
Nishi, H., Tsuchizawa, T., Kou, R., Shinojima, H., Yamada, T., Kimura, H., et al.
(2012). Monolithic integration of a silica AWG and Ge photodiodes on Si
photonic platform for one-chip WDM receiver. Opt. Express 20, 9312–9321.
doi:10.1364/OE.20.009312
Shimura, D., Horikawa, T., Okayama, H., Jeong, S.-H., Tokushima, M., Sasaki, H.,
et al. (2014). “High precision Si waveguide devices designed for 1.31µm and
1.55µm wavelengths on 300mm-SOI,” in Conference on Group IV Photonics.
Vancouver: IEEE.
Tsuchizawa, T., Yamada, K., Watanabe, T., Sungbong, P., Nishi, H., Kou, R., et al.
(2011). Monolithic integration of silicon-, germanium-, and silica-based optical
devices for telecommunications applications. IEEE J. Sel. Top. Quant. Electron.
17, 516–525. doi:10.1109/JSTQE.2010.2089430
Uenuma, M., and Motooka, T. (2009). Temperature-independent silicon waveguide
optical filter. Opt. Lett. 34, 599–601. doi:10.1364/OL.34.000599
Vivien, L., and Pavesi, L. (eds). (2013). Handbook of Silicon Photonics. New York:
Taylor and Francis.

www.frontiersin.org

Temperature-insensitive Si-SiN-waveguide MZI

Winzer, P. J., and Leuthold, J. (2001). Return-to-zero modulator using a single NRZ
drive signal and an optical delay interferometer. IEEE Photon. Technol. Lett. 13,
1298–1300. doi:10.1109/68.969887
Xia, F., Rooks, M., Sekaric, L., and Vlasov, Y. (2007). Ultra-compact high order
ring resonator filters using submicron silicon photonic wires for on-chip optical
interconnects. Opt. Express 15, 11934–11941. doi:10.1364/OE.15.011934
Conflict of Interest Statement: The authors declare that the research was conducted
in the absence of any commercial or financial relationships that could be construed
as a potential conflict of interest.
Received: 16 January 2015; accepted: 16 March 2015; published online: 30 March 2015.
Citation: Hiraki T, Fukuda H, Yamada K and Yamamoto T (2015) Small sensitivity to
temperature variations of Si-photonic Mach–Zehnder interferometer using Si and SiN
waveguides. Front. Mater. 2:26. doi: 10.3389/fmats.2015.00026
This article was submitted to Optics and Photonics, a section of the journal Frontiers
in Materials.
Copyright © 2015 Hiraki, Fukuda, Yamada and Yamamoto. This is an open-access
article distributed under the terms of the Creative Commons Attribution License (CC
BY). The use, distribution or reproduction in other forums is permitted, provided the
original author(s) or licensor are credited and that the original publication in this
journal is cited, in accordance with accepted academic practice. No use, distribution or
reproduction is permitted which does not comply with these terms.

March 2015 | Volume 2 | Article 26 | 27

ORIGINAL RESEARCH
published: 05 May 2015
doi: 10.3389/fmats.2015.00036

Ultrahigh temperature-sensitive
silicon MZI with titania cladding
Jong-Moo Lee 1,2*
1
Electronics and Telecommunications Research Institute, Daejeon, South Korea, 2 School of Advanced Device Technology,
University of Science and Technology, Daejeon, South Korea

We present a possibility of intensifying temperature sensitivity of a silicon Mach-Zehnder
interferometer (MZI) by using a highly negative thermo-optic property of titania (TiO2 ). Temperature sensitivity of an asymmetric silicon MZI with a titania cladding is experimentally
measured from +18 to −340 pm/°C depending on design parameters of MZI.
Keywords: silicon, photonics, temperature, sensor, titania

Introduction

Edited by:
Jifeng Liu,
Thayer School of Engineering, USA
Reviewed by:
Raul J. Martin-Palma,
Universidad Autonoma de Madrid,
Spain
Venu Gopal Achanta,
Tata Institute of Fundamental
Research, India
*Correspondence:
Jong-Moo Lee,
Electronics and Telecommunications
Research Institute, 161 Gajong-dong,
Yusong-gu, Daejeon 305-350, Korea
jongmool@etri.re.kr

Specialty section:
This article was submitted to Optics
and Photonics, a section of the
journal Frontiers in Materials
Received: 30 January 2015
Accepted: 08 April 2015
Published: 05 May 2015
Citation:
Lee J-M (2015) Ultrahigh
temperature-sensitive silicon MZI with
titania cladding.
Front. Mater. 2:36.
doi: 10.3389/fmats.2015.00036

Frontiers in Materials | www.frontiersin.org

There have been many efforts to adjust temperature-dependent wavelength shift (TDWS) of
a photonic waveguide device using a cladding material with a negative thermo-optic coefficient (TOC) differently from a core material with a positive TOC (Kokubun et al., 1998; Lee
et al., 2007, 2008; Alipour et al., 2010; Guha et al., 2013; Bovington et al., 2014; Lee, 2014).
Polymers have been popularly used as the cladding material with a negative TOC (Kokubun
et al., 1998; Lee et al., 2007, 2008), and titania (TiO2 ) is recently attracting attention with a
highly negative TOC (Alipour et al., 2010; Guha et al., 2013; Bovington et al., 2014; Lee, 2014)
and its merit of complementary-metal-oxide-semiconductor (CMOS) compatibility in fabrication when it is used for a silicon photonic waveguide device. Silicon has a very high TOC of
1.8 × 10−4 /°C and there have been many efforts to reduce the high TDWS of silicon photonic
devices such as a ring resonator by using polymer (Kokubun et al., 1998; Lee et al., 2007,
2008) or titania cladding (Alipour et al., 2010; Guha et al., 2013; Lee, 2014) with a highly
negative TOC.
In case of silicon photonic Mach-Zehnder interferometer (MZI), there were reports showing the
way to reduce the TDWS of MZI without using a cladding with a negative TOC (Uenuma and
Moooka, 2009; Guha et al., 2010; Dwivedi et al., 2013). TDWS of silicon MZI was shown to be
reduced by using different widths of waveguide (Uenuma and Moooka, 2009; Guha et al., 2010)
or by using different polarization (Dwivedi et al., 2013) in each of the MZI arm, respectively. The
difference in each of the MZI arm can induce a different temperature-dependent phase change for
the each arm, resulting reduction in TDWS of MZI.
The previous efforts of silicon photonic devices using a negative thermo-optic cladding have
been focused on reducing the TDWS, but there have been demands also on high TDWS in
such applications of low power temperature tuning (Masood et al., 2013) and integrated-photonic
temperature sensors (Irace and Breglio, 2003; Kim et al., 2010; Deng et al., 2014). So, it would also be
attractive if there is a method to intensify the TDWS of the silicon device by using a cladding material
such as titania with a very high TOC. There have been MZIs using titania for chemical sensing (Qi
et al., 2002; Celo et al., 2009), but no reports on temperature sensors using titania cladding to the
best of our knowledge.
In this regard, here, we combine the method used to in reducing TDWS of silicon MZI with
different dimension for each arm and the method of adding titania cladding on the silicon MZI to
show the possibility of ultrahigh temperature-sensitive silicon MZI.

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Lee

cladding. We deposited 400 nm thickness of titania cladding on
the fabricated device without an upper cladding, using electronbeam evaporation. The initial vacuum level of the electron-beam
evaporation was 5 × 10−7 Torr, and it was kept at 8 × 10−5 Torr
with O2 during the deposition. The temperature of a plate holding
SOI chip was maintained at 150°C during the evaporation, and the
speed of deposition was about 3Å/s. The refractive index of titania
was measured using ellipsometry as 2.13 at 1550 nm.

Experiment and Results
Design and Fabrication
Temperature dependence of a silicon MZI can be adjusted by
asymmetric geometry of two waveguide arms with different effective refractive indexes induced by different cross-sectional dimension as in reference Uenuma and Moooka (2009) and Guha et al.
(2010). Silicon MZIs in this experiment are designed with variations in the length of MZI arm and cross-sectional dimension of
each MZI arm as in Figure 1. Figure 1A shows AsyL, which is
for asymmetry in the length of each MZI arm of 80 μm, L for the
common length of MZI arm, which is varied from 110 to 360 μm,
w0 for the common width of waveguide core, which is 450 nm,
w1 for the cross-sectional dimension of a waveguide core which
is 1350 nm and shaped as a rib waveguide shown in Figure 1B or
450 nm and shaped as a channel waveguide for a comparison, and
w2 for the cross-sectional dimension of a waveguide core which is
350 or 450 nm for a comparison. Figures 1B,C show the crosssectional structure of the waveguide with a silica cladding and
a titania cladding, respectively. The rib waveguide is formed by
shallow etch of 70 nm for the width w1, and there are tapers at
the both ends of the rib waveguide for an adiabatic transfer to the
channel waveguide with the width of w0.
Figure 2 shows a microscopic image and scanning electron
microscope (SEM) image of the fabricated MZIs. There are many
variations for the length, L, in design, but we limit our discussion here to the two extreme case of L, 110 and 360 μm. Silicon waveguide core with the width of 450 nm was patterned by
DUV lithography on a silicon-on-insulator (SOI) wafer with a
220-nm thick silicon layer on a 2-μm thick buried oxide (BOX)
layer. The fabrication of the devices except a deposition of titania
cladding were processed using a standard CMOS fabrication process through ePIXfab. There were two types of fabricated device:
one with a silica (SiO2 ) cladding and the other without an upper

Measured Results
One pair of single-mode fibers is coupled to the silicon devices
for measurement through grating couplers which are with 630nm pitch and 70 nm depth of the shallow etch. Figure 3 shows
normalized transmission spectra of a silicon MZI with a silica

FIGURE 2 | A microscope image and a SEM image of silicon MZIs
fabricated in this experiment.

FIGURE 1 | (A) Schematic diagram of asymmetric MZI with arms different in
length and cross-sectional dimension of waveguide, and schematic diagram
of the cross-section of the arms in (B) with silica cladding and (C) with titania
cladding, respectively.

Frontiers in Materials | www.frontiersin.org

FIGURE 3 | Normalized transmission spectra of MZI with silica
cladding as the temperature varied from 15 to 55°C when L = 360 μm,
w1 = 1350 nm, and w2 = 350 nm.

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Lee

cladding when the temperature varied from 15 to 55°C. The
main design parameters of the silicon MZI are 360 μm for L,
1350 nm for w1, and 350 nm for w2. Figure 3 shows TDWS of the
silicon MZI is +48 m/°C, which was reduced from +74 pm/°C of
a ring resonator included in the same chip for a comparison. The
normalized transmissions can be regarded as the insertion loss
of the silicon MZI, because they were calculated by subtracting
the amount of fiber-to-fiber transmission of a straight silicon
waveguide from the amount of fiber-to-fiber transmission of the
silicon MZI device. Figure 3 shows that the insertion loss through
the silicon MZI is negligibly small.
Figure 4 shows normalized transmission spectra of a silicon
MZI, whose design is the same as in Figure 3 but with a titania
cladding instead of the silica cladding, when the temperature
varied from 25 to 35°C. Figure 4 shows TDWS of the silicon MZI
is intensified with opposite sign by the titania cladding as high as
−340 pm/°C, which is about seven times bigger than the TDWS of
the same design of MZI with a silica cladding and five times bigger
than the TDWS of the ring resonator with a silica cladding.
Figure 5 shows the relative wavelength shift of various MZIs
with silica or titania cladding in this experiment compared to the
TDWS of the ring resonator with the silica cladding. The radius of
the ring resonator with the silica cladding was 5 μm and TDWS
of the ring resonator was measured at +74 pm/°C as in Figure 6.
TDWS of a silicon MZI with the same cross-section dimension
of 450 nm and titania cladding was measured as +18 pm/°C as
in Figure 5. TDWS of another titania-covered silicon MZI with
1350 nm for w1, 450 nm for w2, and 110 μm for L was measured
as −70 pm/°C as in Figure 5.

The input and output fibers are coupled to the silicon waveguide through grating couplers with the pitch of 630 nm. The fiber
was coupled at the vertical angle of 10° for the waveguide with
silica cladding and 15° for the waveguide with titania cladding.
There was not a big difference in the coupling loss of the grating
couplers for the silica cladding and titania cladding. It was about
5 dB/facet for the silica cladding and 5.5 dB/facet for the titania
cladding. The slightly excessive loss of the grating coupler in case
of titania cladding is expected to be reduced by optimizing the
design of gratings or the thickness of titania if it is required. The
normalized transmission in Figures 3 and 4 were calculated by
subtracting the amount of fiber-to-fiber transmission of a straight
silicon waveguide from the amount of fiber-to-fiber transmission
of the silicon MZI device for each case of silica cladding and titania
cladding, respectively. So, the normalized transmission spectra
show the insertion loss of MZI compared to a straight waveguide.
The refractive index of titania cladding was measured using
ellipsometry as 2.13 at 1550 nm, and TOC of titania film was

Discussion
The experimental results show that we can adjust TDWS of the
titania-covered silicon MZIs with proper design and can intensify the temperature sensitivity highly enough to be useful in
applications requiring an ultrahigh temperature sensitivity such
as thermo-optic tuning devices or photonic temperature sensors.

FIGURE 5 | Relative wavelength shift depending on temperature of
MZI with silica and titania cladding, respectively, in comparison with
+74 pm/°C or a ring resonator with silica cladding. w350 and w450 are
for 350 and 450 nm width, respectively, of the MZI narrow arm. L110 and
L360 are for 110 and 360 μm length, respectively, of the MZI arm.

FIGURE 4 | Normalized transmission spectra of MZI with titania
cladding as the temperature varied from 25 to 35°C when L = 360 μm,
w1 = 1350 nm, and w2 = 350 nm.

Frontiers in Materials | www.frontiersin.org

FIGURE 6 | Normalized transmission spectra of the ring resonator with
silica cladding mentioned in Figure 5, as the temperature varied from
15 to 55°C.

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Ultrahigh temperature-sensitive MZI

Lee

not directly measured but estimated from −5 to −7 × 10−4 /°C
by the measured TDWS of a ring resonator with titania cladding
as in reference Lee (2014). The absolute value of TOC of titania
is several times higher than TOC of silicon or polymer, and
that is the reason we used it as highly negative thermo-optic
cladding in this experiment. The reason for the highly negative
TOC of the titania cladding and the variation of TOC is not fully
understood yet, and finding the reason remains for our future
research.

thermo-optic titania cladding. We experimentally showed temperature sensitivity of an asymmetric silicon MZI with a titania
cladding could be adjusted from +18 to −340 pm/°C depending
on design parameters such as the width and length of MZI. We
believe these results show the possibility of ultrahigh temperaturesensitive silicon MZI for new applications requiring ultrahigh
temperature sensitivity such as thermo-optic tuning devices or
photonic temperature sensors.

Acknowledgments

Conclusion

We would like to thank ePIXfab (www.epixfab.eu) for the fabrication of SOI waveguide before our deposition of titania cladding.
This work was supported by Korean IT R&D program MOTIE
[N019800001] and [10044735].

We experimentally showed that TDWS of a silicon MZI can
be reduced or intensified by proper design of the width and
length of arms of MZI when it is used with a highly negative

Kokubun, Y., Yoneda, S., and Matsuura, S. (1998). Temperature-independent optical
filter at 1.55μm wavelength using a silica-based athermal wavelength. Electron.
Lett. 34, 367–369. doi:10.1049/el:19980245
Lee, J. M. (2014). Influence of titania cladding on SOI grating coupler and 5 μmradius ring resonator. Opt. Commun. 338, 101–105. doi:10.1016/j.optcom.2014.
10.039
Lee, J. M., Kim, D. J., Ahn, H. Y., Park, S. H., and Kim, G. O. (2007). Temperature
dependence of silicon nanophotonic ring resonator with a polymeric overlayer.
J. Lightwave Technol. 25, 2236–2243. doi:10.1109/JLT.2007.899792
Lee, J. M., Kim, D. J., Kim, G. H., Kwon, O. K., Kim, K. J., and Kim, G. O. (2008).
Controlling temperature dependence of silicon waveguide using slot structure.
Opt. Express 16, 1645–1652. doi:10.1364/OE.16.001645
Masood, A., Pantouvakiy, M., Lepagey, G., Verheyeny, P., Campenhouty, J. V.,
Absily, P., et al. (2013). “Comparison of heater architectures for thermal control
of silicon photonic circuits,” in IEEE 10th International Conference, Group IV
Photonics (GFP), ThC2; 2013 Aug 28-30, (Seoul).
Qi, Z., Matzuda, N., Itoh, K., Murabayashi, M., and Labers, C. R. (2002). A design
for improving the sensitivity of Mach-Zehnder interferometer to chemical
and biological measurands. Sens Actuators B Chem 81, 254–258. doi:10.1016/
S0925-4005(01)00960-1
Uenuma, M., and Moooka, T. (2009). Temperature-independent silicon waveguide
optical filter. Opt. Lett. 34, 599–601. doi:10.1364/OL.34.000599

References
Alipour, P., Atabaki, A. H., Eftekhar, A. A., and Adibi, A. (2010). “Athermal
performance in titania-clad microring resonators on SOI,” in OSA Technical
Digest. Integrated Photonics Research, Silicon and Nanophotonics and Photonics
in Switching, Paper IMC6; 2010 Jul 25-28, (Monterey, CA: Optical Society of
America).
Bovington, J., Wu, R., Cheng, K. T., and Bowers, J. E. (2014). Thermal stress
implications in athermal TiO2 waveguides on a silicon substrate. Opt. Express
21, 661–666. doi:10.1364/OE.22.000661
Celo, D., Post, E., Summers, M., Smy, T., Brett, M. J., and Albert, J. (2009).
Interferometric sensing platform with dielectric nanostructured thin films. Opt.
Express 17, 6655–6664. doi:10.1364/OE.17.006655
Deng, Q. Z., Li, X. B., Chen, R. B., and Zhou, Z. (2014). “Low-cost silicon photonic
temperature sensor using broadband light source,” in IEEE 11th International
Conference, Group IV Photonics (GFP), WP23; 2014 Aug 27-29, (Paris).
Dwivedi, S., D’heer, H., and Bogaerts, W. (2013). A compact all-silicon temperature
insensitive filter for WDM and bio-sensing applications. IEEE Photonics Technol.
Lett. 25, 2167–2170. doi:10.1109/LPT.2013.2282715
Guha, B., Cardenas, J., and Lipson, M. (2013). Athermal silicon microring resonators with titanium oxide cladding. Opt. Express 21, 26557–26563. doi:10.
1364/OE.21.026557
Guha, B., Gondarenko, A., and Lipson, M. (2010). Minimizing temperature sensitivity of silicon Mach-Zehnder interferometers. Opt. Express 18, 1879–1887.
doi:10.1364/OE.18.001879
Irace, A., and Breglio, G. (2003). All-silicon optical temperature sensor based
on multi-mode interference. Opt. Express 11, 2807–2812. doi:10.1364/OE.11.
002807
Kim, G. D., Lee, H. S., Park, C. H., Lee, S. S., Lim, B. T., Bae, H. K., et al. (2010).
Silicon photonic temperature sensor employing a ring resonator manufactured
using a standard CMOS process. Opt. Express 18, 22215–22221. doi:10.1364/OE.
18.022215

Frontiers in Materials | www.frontiersin.org

Conflict of Interest Statement: The author declares that the research was conducted in the absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Copyright © 2015 Lee. This is an open-access article distributed under the terms of the
Creative Commons Attribution License (CC BY). The use, distribution or reproduction
in other forums is permitted, provided the original author(s) or licensor are credited
and that the original publication in this journal is cited, in accordance with accepted
academic practice. No use, distribution or reproduction is permitted which does not
comply with these terms.

31

May 2015 | Volume 2 | Article 36

ORIGINAL RESEARCH
published: 27 April 2015
doi: 10.3389/fmats.2015.00034

Silicon-nitride-based integrated
optofluidic biochemical sensors
using a coupled-resonator optical
waveguide
Jiawei Wang, Zhanshi Yao and Andrew W. Poon *
Photonic Device Laboratory, Department of Electronic and Computer Engineering, The Hong Kong University of Science and
Technology, Hong Kong, China

Edited by:
Dan-Xia Xu,
National Research Council Canada,
Canada
Reviewed by:
Koji Yamada,
Nippon Telegraph and Telephone
Corporation, Japan
Weidong Zhou,
University of Texas at Arlington, USA
Robert Halir,
Universidad de Málaga, Spain
*Correspondence:
Andrew W. Poon,
Photonic Device Laboratory,
Department of Electronic and
Computer Engineering, The Hong
Kong University of Science and
Technology, Clear Water Bay,
Kowloon, Hong Kong, China
eeawpoon@ust.hk

Specialty section:
This article was submitted to Optics
and Photonics, a section of the
journal Frontiers in Materials
Received: 31 January 2015
Accepted: 01 April 2015
Published: 27 April 2015
Citation:
Wang J, Yao Z and Poon AW (2015)
Silicon-nitride-based integrated
optofluidic biochemical sensors using
a coupled-resonator optical
waveguide
Front. Mater. 2:34.
doi: 10.3389/fmats.2015.00034

Frontiers in Materials | www.frontiersin.org

Silicon nitride (SiN) is a promising material platform for integrating photonic components
and microfluidic channels on a chip for label-free, optical biochemical sensing applications
in the visible to near-infrared wavelengths. The chip-scale SiN-based optofluidic sensors
can be compact due to a relatively high refractive index contrast between SiN and the
fluidic medium, and low-cost due to the complementary metal-oxide-semiconductor
(CMOS)-compatible fabrication process. Here, we demonstrate SiN-based integrated
optofluidic biochemical sensors using a coupled-resonator optical waveguide (CROW) in
the visible wavelengths. The working principle is based on imaging in the far field the outof-plane elastic-light-scattering patterns of the CROW sensor at a fixed probe wavelength.
We correlate the imaged pattern with reference patterns at the CROW eigenstates. Our
sensing algorithm maps the correlation coefficients of the imaged pattern with a library of
calibrated correlation coefficients to extract a minute change in the cladding refractive
index. Given a calibrated CROW, our sensing mechanism in the spatial domain only
requires a fixed-wavelength laser in the visible wavelengths as a light source, with the
probe wavelength located within the CROW transmission band, and a silicon digital
charge-coupled device/CMOS camera for recording the light scattering patterns. This is
in sharp contrast with the conventional optical microcavity-based sensing methods that
impose a strict requirement of spectral alignment with a high-quality cavity resonance
using a wavelength-tunable laser. Our experimental results using a SiN CROW sensor
with eight coupled microrings in the 680 nm wavelength reveal a cladding refractive
index change of ~1.3 × 10−4 refractive index unit (RIU), with an average sensitivity of
~281 ± 271 RIU−1 and a noise-equivalent detection limit of 1.8 × 10−8 ~ 1.0 × 10−4 RIU
across the CROW bandwidth of ~1 nm.
Keywords: silicon nitride, biochemical sensor, integrated optofluidics, coupled-resonator optical waveguide,
microring resonators, CMOS-compatible, elastic light scattering, visible wavelengths

Introduction
In recent years, the increasing demands of medical diagnostics outside a clinic or a laboratory and
self-monitoring for personal healthcare have highly motivated the rapid research and development
of portable, low-cost biochemical sensors (Estevez et al., 2012). Particularly, miniaturized, label-free

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biochemical sensors are highly desired in order to be readily
deployed at or carried to the sensing environment and to readout in real-time, quantitative biochemical information about the
environment (Vollmer et al., 2008). Among various demonstrated
chip-scale photonic biochemical sensors, optical microresonatorbased biosensors featuring optical resonances with a high quality
(Q) factor (103 ~ 104 ) promise a high sensitivity [few tens to
hundreds of nanometer resonance shift per refractive index unit
(RIU)], a low detection limit (10−7 ~ 10−4 RIU) and a compact
footprint (few to hundreds of micrometer square) (De Vos et al.,
2007; Ciminelli et al., 2013; Sedlmeir et al., 2014). However, such
high-Q microcavity-based sensors working in the spectral domain
are constrained by a narrow resonance bandwidth as the sensing
window, which requires a strict resonance alignment and thus
may compromise the reliability of the sensor system. Besides, the
sensing implementation typically requires a precision wavelengthscanning setup, such as a wavelength-tunable laser, which may
limit the portability of the sensor system.
Other than microcavity-based biochemical sensors, integrated
interferometric optical biochemical sensors also attract increasing
attentions. Various kinds of interferometer structures, including
Mach–Zehnder interferometers (MZI) (Densmore et al., 2008;
Kozma et al., 2009; Duval et al., 2013; Halir et al., 2013; Dante
et al., 2015), Young interferometers (Ymeti et al., 2007), and
Hartman interferometers (Xu et al., 2007) have been adopted as
integrated interferometric biochemical sensors, demonstrating a
high sensitivity (102 ~ 104 rad/RIU) along with a low detection
limit (10−7 ~ 10−5 RIU). One key merit of such integrated interferometric sensors is that they require a relatively simple configuration, which typically comprises a fixed-wavelength laser source
and a photodetector. However, these interferometric sensors are
not tolerant to equipment noises that cause output intensity variations, such as laser intensity variations.
Previously, our research group has proposed a coupledresonator optical waveguide (CROW)-based biochemical sensing
scheme using what we termed “pixelized pattern detection” in
the spatial domain (Lei and Poon, 2011). The scheme employs
the discrete transition of the CROW eigenstate excited at a fixed
laser wavelength upon a small change in the cladding refractive index, Δn, and detects the resulting change in mode-fieldintensity distribution by far-field measurement of the out-of-plane
elastic-light-scattering intensity patterns. Such a sensing scheme
in principle only requires relatively simple optical sources and
imaging systems including a fixed-wavelength laser and a camera. Recently, we have experimentally demonstrated a proof of
concept of such a chip-scale CROW-based sensor on the siliconon-insulator (SOI) platform in the 1550 nm telecommunication
wavelengths (Wang et al., 2014). We have extended the scheme
by detecting the continuous modulation of the CROW modefield-intensity distribution at a fixed wavelength upon a Δn by
correlating the elastic-light-scattering patterns with reference patterns at the CROW eigenstates. Compared with interferometric
sensors, the correlation analysis allows our sensing scheme to
be more tolerant to equipment noises that are common to all
pixels of the CROW sensor yet do not cause a spectral shift,
including laser intensity variations. Our previous experiment
demonstrated a Δn of ~1.5 × 10−4 RIU and a noise-equivalent
detection limit (NEDL) of 2 × 10−7 ~ 9 × 10−4 RIU. However, the
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choice of the SOI platform and the experimental setup configuration (including a 1550 nm laser, an optical amplifier and an
InGaAs camera) render our previous work not practical for pointof-care optical biochemical sensing applications. Particularly, in
order to leverage the wide availability of smartphones for biochemical sensing (Lakshminarayanan et al., 2015), it would be
advantageous to switch the operational wavelength of the sensor
from the telecommunication wavelengths to the visible or nearinfrared wavelengths that can be readily recorded using highresolution silicon charge-coupled device (CCD)/complementary
metal-oxide-semiconductor (CMOS) cameras.
In this paper, we report our experimental demonstration of
the CROW-based biochemical sensors in the visible wavelengths
in the silicon-nitride (SiN) platform. The SiN platform is transparent to the visible and near-infrared wavelengths (Gorin et al.,
2008; Subramanian et al., 2013) and its fabrication process is
CMOS-compatible. After the CROW calibration steps, our sensing scheme in principle only requires a fixed-wavelength, lowoutput-power, visible laser source, and a silicon CCD/CMOS
camera for recording out-of-plane light-scattering patterns from
the top-view. This offers a promising opportunity to integrate
the CROW sensor with a smartphone that is equipped with a
compact laser source and a high-resolution camera with a properly
designed optical interface for future smartphone-based point-ofcare applications.

Principle and Methods
Principle and the Sensing Algorithm
Figure 1 illustrates the principle of the CROW-based biochemical
sensor following our previous work (Wang et al., 2014). Here,
we outline the key concepts of the principle for understanding
this work. Figure 1A schematically shows a SiN CROW sensor comprising eight coupled microring resonators with identical design, coupled to input and output bus waveguides in an
add-drop filter configuration. For a perfect CROW comprising C coupled identical single-mode resonators, the inhomogeneously broadened transmission spectrum features a combination
of split mode resonances, with each mode slightly shifted from the
original resonance frequency due to inter-cavity-coupling effect.
Therefore, the eigenstate number N within each transmission
band always equals to the resonator number C. While a perfect
CROW exhibits distinctive mode-field-amplitude distributions
at eigenstates, the pair of symmetric and anti-symmetric splitmodes at different eigenfrequencies have non-distinctive modefield-intensity distributions. In practice, a CROW inevitably suffers from fabrication imperfections. The coupled resonators are
no longer identical nor are identically coupled. The symmetry breaking between the pair of symmetric and anti-symmetric
split-modes therefore results in distinctive mode-field-intensity
distributions at all discernable eigenstates. The resulting phase
disorders and coupling disorders can result in the split mode
resonances to be spectrally overlapped. Therefore, in the presence
of structural non-uniformity, N could be equal to or smaller than
C (N ≤ C).
Figure 1B schematically illustrates the inhomogeneously
broadened transmission bands upon applying cladding refractive indices n0 and n0 + Δn, for an imperfect eight-microring
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characterizing an imperfect eight-microring CROW, including the
inhomogeneously broadened transmission bands upon a buffer solution
(n0 ) and a test solution (n0 + Δn), and pixelized mode-field-intensity
distributions at eigenstate wavelength λ j upon n0 , A(λ j ). Insets: pixelized
mode-field-intensity distributions at probe wavelength, λ p , (i) upon n0
[B(λ p )]; and (ii) upon n0 + Δn [T(λ p )].

FIGURE 1 | Principle of SiN CROW-based biochemical sensors
using out-of-plane elastic light scattering at the visible
wavelengths. (A) Schematic of a SiN CROW-based sensor integrated
with a microfluidic channel. An objective lens and a CMOS/CCD camera
are applied on top of the optofluidic chip in order to image the
out-of-plane elastic-light-scattering pattern. (B) Illustration of

CROW exhibiting a complete set of eight distinctive eigenstate
mode-field-intensity distributions. With the mode-field intensity
of each microring integrated as a pixel, we denote the pixelized
one-dimensional pattern at the eigenstate as {Aj }, with j indexing the eigenstate. Any mode-field-amplitude distribution at an
arbitrary wavelength, λp , within the CROW transmission band
upon n0 can be expressed by a linear superposition of the complete set of eigenstate mode-field-amplitude distributions upon
n0 . Therefore, we are able to uniquely identify any pixelized
mode-field-intensity profile at λp upon n0 , B(λp ), as shown in
inset (i), with {Aj } by a correlation analysis. Upon a small Δn
applied homogenously to the cladding, we can uniquely identify by the correlation analysis any pixelized mode-field-intensity
distribution at λp upon n0 + Δn, T(λp ), as shown in inset (ii),
with {Aj }.
As in our previous work (Wang et al., 2014), we adopt the Pearson’s correlation coefficient, ρ, in order to analyze the degree of
correlation between a pixelized pattern at an arbitrary probe
wavelength λp , B(λp ), and the pixelized patterns at the eigenstate
wavelengths λj , A(λj ). For a CROW with a number of coupled
single-mode cavities, C, and a number of discernable eigenstates,
N (≤C), we define ρ at λp for A(λj ) as follows:
C
∑

ρj (λp ) = √

reference wavelength λ0 centered at the CROW transmission
band. The library is calibrated over a range of Δn values, Δnd ,
given by an integer{multiple}of a minimum refractive index change
( )
interval Δni . The ρ′j λ0 thus comprises a library of data array
of N (rows) × M (columns), where M is given by Δnd /Δni .
For sensing, we first measure the pixelized mode-field-intensity
pattern in a buffer solution at a fixed probe wavelength λp (which
is generally offset from λ0 ) as B(λp ) (Figure 1B). We correlate
B(λp ) with the eigenstate patterns {Aj } in order to extract
{ρj (λp )}. }
We look for the closest match of {ρj (λp )} with the library
{
( )
ρ′j λ0 , using only the principal (largest) component, ρp ,
and the second-principal (second-largest) component, ρs , of
{ρj (λp )} in order to streamline the pattern recognition process

(A(i, λj ) − A(λj ))(B(i, λp ) − B(λp ))

i=1
C
∑

respectively. The bar sign denotes the mean of the entire pixelized
pattern over C pixels.
We adopt the Pearson’s correlation coefficient approach to
describe the linear dependence of the measured and calibrated
intensity distributions. The Pearson’s correlation approach is
insensitive to both level and scale variations of the intensity
distributions. Therefore, the approach is tolerant to equipment
noise sources, such as uniform background light imaged onto the
camera and the intensity variation of the laser source, which are
common to all pixels and do not cause a spectral shift. However,
this approach still suffers from the noises that cause a spectral
shift, such as a wavelength drift of the laser source and thermal
variations in the test environment.
Here, we detail our sensing algorithm following our previous
work (Wang et al., 2014). Figure 2 shows a flow chart illustrating
our sensing algorithm including calibration.
{ ( )} We first generate a
library of correlation coefficients ρ′j λ0 , defined at a fixed

(A(i, λj ) − A(λj ))

i=1

√
2

C
∑

2

(B(i, λp ) − B(λp ))

i=1

(1)
where j = 1, 2, . . ., N is the eigenstate number, and i = 1, 2, . . .,
C is the cavity (pixel) number. A(i, λj ) and B(i, λp ) are the pixel
values normalized to the total intensity of the entire patterns,

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We study the effects of these empirical inputs on the device
parameters, including the waveguide effective refractive index,
neff , and the inter-cavity coupling coefficient, κ. We calculate using
the numerical finite-element method (FEM) (COMSOL RF module) the neff of a SiN channel ridge waveguide for the transversemagnetic (TM)-polarized mode, as a function of waveguide width
around 427.5 nm at a fixed waveguide height of 300 nm upon a
water upper-cladding. We adopt the measured material refractive
index of the deposited 300 nm-thick SiN film as a function of
wavelength using ellipsometry. The mean value of the calculated
neff is 1.5994 ± 0.0003 at 686 nm. We choose the TM polarization
mode in order to obtain a large evanescent field exposure near
the waveguide top surface for better light–analyte interaction.
We calculate the coupling coefficient in each directional coupling
region as a function of the coupling gap spacing, assuming the
waveguide width is fixed at 427.5 nm. We estimate the waveguide
propagation loss upon a water upper-cladding to be relatively
high at ~17 dB/cm based on our measurements. We attribute this
primarily to surface-roughness-induced scattering losses from the
waveguide sidewall. We apply the designed racetrack arc radius
and interaction length into the modeled CROW. We find from
our FEM calculations a linear relationship between Δn and the
resulting effective refractive index change Δneff , which we apply
to our transfer-matrix modeling (see Supplementary Materials S1
and S2).

Device Fabrication

We fabricate the CROW devices in a 4′′ silicon wafer. The silicon
wafer is first grown with a ~2 μm-thick thermal oxide. We grow
nitrogen-rich SiN by plasma-enhanced chemical vapor deposition (PECVD) (SiH4 :NH3 = 25:40 (sccm), 300°C, 13.56 MHz).
The thickness of SiN layer is ~300 nm. We fabricate the CROW
device pattern by electron-beam lithography (JEOL JBX-6300FS)
using a positive electron-beam resist ZEP-520A. We transfer the
device pattern to the SiN layer by inductively coupled plasma
etching with C4 F8 and SF6 gases (STS ICP DRIE Silicon Etcher).
Figure 3A shows the optical micrograph of the fabricated SiN
eight-microring CROW device. The racetrack microring comprises two half circles with a radius of 20 μm and two straight
waveguides with an interaction length (Lc ) of 4 μm. We design the
waveguide width to be 450 nm and the coupling gap spacing to
be 100 nm. Figure 3B shows a zoom-in-view optical microscope
image of the CROW. Figure 3C shows a SEM picture of the
coupling region.
We fabricate a microfluidic chamber on a polydimethylsiloxane
(PDMS) layer. We pattern a SU8 film by contact photolithography as a mold in order to form the PDMS microfluidic channel
by imprinting. The designed dimension of microfluidic channel
is 8 mm × 2 mm × 50 μm (length, width, and height). We use
a puncher to make two holes, each with a diameter of 1 mm,
as an inlet and outlet for solution delivery. The diced silicon
chip and the PDMS microfluidic layer are treated with oxygen plasma and directly bonded, with the microfluidic channel encompassing the CROW sensor. The bonded PDMS–SiN
interface is stable enough for repeating the sensing experiments
for many times under a relatively high fluidic pump pressure.
Figure 3D schematically shows the cross-sectional view of the
optofluidic chip.

FIGURE 2 | A flow chart showing the sensing algorithm including
calibration of CROW-based sensors.

(Wang et al., 2014). We thus obtain a unique equivalent refractive
index change for the buffer solution, ΔnB , which is only due to
the offset between λp and λ0 . We repeat the same procedure for
measuring the pattern at λp upon the test solution, T(λp ), and
obtain another unique equivalent refractive index change ΔnT .
Finally, we obtain Δn = ΔnT − ΔnB .

Transfer-Matrix Modeling of Imperfect CROW
Sensors
We model imperfect SiN CROWs in 680 nm wavelengths using
transfer-matrix method with empirical inputs (Wang et al., 2014)
(see Supplementary Materials S1 and S2). We measure and accumulate statistics of the measured waveguide widths and coupling
gap widths from scanning-electron microscope (SEM) characterization of our fabricated devices. We sample six waveguide widths
and three coupling gap width in one coupling region, and measure
a total of eighteen coupling regions in two representative eightmicroring CROW devices (see Supplementary Material S3). The
statistics of the waveguide widths and the coupling gap widths
approximately follow two Gaussian distributions. We extract the
fabricated waveguide width of 427.5 ± 1.1 nm and coupling gap
spacing of 129.1 ± 1.0 nm. In the modeling, we assume that the
two Gaussian distributions are independent, and we generate a set
of varied waveguide widths and coupling gap spacing randomly
distributed across the CROW using the Gaussian number generator in Matlab.
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FIGURE 3 | (A) Optical micrograph of the fabricated eight-microring
CROW. (B) Zoom-in-view picture of the CROW.
(C) Scanning-electron microscope image of an inter-cavity coupling
region of the CROW. (D) A cross-sectional view of the SiN chip

integrated with a microfluidic channel. (E) Schematic of the
experimental setup. HWP, half-wave plate; PBS, polarizing beam
splitter; LWD OB, long-working-distance objective lens; OB, objective
lens; PD, photodetector; MMLF, multimode lensed fiber.

spectra with a sum of multiple inverted Lorentzian lineshapes,
each centered at the resonance (eigenstate) wavelength. The overall transmission band shift is taken as the average value of the
spectral shifts of all the eigenstates.

Experimental Method
Figure 3E schematically shows the experimental setup. The
wavelength-tunable laser light in the 680 nm wavelengths is endfired into a tapered 3 μm-wide SiN waveguide through an objective lens (NA = 0.65). The laser power before coupling into the
chip is ~2 mW. The polarization is controlled by a half-wave
plate before a polarizing beam splitter. The output light from the
throughput- or drop-port is collected using a multimode lensed
fiber to a silicon power meter and a lock-in amplifier.
For elastic-light-scattering pattern imaging from the top view,
we use a long-working-distance microscope objective lens (20×
Mitutoyo Plan Apo, NA = 0.42) and a CCD camera (Diagnostic Instruments, Inc., RT3) with 1600 × 1200 pixels (7.4 μm-sized
pixels). The camera has an effective differential cooling of −43°C
and an 8-bit analog-to-digital conversion in data readout. We fix
the exposure time as 60 ms and the gain of ~1. For background
subtraction, we set the probe wavelength in between the CROW
transmission bands in order to obtain a background image.
In order to acquire the library of calibrated correlation coefficients, we scan the laser wavelength in steps of 0.02 nm over
~2 free spectral ranges (FSRs) of the CROW sensor. We record
at each wavelength eight successive images over a time period of
4 s (at 2 frames/s). We take average of these successive images in
order to reduce the systematic equipment noise contribution. In
the sensing tests, we inject the buffer and test solutions, and start
recording the images after the scattering pattern is stabilized upon
an essentially static fluidic medium. We record over 50 successive
images during a time period of 25 s at a fixed probe wavelength.
In order to calibrate the spectral sensitivity of the CROW, we
prepare NaCl solutions with mass concentrations from 1 to 5%
(in steps of 1%) and test the transmission band spectral shifts
upon a Δn. Between each measurement, we rinse the chip by
injecting deionized (DI) water using a fluidic pump. We obtain the
resonance spectral shifts by fitting the throughput-transmission

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Results
Modeling Results
Figure 4 shows the modeling results for N = C (see Supplementary Material S4 for modeling results corresponding to the
case N < C). Figure 4A schematically shows an imperfect SiN
CROW with varied waveguide width and coupling gap width of
each microring. Inset shows the numerically calculated waveguide mode-field-amplitude profile in the TM mode at 686 nm
wavelength. Figure 4B shows the modeled throughput- and droptransmission spectra of an imperfect eight-microring CROW.
We define the CROW transmission bandwidth, ΔλBW , as the
spectral range between the first and last discernable eigenstates
within the transmission band. Figure 4C shows the modeled
pixelized patterns at the
eigenstates. Figure 4D shows
{ eight
( )}
the calculated library ρ′j λ0
as a function of Δn, with
Δnd = 2.523 × 10−2 and Δni = 3.6 × 10−4 RIU. Figure 4E shows
the calculated
correlation coefficients per unit Δn,
( (differential
))
given as |d ρ′j λ0 /d (Δn) |.
We define the CROW sensitivity (in units of RIU−1 ) at
an(arbitrary
λp within the transmission band as the larger
( ))
′
|d ρj λp /d (Δn) | of the ρp and ρs . Figure 4F shows the
modeled sensitivity as a non-linear function of λp . The sensitivity
in the transmission band spans a range from ~73 to ~1440 RIU−1 ,
with an average sensitivity of ~553 ± 290 RIU−1 . We quantify
the non-uniformity of the sensitivity by the ratio of SD value
to average sensitivity value. A lower ratio value suggests a more
uniform sensitivity. The extracted non-uniformity ratio from

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Silicon-nitride coupled-microresonator biochemical sensors

FIGURE 4 | (A) Schematic of an imperfect CROW model with non-uniform
waveguide widths (W 0 , W 1 , W 2 , . . . W N+1 ) and coupling gap spacing
(g1 , g2 , . . . gN+1 ). Inset (i): numerically calculated waveguide mode-field
amplitude profile in the TM mode. (B) Modeled throughput- and drop-port
transmission spectra of an imperfect eight-microring CROW using
transfer-matrix modeling. Green and red dashed-lines indicate the reference
wavelength λ 0 of 688.06 nm and the probe wavelength λ p of 688.14 nm,

respectively. (C) Modeled normalized pixelized intensity patterns at the eight
eigenstates, I–VIII. (D) Calculated library of the correlation coefficients
ρ ′1 − ρ ′8 as a function of Δn at λ 0 , with Δnd = 2.523 × 10−2 and
Δni = 3.6 × 10−4 RIU. (E) Calculated library of the
( differential
) correlation
coefficients as a function of Δn at λ 0 , given as |d ρ ′j (λ 0 ) /d (Δn) |.
(F) Calculated sensitivity as a function of λ p . The red dashed-line indicates a
sensitivity of 772 RIU−1 at λ p = 688.14 nm.

agrees with the arbitrarily chosen Δn value. We attribute the
deviation of 2 × 10−5 RIU to the interpolation error. In principle,
the maximum error upon the sampling interval in the library is
given by ± Δni /2, which is ~1.8 × 10−4 RIU given the assumed
Δni value.

Figure 4F is ~0.52. Although such a sensitivity variation is not
ideal, we can obtain a practical sensitivity within a wide enough
wavelength window without fine-tuning the probe wavelength.
As an example, we can set a practical sensitivity of ~100 RIU−1
in order to sense a Δn down to 10−5 RIU (assuming a noiseinduced uncertainty of correlation coefficients of ~±10−3 ). From
Figure 4F, the width of the probe wavelength window with a sensitivity >100 RIU−1 is 1.1 nm. We consider this sufficiently wide
for sensing with a practical sensitivity at an arbitrarily set probe
wavelength. If a higher practical sensitivity of, say, 200 RIU−1
is desired, the width of the probe wavelength window with a
sensitivity >200 RIU−1 narrows to ~1.06 nm.
Here, we arbitrarily choose λp at 688.14 nm near the center
of the CROW transmission band (Figure 4B) in order to model
the sensing test. The sensitivity at λp is ~772 RIU−1 . Figure 5
illustrates the modeled sensing results. Figure 5A shows the
modeled pixelized patterns at λp , B(λp ) and T(λp ), assuming a
water buffer (n0 = 1.331) and an arbitrarily chosen Δn value of
2.50 × 10−3 RIU, respectively. Figure 5B shows the two sets of
correlation coefficients extracted from the two modeled pixelized
patterns without and with Δn. The ρp and ρs without Δn are ρ4 and
ρ5 , respectively. The ρp and ρs with Δn are ρ5 and ρ3 , respectively.
Figures
show the zoom-in view of the calculated library
{ ( )5C,D
}
ρ′j λ0 as a function of Δn. Insets show the detailed mappings
of ρp and ρs with the library. We extract using linear interpolation from the library Δn = ΔnT − ΔnB = 2.52 × 10−3 RIU, which

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Calibrating the CROW Sensor in a Buffer Solution
Figure 6 summarizes the characterization results upon a buffer
solution (DI water). Figure 6A shows the measured TM-polarized
transmission spectra with DI water upper-cladding. The measured
FSR of ~1.80 nm is consistent with the microring circumference.
The CROW exhibits an inhomogeneously broadened transmission band, with a ΔλBW of ~1.10 nm. We discern eight eigenstates
within each transmission band (labeled by I to VIII for the first
transmission band, and I′ –VIII′ for the second transmission band
in Figure 6A).
Figure 6B shows the measured elastic-light-scattering images
at eigenstates I–VIII. We observe a non-uniform scattering image
profile across each microring. We attribute this to the extra
modulation of the surface roughness and local defects to the
intrinsic mode-field-intensity distributions. We notice an obvious
“local hotspot” in the coupling region between microring 3 and
microring 4 in all the light-scattering images. We attribute that
to the larger surface roughness localized in the coupling region
between microring 3 and microring 4. We integrate within a certain window the elastic-light-scattering intensity of each microring to form a single pixel. The window excludes the coupling
region in order to avoid scattering-induced crosstalks between

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the calculated library of ρ ′j as a function of Δn. Dashed-lines indicate the
mapping of ΔnB , ΔnT for buffer solution and test solution, respectively.
Insets (i)–(iv): Mapping of ρ p and ρ s with the library to extract ΔnB
and ΔnT .

FIGURE 5 | (A) Modeled normalized pixelized patterns at λ p (688.14 nm)
upon n0 and n0 + Δn. (B) Calculated correlation coefficients at
λ p = 688.14 nm upon n0 and n0 + Δn. The dashed-line and the
dotted-line boxes indicate ρ p and ρ s , respectively. (C,D) Zoom-in view of

the coupled waveguides and local hotspots. Here we normalize
the patterns with the estimated contributions of the surfaceroughness-induced scattering as a step for pattern correction (see
Supplementary Material S5). Figure 6C shows the corrected pixelized mode-field-intensity patterns at the eight eigenstates. We
use the corrected pixelized patterns for sensing.
Figure 6D shows the measured library of the calibrated correlation coefficients as a function of Δn. Here, we calibrate the
sensor by scanning the laser wavelength over ±Δλ (Δλ = 0.7 nm)
about the center of the CROW transmission band spanning a
FSR upon a fixed buffer solution (DI water), with a minimum
wavelength step of 0.02 nm. This interval corresponds to a Δni of
~3.5 × 10−4 RIU, based on the calibrated linear spectral sensitivity of ~57.30 nm/RIU of the CROW sensor (see Supplementary
Material S6). We also convert Δλ back to Δn using the calibrated
linear spectral sensitivity. The corresponding range of Δnd is
~±1.2 × 10−2 RIU.
Figure 6E shows the calculated |dρ′j /d (Δn) | as a function of
Δn. Figure 6F shows the calculated sensitivity as a function of
λp over the λ0 ± Δλ range. The calculated sensitivity value shows
highly non-uniform profiles. The sensitivity ranges from ~15 to
~1420 RIU−1 , with an average value of ~281 ± 271 RIU−1 . The
extracted non-uniformity ratio from Figure 6F is ~0.96. The width
of the probe wavelength window with a sensitivity >100 RIU−1
is 0.88 nm. Whereas, the width of the probe wavelength window
with a sensitivity >200 RIU−1 narrows to ~0.48 nm, which is

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still relatively tolerant to set a probe wavelength. In conventional
microcavity-based sensing methods, the sensitivity is only applicable within the high-Q transmission band (~0.1 nm in De Vos
et al., 2007), which is generally much narrower than our probe
wavelength window.
We define the NEDL at λp as the uncertainty of extracted Δn.
We repeat the extraction of Δn values based on ρp and ρs at each
λp for eight times and calculate the SD of the eight extracted
Δn values. Figure 6G shows the extracted NEDL values as a
function of λp, which shows a high dependence on the choice of
λp. The NEDL values range from ~2 × 10−8 to ~1 × 10−4 RIU.
We observe particularly low NEDL values (~10−8 RIU) at λp
aligning with the eigenstate wavelengths. We attribute the low
NEDL at each eigenstate to the particularly low uncertainty of
ρp (~±10−6 − 10−4 ) close to 1 at each eigenstate. Upon eight
repeated tests at a fixed probe wavelength at each eigenstate, the
measured pixelized patterns only slightly deviate from the calibrated eigenstate distributions due to the low noise in the cooled
silicon CCD camera and the low thermo-optic coefficient of SiN.
The low uncertainties of ρp at each eigenstate are converted into
particularly low NEDL values.
In order to quantify the sensing resolution, here we define
the resolution of the CROW sensor as the lowest refractive
index change that can be sensed reliably and repeatedly. In
practice, there are two main limiting factors to the resolution.
One is the interpolation error in extracting Δn. The other is

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coefficients ρ ′1 − ρ ′8 as a function of Δn. White dashed-lines indicate the
ΔnB values at λ p1 , λ p2 , and λ p3 . (E) Calculated
correlation
( differential
)

FIGURE 6 | (A) Measured TM-polarized throughput- and drop-port
transmission spectra of the eight-microring SiN CROW with DI water
upper-cladding. Green and red dashed-lines indicate the reference
wavelength λ 0 (686.86 nm) and three probe wavelengths, λ p1 (687.06 nm),
λ p2 (687.38 nm), and λ p3 (686.42 nm). (B) Measured elastic-light-scattering
images with DI water upper-cladding at the eight eigenstates I–VIII. The
white-line box indicates the integration window for pixelization.
(C) Normalized pixelized mode-field intensity patterns at the eight CROW
eigenstates I–VIII. (D) Calculated library of the calibrated correlation

coefficients as a function of Δn, given as |d ρ ′j (λ 0 ) /d (Δn) |.

(F) Calculated sensitivity as a function of λ p . Red dashed-lines indicate
sensitivities of 214, 279, and 541 RIU−1 at probe wavelengths λ p1 , λ p2 , and
λ p3 , respectively. (G) Extracted noise-equivalent detection limit (NEDL) as a
function of λ p . Red dashed-lines indicate NEDL values of ~4 × 10−6 ,
~2 × 10−8 , and ~1 × 10−6 RIU at λ p1 , λ p2 , and λ p3 , respectively. Green
dashed-lines indicate the eight eigenstate wavelengths λ j .

the NEDL taking into account all the noise sources that our
correlation approach is not tolerant to. Therefore, given a calibration interval of Δni (3.5 × 10−4 RIU), the worst resolution is
~1.8 × 10−4 RIU given ± Δni /2 (1.8 × 10−4 RIU) and the NEDL
(~1.8 × 10−8 − 1.0 × 10−4 RIU in Figure 6G). The interpolationerror-limited resolution (±Δni /2) suggests that a Δn below Δni /2
may not be tested reliably or repeatedly. The resolution can be
improved by adopting a finer Δni .
We also calibrate the CROW sensor in the adjacent transmission band (see Supplementary Material S7). The pixelized
mode-field intensity patterns at eigenstates I′ –VIII′ show a
high similarity with the corresponding patterns at eigenstates
I–VIII, respectively. The extracted sensitivity and NEDL range

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are both close to the calibrated results of the first transmission
band.

Blind Sensing Test Results
We implement blind sensing tests at three different probe wavelengths (λp1 , λp2 , and λp3 ) within the CROW transmission band.
We prepare one buffer solution (DI water) and three NaCl solutions, X, Y, and Z, with different mass concentration values
unknown to the researcher conducting the sensing tests. We study
the images upon the buffer solution at the initial stage and upon
rinsing after each sensing test. We confirm that the pixelized pattern returns to the baseline pattern (see Supplementary Material
S8). Table 1 summarizes the experimental sensing results.

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TABLE 1 | Sensing results at the three probe wavelengths upon the buffer solution and the three test solutions.
λp

ρp

Solution

λ p1 (687.06 nm)

Buffer (DI water)
X (NaCl)
Y (NaCl)
Z (NaCl)

λ p2 (687.38 nm)

Buffer (DI water)
X (NaCl)
Y (NaCl)
Z (NaCl)

λ p3 (686.42 nm)

Buffer (DI water)
X (NaCl)
Y (NaCl)
Z (NaCl)

ρ3
ρ6
ρ4
ρ3

(0.927 ± 0.003)
(0.924 ± 0.003)
(0.946 ± 0.002)
(0.903 ± 0.002)

ρ 1 (0.99996 ± 0.00003)
ρ 4 (0.941 ± 0.007)
ρ 2 (0.841 ± 0.012)
ρ 1 (0.989 ± 0.001)
ρ7
ρ1
ρ2
ρ7

(0.851 ± 0.001)
(0.205 ± 0.015)
(0.519 ± 0.006)
(0.783 ± 0.005)

ρs

ΔnB or ΔnT
(× 10−3 RIU)

Δn (× 10−3 RIU)

Sensed
concentration (%)

ρ6
ρ2
ρ5
ρ6

(0.866 ± 0.002)
(0.838 ± 0.003)
(0.854 ± 0.003)
(0.854 ± 0.008)

~−3.54 ± 0.02
~4.21 ± 0.01
~−2.46 ± 0.03
~−3.41 ± 0.01

–
~7.75 ± 0.02
~1.08 ± 0.04
~0.13 ± 0.03

–
~4.35 ± 0.01
~0.61 ± 0.03
~0.073 ± 0.014

ρ7
ρ5
ρ6
ρ7

(0.622 ± 0.001)
(0.875 ± 0.005)
(0.807 ± 0.012)
(0.622 ± 0.003)

~−9.1800 ± 0.0001
~−0.79 ± 0.02
~−8.16 ± 0.02
~−9.14 ± 0.01

–
~8.39 ± 0.02
~1.03 ± 0.02
~0.05 ± 0.01

–
~4.70 ± 0.01
~0.58 ± 0.01
~0.03 ± 0.01

–
–
~1.04 ± 0.01
~0.13 ± 0.01

–
–
~0.58 ± 0.01
~0.069 ± 0.005

ρ 2 (0.485 ± 0.002)
ρ 7 (−0.013 ± 0.014)
ρ 8 (0.305 ± 0.009)
ρ 2 (0.477 ± 0.005)

~7.775 ± 0.003
–
~8.81 ± 0.01
~7.90 ± 0.01

Prepared concentration values: X: (4.5 ± 0.1)%, Y: (0.60 ± 0.02)%, Z: (0.070 ± 0.002)%.

Sensing at an Arbitrarily Set Probe Wavelength λp1

attribute this deviation to a not sufficiently fine calibration of the
library and the error from linear interpolation. The calibrated
response of ρp around the eigenstate is in the proximity to the
maximum (unity). The limited sampling resolution of Δni may not
be sufficient to describe the response around an extremum.

Figure 7 shows the sensing results at an arbitrarily set probe wavelength λp1 (687.06 nm) near the center of the CROW transmission
band. The sensitivity at λp1 is ~214 RIU−1 (see Figure 6F). The
NEDL at λp1 is ~4 × 10−6 RIU (see Figure 6G). Figure 7A shows
the measured elastic-light-scattering images of the CROW upon
the buffer solution and the three test solutions at λp1 . Figure 7B
shows the corresponding pixelized patterns. Figure 7C shows the
corresponding calculated correlation coefficients. Figures 7D–G
show the mapping of ρp and ρs in the buffer solution and the
three test solutions with the library. Insets (i)–(viii) show the
mapping ρp and ρs to the corresponding ΔnB or ΔnT using linear
interpolations in between Δni .
We acquire for solution X a ΔnX of ~(7.75 ± 0.02) × 10−3 RIU
and for solution Y a ΔnY of ~(1.08 ± 0.04) × 10−3 RIU, both corresponding to a relatively large Δn but still within Δnd . We acquire
for solution Z (Figure 7G) a ΔnZ of ~(1.3 ± 0.3) × 10−4 RIU. For
all three solutions, we convert from the measured Δn values the
sensed concentration values (see Table 1), which show a good
agreement with the prepared values.

Sensing at λp3 Near Eigenstate VII
Figure 9 shows the sensing results at another specifically chosen
probe wavelength λp3 (686.42 nm). We specifically set λp3 at the
blue-edge of the transmission band near eigenstate VII. The sensitivity at λp3 is ~541 RIU−1 (see Figure 6F). The NEDL at λp3 is
~1 × 10−6 RIU (see Figure 6G). We consider λp3 as a near optimized choice with a relatively high sensitivity and a low NEDL.
Figure 9A shows the measured elastic-light-scattering images
upon the buffer solution and the three test solutions. Figure 9B
shows the corresponding pixelized patterns. Figure 9C shows the
corresponding calculated correlation coefficients. Figures 9D–F
show the mapping of ρp and ρs values with the library (see
Supplementary Material S9 for detailed mappings).
For solution X, however, we observe an almost dark scattering
pattern, which suggests that λp3 upon solution X is relatively
shifted out of the transmission band. Both the extracted ρp and
ρs values out of ρj (λp3 ) upon solution X are particularly low. By
mapping the extracted ρj (λp3 ) values with the library, we find no
match to indicate the corresponding Δnx. Therefore, in the case
that there is a chance to measure a large Δn near Δnd (in the order
of 10−2 ~ 10−3 RIU in this case), it is better to position λp close
to the red-side of the transmission band in order to leverage the
dynamic range given by ΔλBW in full.
For solution Y, we acquire a ΔnY of ~(1.04 ± 0.02) × 10−3 RIU.
For solution Z, we acquire a ΔnZ of ~(1.24 ± 0.1) × 10−4 RIU.
Both sensing results agree with the prepared concentrations of
solutions Y and Z. Compared with the sensing result of solution
Z at λp1 , we obtain a more accurate value of ΔnZ with a much
improved uncertainty. We attribute this to a higher sensitivity and
a lower NEDL at λp3 than those at λp1.

Sensing at λp2 Aligned with Eigenstate I
Figure 8 shows the sensing results at a specifically chosen probe
wavelength λp2 (687.38 nm) aligned with eigenstate I. The sensitivity at λp2 is ~279 RIU−1 (see Figure 6F). The NEDL at λp2 is
~2 × 10−8 RIU (see Figure 6G), which is much lower compared
with that at λp2 . Figure 8A shows the measured elastic-lightscattering images upon the buffer solution and the three test
solutions. Figure 8B shows the corresponding pixelized patterns.
Figure 8C shows the corresponding calculated correlation coefficients. Figures 8D–G show the mapping of ρp and ρs values in
the buffer solution and the three test solutions with the library (see
Supplementary Material S9 for detailed mappings).
We acquire for solution X a ΔnX of ~(8.40 ± 0.02) × 10−3 RIU
and for solution Y a ΔnY of ~(1.03 ± 0.02) × 10−3 RIU. Both
sensing results agree with the prepared concentrations of solutions X and Y. For solution Z (Figure 8G), we acquire a ΔnZ of
~(0.5 ± 0.1) × 10−4 RIU, corresponding to a mass concentration
of ~(0.03 ± 0.01)%. This, however, shows a significant deviation from the prepared concentration [~(0.070 ± 0.002)%]. We

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Discussion
Here, we benchmark our work with other silicon- and SiN-based
on-chip optical biochemical sensors that have been demonstrated

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Silicon-nitride coupled-microresonator biochemical sensors

buffer solution and solutions X, Y and Z at λ p1 . Dashed-line and dotted-line
boxes indicate ρ p and ρ s , respectively. (D–G) Zoom-in view of the library to
extract Δn. (D) Upon the buffer solution. (E) Upon solution X. (F) Upon solution
Y. (G) Upon solution Z. Insets (i)–(viii): Mapping of ρ p and ρ s upon the buffer
solution and solutions X, Y, and Z.

FIGURE 7 | (A) Measured elastic-light-scattering images of CROW upon the
buffer solution and the three blind-test solutions X, Y, and Z at an arbitrarily set
probe wavelength λ p1 . The white-line box indicates the integration window for
pixelization. (B) Normalized pixelized patterns upon the buffer solution and
solutions X, Y, and Z at λ p1 . (C) Calculated correlation coefficients upon the

in recent years, including our previous work (Wang et al., 2014),
as summarized in Table 2. All of the work including this work
have attained a detection limit of 10−7 ~ 10−4 RIU. Two of the
microcavity-based sensors (Ghasemi et al., 2013; Doolin et al.,
2015) and three of the MZI-based sensors (Duval et al., 2013;
Misiakos et al., 2014; Dante et al., 2015) operate on the SiN-based
platform in the visible wavelengths.
Most of the reported microcavity-based sensors in the literature (except Ghasemi et al., 2013; Doolin et al., 2015) operate
in the telecommunication wavelengths (1.3/1.55 μm) and require
a wavelength-tunable laser and a non-silicon photodetector.
Whereas, our CROW sensor operating in the visible wavelengths
only requires in principle a fixed-wavelength visible laser diode
and a silicon CCD/CMOS camera after the library preparation.
In terms of the sensor calibration, the main difference between
our library preparation and the conventional calibration process
for a microcavity-based sensor is the recording of the pixelized
patterns instead of single intensity values. A typical calibration
for a conventional microcavity-based sensor [e.g., De Vos et al.
(2007) and Iqbal et al. (2010)] involves scanning laser wavelength
across a narrow transmission band. As an example, in the work of
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De Vos et al., calibrating the spectral sensitivity of a microring sensor of Q ~ 20,000 involved measuring the microring transmission
spectrum three times for each of the four given NaCl solutions
with different concentrations (De Vos et al., 2007). In contrast,
our library preparation involves scanning laser wavelength across
the CROW transmission band, recording the pixelized patterns at
each wavelength step corresponding to the refractive index interval Δni and deriving the corresponding correlation coefficients
with the eigenstate patterns. The pattern recording and additional
computation of the correlation coefficients render our library
preparation more reliable and tolerant to the equipment noises
that are common to all pixels compared with recording single
intensity values multiple times.
A major issue requiring further developments is the significant variation of sensitivity values upon different probe wavelengths. We can modify the CROW design in order to attain
a more uniform sensitivity (see Supplementary Material S10).
Our modeling results suggest that an imperfect CROW with a
reduced cavity size along with an enhanced inter-cavity coupling
coefficient offers a more uniform sensitivity. Upon a small cavity
radius R = 10 μm and a strong inter-cavity coupling coefficient
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Silicon-nitride coupled-microresonator biochemical sensors

FIGURE 8 | (A) Measured elastic-light-scattering images of CROW upon
the buffer solution and the three blind-test solutions X, Y, and Z at a
specifically chosen probe wavelength λ p2 at eigenstate I. The white-line
box indicates the integration window for pixelization. (B) Normalized
pixelized patterns upon the buffer solution and solutions X, Y, and Z at

λ p2 . (C) Calculated correlation coefficients upon the buffer solution and
solutions X, Y, and Z at λ p2 . Dashed-line and dotted-line boxes indicate
ρ p and ρ s , respectively. (D–G) Zoom-in view of the library to extract Δn.
(D) Upon the buffer solution. (E) Upon solution X. (F) Upon solution Y.
(G) Upon solution Z.

FIGURE 9 | (A) Measured elastic-light-scattering images of CROW upon
the buffer solution and the three blind-test solutions X, Y, and Z at a
specifically chosen probe wavelength λ p3 near eigenstate VII. The
white-line box indicates the integration window for pixelization.
(B) Normalized pixelized patterns upon the buffer solution and solutions X,

Y and Z at λ p3 . (C) Calculated correlation coefficients upon the buffer
solution and solutions X, Y, and Z at λ p3 . Dashed-line and dotted-line
boxes indicate ρ p and ρ s , respectively. (D–F) Zoom-in view of the library
to extract Δn. (D) Upon the buffer solution. (E) Upon solution Y. (F) Upon
solution Z.

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Silicon-nitride coupled-microresonator biochemical sensors

TABLE 2 | Summary of silicon- and silicon-nitride-based on-chip optical biochemical sensors.
Device config.

Reference

Material
platform

Operational
wavelength (nm)

Footprint
(μm2 )

Q-factor

Sensitivity
(RIU−1 )

Detected
Δn (RIU)

Detection
limit (RIU)

MZI

Densmore et al. (2008)
Duval et al. (2013)
Misiakos et al. (2014)
Dante et al. (2015)

SOI
Si3 N4
SiN
Si3 N4

~1550
658
~600–900
660

~40,000
~108
~107
~60,000

N/A
N/A
N/A
N/A

920 π rad
4950 π rad
581 rad
6000 π rad

~7 × 10−4
~3 × 10−4
~4 × 10−5
~2 × 10−4

~1 × 10−5
~2 × 10−7
~1 × 10−5
~4 × 10−7

Microdisk

Wang et al. (2013)
Doolin et al. (2015)

SOI
Si3 N4

~1550
~770

~3
~900

~100
10000

130 nm
200 nm

~9 × 10−3
~4 × 10−4

~8 × 10−4
~10− 6

Microring with
slot-waveguide

Barrios et al. (2007)
Claes et al. (2009)
Carlborg et al. (2010)

Si3 N4
SOI
Si3 N4

~1300
~1550
~1300

~20,000
~240
~20,000

1800
~450
–

212 nm
298 nm
248 nm

~10−3
~4 × 10−3
~3 × 10−4

~2 × 10−4
~4.2 × 10−5
~5 × 10−6

Microring

De Vos et al. (2007)
Iqbal et al. (2010)
Ghasemi et al. (2013)
Liu et al. (2014)

SOI
SOI
SiN
SOI

~1550
~ 1550
~656
~1550

~110
~900
~400
~1600

20,000
43,000
–
15000

70 nm
163 nm
48 nm
6000 rad

~9 × 10−4
~10−6
–
~4 × 10−4

~10−5
–
–
~2.5 × 10−6

Eight-microring
CROW in the
spatial domain

Wang et al. (2014)
(This work)

SOI
SiN

~1550
~680

~1716
~14080

N/A
N/A

~199
~281 ± 271

~1.5 × 10−4
~1.3 × 10−4

2 × 10−7 ~ 9 × 10−4
2 × 10−8 ~ 1 × 10−4

κ ~ 0.9, we obtain for an imperfect eight-microring CROW a
modeled sensitivity of ~384 ± 153 RIU−1 , with an improved nonuniformity ratio of ~0.40 compared to the modeled ratio of
~0.52 following our experimental device parameters. Assuming
a practical sensitivity of ~100 RIU−1 , the width of the modeled probe wavelength window with a sensitivity >100 RIU−1
is 2.2 nm, which is much improved compared to the modeled
width of 1.1 nm following the experimental device parameters. If
a higher practical sensitivity of 300 RIU−1 is desired, the modeled
probe wavelength window width with a sensitivity >300 RIU−1
is ~1.56 nm, which is still sufficiently wide for practical applications. Based on our current imperfect CROW model, we can
further design the CROW with tailored non-uniform parameters
to optimize the sensitivity and sensitivity variation.
In summary, we demonstrated a SiN CROW-based sensing
scheme in the spatial domain in the visible wavelengths. Given
a calibrated CROW sensor, this sensing scheme in principle
only requires a low-power, fixed-wavelength laser source in the
visible wavelengths and a silicon CCD or CMOS camera to
image the elastic-light-scattering patterns in the far field. Our
proof-of-concept experiment using an eight-microring CROW
on the SiN-on-silica platform showed an average sensitivity of

~281 ± 271 RIU−1 and a NEDL of 2 × 10−8 ~ 1 × 10−4 RIU. Our
blind sensing tests using NaCl solutions showed a detection
of ~1.26 × 10−4 RIU. Therefore, we have shown that such a
chip-scale, microresonator-based SiN CROW sensor operating
in the visible wavelengths is promising as a potentially highperformance, portable, and low-cost optical biochemical sensor
for applications such as point-of-care biochemical analyses and
self-monitoring of personal healthcare using smartphones.

References

Dante, S., Duval, D., Fariña, D., González-Guerrero, A. B., and Lechuga, L. M.
(2015). Linear readout of integrated interferometric biosensors using a periodic wavelength modulation. Laser Photon. Rev. 9, 248–255. doi:10.1002/lpor.
201400216
De Vos, K., Bartolozzi, I., Schacht, E., Bienstman, P., and Baets, R. (2007). Siliconon-insulator microring resonator for sensitive and label-free biosensing. Opt.
Express 15, 7610–7615. doi:10.1364/oe.15.007610
Densmore, A., Xu, D.-X., Janz, S., Waldron, P., Mischki, T., Lopinski, G.,
et al. (2008). Spiral-path high-sensitivity silicon photonic wire molecular sensor
with temperature-independent response. Opt. Lett. 33, 596–598. doi:10.1364/ol.
33.000596
Doolin, C., Doolin, P., Lewis, B. C., and Davis, J. P. (2015). Refractometric sensing
of Li salt with visible-light Si3N4 microdisk resonators. Appl. Phys. Lett. 106,
081104. doi:10.1063/1.4913618
Duval, D., Osmond, J., Dante, S., Dominguez, C., and Lechuga, L. M. (2013). Grating couplers integrated on Mach-Zehnder interferometric biosensors operating

Acknowledgments
This work is supported by grants from the Research Grants
Council of the Hong Kong Special Administrative Region, China
(Project No. 618010, 617612, and 617913). The authors acknowledge the HKUST Nanoelectronics Fabrication Facility for fabricating the optofluidic chip.

Supplementary Material
The Supplementary Material for this article can be found
online at http://www.frontiersin.org/Journal/10.3389/fmats.2015.
00034/abstract

Barrios, C. A., Gylfason, K. B., Sánchez, B., Griol, A., Sohlström, H., Holgado,
M., et al. (2007). Slot-waveguide biochemical sensor. Opt. Lett. 32, 3080–3082.
doi:10.1364/ol.32.003080
Carlborg, C. F., Gylfason, K. B., Kazmierczak, A., Dortu, F., Polo, M. B., Catala,
A. M., et al. (2010). A packaged optical slot-waveguide ring resonator sensor
array for multiplex label-free assays in labs-on-chips. Lab. Chip 10, 281–290.
doi:10.1039/b914183a
Ciminelli, C., Campanella, C. M., Dell’olio, F., Campanella, C. E., and Armenise,
M. N. (2013). Label-free optical resonant sensors for biochemical applications.
Prog. Quant. Electron. 37, 51–107. doi:10.1016/j.pquantelec.2013.02.001
Claes, T., Molera, J. G., De Vos, K., Schachtb, E., Baets, R., and Bienstman, P.
(2009). Label-free biosensing with a slot-waveguide-based ring resonator
in silicon on insulator. Photonics J. IEEE 1, 197–204. doi:10.1109/jphot.2009.
2031596

Frontiers in Materials | www.frontiersin.org

43

April 2015 | Volume 2 | Article 34

Wang et al.

Silicon-nitride coupled-microresonator biochemical sensors

in the visible range. Photonics J. IEEE 5, 3700108. doi:10.1109/jphot.2013.
2251873
Estevez, M.-C., Alvarez, M., and Lechuga, L. M. (2012). Integrated optical devices
for lab-on-a-chip biosensing applications. Laser Photon. Rev. 6, 463–487. doi:10.
1002/lpor.201100025
Ghasemi, F., Eftekhar, A. A., Gottfried, D. S., Song, X., Cummings, R. D., and Adibi,
A. (2013). “Self-referenced silicon nitride array microring biosensor for toxin
detection using glycans at visible wavelength,” in SPIE BiOS (San Francisco, CA:
International Society for Optics and Photonics).
Gorin, A., Jaouad, A., Grondin, E., Aimez, V., and Charette, P. (2008). Fabrication
of silicon nitride waveguides for visible-light using PECVD: a study of the
effect of plasma frequency on optical properties. Opt. Express 16, 13509–13516.
doi:10.1364/oe.16.013509
Halir, R., Vivien, L., Le Roux, X., Xu, D.-X., and Cheben, P. (2013). Direct and
sensitive phase readout for integrated waveguide sensors. Photonics J. IEEE 5,
6800906. doi:10.1109/jphot.2013.2276747
Iqbal, M., Gleeson, M. A., Spaugh, B., Tybor, F., Gunn, W. G., Hochberg, M., et al.
(2010). Label-free biosensor arrays based on silicon ring resonators and highspeed optical scanning instrumentation. IEEE J. Sel. Top. Quantum Electron. 16,
654–661. doi:10.1109/jstqe.2009.2032510
Kozma, P., Hamori, A., Cottier, K., Kurunczi, S., and Horvath, R. (2009). Grating
coupled interferometry for optical sensing. Appl. Phys. B 97, 5–8. doi:10.1007/
s00340-009-3719-1
Lakshminarayanan, V., Zelek, J., and Mcbride, A. (2015). “Smartphone Science” in
eye care and medicine. Opt. Photonics News 26, 44–51. doi:10.1364/opn.26.1.
000044
Lei, T., and Poon, A. W. (2011). Modeling of coupled-resonator optical
waveguide (CROW) based refractive index sensors using pixelized spatial detection at a single wavelength. Opt. Express 19, 22227–22241. doi:10.1364/oe.19.
022227
Liu, J., Zhou, X., Qiao, Z., Zhang, J., Zhang, C., Xiang, T., et al. (2014). Integrated optical chemical sensor based on an SOI ring resonator using phaseinterrogation. Photonics J. IEEE 6, 1–7. doi:10.1109/jphot.2014.2352973
Misiakos, K., Raptis, I., Makarona, E., Botsialas, A., Salapatas, A., Oikonomou, P.,
et al. (2014). All-silicon monolithic Mach-Zehnder interferometer as a refractive

Frontiers in Materials | www.frontiersin.org

index and bio-chemical sensor. Opt. Express 22, 26803–26813. doi:10.1364/OE.
22.026803
Sedlmeir, F., Zeltner, R., Leuchs, G., and Schwefel, H. G. (2014). High-Q MgF2
whispering gallery mode resonators for refractometric sensing in aqueous
environment. Opt. Express 22, 30934–30942. doi:10.1364/oe.22.030934
Subramanian, A., Dhakal, A., Peyskens, F., Selvaraja, S., Baets, R., Neutens, P., et al.
(2013). Low-loss singlemode PECVD silicon nitride photonic wire waveguides
for 532–900 nm wavelength window fabricated within a CMOS pilot line.
Photonics J. IEEE 5, 2202809. doi:10.1109/jphot.2013.2292698
Vollmer, F., Arnold, S., and Keng, D. (2008). Single virus detection from the
reactive shift of a whispering-gallery mode. Proc. Natl. Acad. Sci. U.S.A. 105,
20701–20704. doi:10.1073/pnas.0808988106
Wang, J., Yao, Z., Lei, T., and Poon, A. W. (2014). Silicon coupled-resonator
optical-waveguide-based biosensors using light-scattering pattern recognition
with pixelized mode-field-intensity distributions. Sci. Rep. 4, 07528. doi:10.1038/
srep07528
Wang, X., Guan, X., Huang, Q., Zheng, J., Shi, Y., and Dai, D. (2013). Suspended
ultra-small disk resonator on silicon for optical sensing. Opt. Lett. 38, 5405–5408.
doi:10.1364/ol.38.005405
Xu, J., Suarez, D., and Gottfried, D. S. (2007). Detection of avian influenza virus
using an interferometric biosensor. Anal. Bioanal. Chem. 389, 1193–1199.
doi:10.1007/s00216-007-1525-3
Ymeti, A., Greve, J., Lambeck, P. V., Wink, T., Van Hövell, S. W., Beumer, T. A., et al.
(2007). Fast, ultrasensitive virus detection using a Young interferometer sensor.
Nano Lett. 7, 394–397. doi:10.1021/nl062595n
Conflict of Interest Statement: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Copyright © 2015 Wang, Yao and Poon. This is an open-access article distributed
under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s)
or licensor are credited and that the original publication in this journal is cited, in
accordance with accepted academic practice. No use, distribution or reproduction is
permitted which does not comply with these terms.

44

April 2015 | Volume 2 | Article 34

ORIGINAL RESEARCH ARTICLE

MATERIALS

published: 07 April 2015
doi: 10.3389/fmats.2015.00028

High-throughput multiple dies-to-wafer bonding
technology and III/V-on-Si hybrid lasers for heterogeneous
integration of optoelectronic integrated circuits
Xianshu Luo 1 , Yulian Cao 2 , Junfeng Song 1 , Xiaonan Hu 2 , Yuanbing Cheng 2 , Chengming Li 2 ,
Chongyang Liu 2 , Tsung-Yang Liow 1 , Mingbin Yu 1 , Hong Wang 2 , Qi Jie Wang 2 and Patrick Guo-Qiang Lo 1 *
1
2

Institute of Microelectronics, Agency for Science, Technology and Research (A*STAR), Singapore, Singapore
Photonics Center of Excellence (OPTIMUS), School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, Singapore

Edited by:
Laurent Vivien, Université Paris-Sud,
France
Reviewed by:
Junichi Fujikata, Photonics Electronics
Technology Research Association,
Japan
Yasuhiko Ishikawa, The University of
Tokyo, Japan
*Correspondence:
Patrick Guo-Qiang Lo, Institute of
Microelectronics, Agency for Science,
Technology and Research (A*STAR),
Singapore Science Park II, 11 Science
Park Road, 117685 Singapore
e-mail: logq@ime.a-star.edu.sg

Integrated optical light source on silicon is one of the key building blocks for optical interconnect technology. Great research efforts have been devoting worldwide to explore various
approaches to integrate optical light source onto the silicon substrate. The achievements
so far include the successful demonstration of III/V-on-Si hybrid lasers through III/V gain
material to silicon wafer bonding technology. However, for potential large-scale integration,
leveraging on mature silicon complementary metal oxide semiconductor (CMOS) fabrication technology and infrastructure, more effective bonding scheme with high bonding yield
is in great demand considering manufacturing needs. In this paper, we propose and demonstrate a high-throughput multiple dies-to-wafer (D2W) bonding technology, which is then
applied for the demonstration of hybrid silicon lasers. By temporarily bonding III/V dies to a
handle silicon wafer for simultaneous batch processing, it is expected to bond unlimited III/V
dies to silicon device wafer with high yield. As proof-of-concept, more than 100 III/V dies
bonding to 200 mm silicon wafer is demonstrated. The high performance of the bonding
interface is examined with various characterization techniques. Repeatable demonstrations
of 16-III/V die bonding to pre-patterned 200 mm silicon wafers have been performed for
various hybrid silicon lasers, in which device library including Fabry–Perot (FP) laser, lateralcoupled distributed-feedback laser with side wall grating, and mode-locked laser (MLL).
From these results, the presented multiple D2W bonding technology can be a key enabler
toward the large-scale heterogeneous integration of optoelectronic integrated circuits.
Keywords: silicon photonics, hybrid lasers, heterogeneous integration, die-to-wafer bonding, optoelectronic
integrated circuits

INTRODUCTION
In the future generation of datacom and computercom, which
demand ever higher bandwidth and lower power, the conventional
electrical interconnection routing the electronic signals becomes
bandwidth-limited along with prohibitively high power consumption (Beausoleil et al., 2008). One solution to the challenge is the
optical interconnect technology (Goodman et al., 1984; Miller,
2000, 2009; Ohashi et al., 2009), in which high bandwidth optical
signals are routed by low-loss optical fiber and waveguides. In contrast to the electrical interconnection (i.e., the copper wire), optical
interconnect has many merits, e.g., high speed, low crosstalk,
immunity to electromagnetic interference, low overall power consumption (Alduino and Paniccia, 2007). Most importantly, with
the up scaling potential, optical interconnect is expected to provide
much higher transmission capacity and longer signal transmission
distance than the electrical interconnect.
Although it was proposed initially 30 years ago (Goodman et al.,
1984), there was no significant development progress with solid
demonstrations of optical interconnect for very-large-scale integration (VLSI). The situation has changed since the concept of
silicon photonics (Pavesi and Lockwood, 2004; Reed and Knights,

www.frontiersin.org

2004; Lipson, 2005; Guillot and Pavesi, 2006; Jalali and Fathpour,
2006; Soref, 2006; Poon et al., 2009a,b; Vivien and Pavesi, 2013;
Xu et al., 2014), which utilizes low-cost silicon material along with
leveraging on the advancement of silicon complementary metal
oxide semiconductor (CMOS) process, integration, and mature
infrastructure. Envisioned by Soref and Lorenzo (1985), silicon
photonics has emerged and progressed steadily. Especially in the
past decade, we have been witnessing rapid growth in research and
development activities along with product development efforts
exploiting silicon photonics technology for the optical interconnect (Pavesi and Lockwood, 2004; Reed and Knights, 2004; Lipson,
2005; Guillot and Pavesi, 2006; Jalali and Fathpour, 2006; Soref,
2006; Fedeli et al., 2008; Poon et al., 2009a,b; Michel et al., 2010;
Reed et al., 2010; Feng et al., 2012; Liow et al., 2013; Vivien and
Pavesi, 2013; Dong et al., 2014a,b; Lim et al., 2014; Xu et al., 2014).
For instance, to minimize the small core silicon waveguide propagation losses, considerable research work has been devoted to
minimize the waveguide sidewall roughness by using the deep
ultra-violet (DUV) photolithography and optimized patterning
technique (Dumon et al., 2004; Bogaerts et al., 2005) and sidewall
smoothing technique [e.g., double thermal oxidations (Sparacin

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Luo et al.

et al., 2005; Xia et al., 2006)]. Indeed, submicrometer-scale silicon
wire waveguides have shown a propagation loss of 2 dB/cm and
less (Xia et al., 2006). Furthermore, owing to the enabling CMOS
fabrication technologies, we have seen the establishment and utilization of a myriad of essential silicon photonic passive and active
components including optical filters (Xiao et al., 2007; Zhou and
Poon, 2007; Guha et al., 2010; Fang et al., 2012), optical switches
(Poon et al., 2009a,b; Van Campenhout et al., 2009; Luo et al.,
2012; Song et al., 2013), low-power-consuming modulators with
up to 50 Gb/s-speed operation (Dong et al., 2009; Reed et al., 2010;
Tu et al., 2013, 2014), and Ge-on-Si photodetectors with bandwidth larger than 40 GHz (Michel et al., 2010; Liow et al., 2013).
It is these demonstrated silicon photonic devices and technologies that make ultimate optical interconnection a viable solution
to address the distance/bandwidth/cost and power-consumption
challenges. To this end, silicon photonics provides nearly all key
building blocks for optical interconnection. Furthermore, the
CMOS-compatible fabrication processes make it possible to integrate both electronics and photonics either through monolithic
or heterogeneous approach. Such significant progress has led the
optical interconnect to become a much more practical technology.
However, silicon-based on-chip optical light source, which is
one of the key components for the light generation for carrying
information, has been the missing piece for optical interconnect.
This is mainly because silicon is transparent in the telecommunication wavelengths (i.e., 1310 and 1550 nm wavelengths) due to the
indirect bandgap, which prohibits efficient light emission from silicon. Thus, to solve the challenge, numerous research efforts have
been devoted to explore various technologies for light source on
silicon chips.
REVIEW ON RESEARCH FOR LASERS ON SILICON

Historically, researchers worldwide have devoted many research
efforts by exploring various possibilities for the development of
lasers on silicon, which mainly focused in the following directions:
(1) silicon material engineering by introducing emissive centers
to assist the efficient light emission (Pavesi et al., 2000; Han
et al., 2001; Rotem et al., 2007a,b; Shainline and Xu, 2007),
(2) strained Ge (Liu et al., 2007, 2009, 2010; Cheng et al., 2009;
Sun et al., 2009b,c; Camacho-Aguilera et al., 2012),
(3) silicon Raman laser (Boyraz and Jalali, 2004, 2005; Rong et al.,
2005a,b, 2007), and
(4) heterogeneous integration of III/V gain materials through
packaging (Chu et al., 2009; Fujioka et al., 2010; Urino et al.,
2011) or wafer bonding (Park et al., 2005; Fang et al., 2007a,b;
Liang et al., 2009a,b, 2010; Stanković et al., 2010; Grenouillet
et al., 2012).
Here, we will limit our review to the heterogeneous integrated silicon lasers. With regard to the silicon laser through
heterogeneous integration of III/V gain materials on silicon, there
are two major types of integration strategies, namely the packaging scheme and the bonding scheme. Research groups from
Japan devoted many efforts for the development of silicon lasers
using packaging methods. Chu et al. (2009) demonstrated the
first wavelength-tunable-laser fabricated with silicon photonic

Frontiers in Materials | Optics and Photonics

III/V-on-Si bonding and hybrid lasers

technology, which comprised a semiconductor optical amplifier (SOA) chip and a silicon photonic chip, and were hybridintegrated by using passive alignment technology. An SiON modesize converter was adopted between the silicon waveguide and III/V
SOA for low coupling loss. Later on, silicon photonic-based optical
interconnects were also demonstrated by integrating lasers, silicon modulators, and Ge photodetectors on single silicon substrate
(Urino et al., 2011). While such demonstrations have shown the
advantage in principal of being capable to integrate various building blocks together for optical interconnection, the main issue
is the complicated fabrication process. It typically requires precise alignment between the SOA and the silicon waveguide, even
with assistance of the mode-size converter. Considering the III/V
gain region of <200 nm in thickness, for instance, it became a
difficult challenge for alignment with acceptable coupling loss.
Such complicated fabrication process is a potential show stopper for future massive production demanding high yield, thus
significantly increases the product cost.
Heterogeneous integration of III/V gain materials on silicon
through wafer bonding technology is another major directional
strategy for silicon lasers. UMR-CNRS and LETI initiated the
research work of III/V laser on silicon wafers for photonic integration by using wafer bonding technology. In 2001, they demonstrated the first InP-based microdisk laser integrated on a silicon
wafer through SiO2 –SiO2 molecular bonding (Seassal et al., 2001).
Although this work did not show complete integration of III/V
optoelectronics with silicon photonics waveguide structures, it
showed the potential of such wafer bonding technology for future
heterogeneous integrated optoelectronic circuit. Following such
demonstration, Hattori et al. (2006) demonstrated an integration scheme of III/V microdisk laser with silicon waveguide in
2006. By aligning the microdisk laser atop silicon waveguide, the
laser emissions can be vertically coupled into the underneath silicon waveguide with 35% coupling efficiency. Such demonstration
showed the capability of the hybrid photonic integration of III/V
laser with silicon waveguide for photonic links application.
The so-called hybrid silicon laser was proposed and first
demonstrated by Park et al. (2005) with optical injection. In
this work, the III/V wafer with AlGaInAs quantum well structure is directly bonded to pre-patterned silicon wafer using lowtemperature oxygen plasma-assisted wafer bonding. The laser
cavity was defined by endface-polished silicon waveguide structure, while the III/V provides the optical gain. As the III/V
optoelectronic structures are fabricated after the wafer bonding
with best precise, only possibly achieved via lithographic process,
alignment to the silicon device layer, thus there is no stringent
alignment requirement to the bonding process, which significantly simplifies the fabrication process and makes the possibility
of wafer-level-oriented manufacturing ability. Subsequently, Fang
et al. (2006) demonstrated an electrically pumped AlGaInAssilicon evanescent laser with continuous-wave (CW) operation
in 2006. Subsequently, various hybrid lasers with different structures and also enhanced laser performances are demonstrated by
various research groups using molecular wafer bonding technology, including Fabry–Pérot lasers (FP) (Ben Bakir et al., 2011;
Dong et al., 2013), racetrack lasers (Fang et al., 2007a,b), distributed Bragg reflector (DBR) lasers (Fang et al., 2008a,b,c),

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Luo et al.

distributed-feedback (DFB) lasers (Fang et al., 2008a,b,c), microring lasers (Liang et al., 2009a,b, 2012), wavelength tunable lasers
(Keyvaninia et al., 2013a,b,c), multiple-wavelength lasers (Van
Campenhout et al., 2008; Kurczveil et al., 2011), and mode-locked
lasers (MLL) (Fang et al., 2008a,b,c).
Besides such direct bonding method, wafer bonding can also
be realized through an adhesive material as the bonding interlayer.
Among all kinds of adhesive bonding materials, divinylsiloxanebisbenzocyclobutene (DVS-BCB or BCB) is the most popular one
for hybrid silicon lasers due to the merits such as the high bonding strength and the sustainability in the subsequent III/V process.
IMEC has used BCB-assisted adhesive bonding method for heterogeneous integration (Roelkens et al., 2005). In 2006, Roelkens et al.
(2006) demonstrated the first electrically injected InP/InGaAsP
laser integrated on silicon waveguide circuit using BCB-assisted
adhesive bonding technology. Similar to Seassal et al. (2001), the
optical laser is purely made with III/V layer with the laser facets
being defined by dry etching. With optimized mode-size converter,
the optical light can be vertically coupled down to the underneath
silicon waveguide with high efficiency. By designing hybrid mode
waveguide comprising silicon waveguide and III/V gain medium,
they also demonstrated a hybrid FP laser (Stanković et al., 2011)
and a DFB laser (Stanković et al., 2012; Keyvaninia et al., 2013a,b,c),
and multiple-wavelength laser (Keyvaninia et al., 2013a,b,c) using
such adhesive bonding technology.
Apart from BCB, some kinds of metal can also be adopted as
the bonding interlayer for adhesive bonding. AuGeNi is one of the
most popular metals for metal bonding as it not only functions
as a bonding media but can also be used for the Ohmic contact
to the InGaAsP structure. Tanabe et al. (2010) demonstrated a
InAs/GaAs quantum-dot laser on Si substrate by metal-assisted
wafer bonding with room temperature operation at 1.3 µm wavelength. Meanwhile, Hong et al. (2010) also demonstrated an FP
laser through selective-area metal bonding using AuGeNi. The silicon waveguide in such demonstration is with 5 µm and 800 nm
thickness. The demonstrated FP laser is with threshold current
density of 1.7 kA/cm and a maximum output power of 3 mW.
However, the drawback of the AuGeNi-assisted bonding is the Au
contamination. Thus, Tatsumi et al. (2012) further demonstrated
an Au-free metal-assisted wafer bonding for lasers on silicon chip.
Besides, Creazzo et al. (2013) also demonstrated another type
of silicon laser by using metal-assisted bonding of III/V epitaxial material directly onto the silicon substrate. The demonstrated
that silicon laser had a threshold of ~50 mA and maximum optical
power of ~8 mW. The benefit of such metal-assisted bonding is the
advantage of effective thermal dissipation, which shows a thermal
resistant of only 21°C/W.
Beyond these two major heterogeneous integration schemes,
there are also other methods for III/V-on-Si lasers, including direct
III/V epitaxy on silicon substrate (Liu et al., 2011; Lee et al., 2012)
and III/V epitaxial layer transfer-printing to silicon wafers (Justice
et al., 2012; Yang et al., 2012). However, while the direct epitaxy
method faces major challenges of high-density dislocations due to
the lattice mismatch between III/V material and silicon after many
years of research, the transfer-printing method for hybrid silicon
laser needs further demonstrations to show the repeatability and
reliability.

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III/V-on-Si bonding and hybrid lasers

From these analyses, it shows that among various approaches,
the hybrid silicon laser through wafer bonding technology can
be considered as the most successful and promising one for silicon photonic heterogeneous integration circuits due to the everdemonstrated advanced performances and the fabrication process
compatibility with silicon photonics. Table 1 summarizes some of
the representative demonstrations of hybrid silicon lasers through
wafer bonding technology.
WAFER BONDING TECHNOLOGIES FOR ON-CHIP SILICON LASERS

In general, there are two mainstreams of wafer bonding methods
applying to heterogeneous integrated silicon photonics, namely
the molecular bonding through interfacial bonds, and the adhesive bonding assisted with another adhesive material as bonding
interface such as polymer or metal. Such wafer bonding technology is a mature process, which is widely applicable for SOI
wafer fabrication, MEMS technology, and optoelectronic device
fabrication. As a lot of review papers already exist (Lasky, 1986;
Maszara, 1991; Tong and Goesele, 1999; Alexe and Gösele, 2004;
Christiansen et al., 2006), we thus only focus the discussion on
the application of hybrid silicon lasers. According to the existing
demonstrations, we further summarize here the major bonding
technologies as below:
(1)
(2)
(3)
(4)

wafer-to-wafer (W2W) molecular bonding,
die-to-wafer (D2W) molecular bonding,
BCB-assisted D2W adhesive bonding, and
metal-assisted adhesive bonding.

The W2W molecular bonding for hybrid silicon lasers is mainly
driven by the UCSB group. Through such plasma-activated lowtemperature W2W molecular bonding (Pasquariello and Hjort,
2002), they, together with their collaborators, have demonstrated
various hybrid silicon lasers, starting from the first-hybrid FP
laser (Fang et al., 2006), followed by racetrack-shaped laser (Fang
et al., 2007a,b), DBR lasers (Fang et al., 2008a,b,c), DFB lasers
(Fang et al., 2008a,b,c), MLL (Fang et al., 2008a,b,c), and multiwavelength arbitrary waveform generation (AWG) laser (Kurczveil
et al., 2011). However, for the conventional III/V-to-Si W2W bonding without thick oxide interlayer, the generated gas by-products
of H2 and H2 O are easily trapped inside the bonding interface and
form the interfacial voids, which subsequently affect the bonding
quality. In order to effectively remove such trapped gases, some
proper outgas channels are designed, such as in-plane outgassing
channels (IPOC) (Kissinger and Kissinger, 1993) or vertical outgassing channels (VOC) (Liang and Bowers, 2008). IPOS is formed
by etching some lateral channels extended to the chip edges, so
that the by-product gases can be directed to outside the bonding
interface to the chip edge during post bonding annealing. However, for some close-loop structures, such as microrings, there is
no way to design such IPOS. In order to solve such issue, VOC
is proposed by etching some array of holes down to the BOX
layer. The generated by-product gases can migrate to the closest
VOC and are absorbed by SOI BOX. As both IPOS and VOC can
be formed during the waveguide etching, there is no particular
design requirement from the fabrication point of view. However,
as the formation of such outgas channels affects the silicon layer

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III/V-on-Si bonding and hybrid lasers

Table 1 | Representative demonstrations of hybrid silicon lasers through wafer bonding technology.
Laser types Bonding type

Waveguide scheme

Fabry–Perot

Molecular

Hybrid WG (75 vs. 3% mode

laser

bonding

confinement within Si WG

Performances
λ (nm)

T (°C)

1577

CW @ 15

Reference

I th (mA) P out (mW) SE (mW/mA) Z t (°C/W)
65

1.8

0.013a

40

Fang et al.
(2006)

and QW)
DBR laser

Molecular

Hybrid waveguide with

bonding

inverse taper (66 vs. 4.4%

1569

CW @ 15

65

11

0.088a

40

Fang et al.
(2008a,b,c)

mode confinement within Si
WG and QW)
D2W molecular

Hybrid waveguide with

bonding with

adiabatic mode transformer

1570

Pulse @ 20

100

7.2

0.021a

–

Ben Bakir
et al. (2011)

oxide interlayer
1553

CW @ 20

40

4

0.025a

W2W molecular

Hybrid waveguide with

bonding with

inverse taper, thermal tunable

–

Keyvaninia
et al.

oxide interlayer

microring for wavelength

(2013a,b,c)

tuning
Metal-assisted

III/V gain material

D2W bonding

butt-coupling with Si

1562

CW @ 20

41

8

0.038

21

Creazzo
et al. (2013)

waveguide through a
waveguide coupler
DFB laser

Molecular

Hybrid waveguide with

bonding

inverse taper (59.2 vs. 5.2%

1600

CW @ 10

25

5.4

0.072a

132

Fang et al.
(2008a,b,c)

mode confinement within Si
WG and QW)
D2D BCB

Hybrid waveguide (70 vs. 3%

adhesive

mode confinement within Si

bonding

WG and QW)

Selective-area

Hybrid WG (94% mode

metal bonding

confinement within Si)

Microdisk

D2W molecular

InP microdisk laser light

laser

bonding

vertically coupling to Si WG

1308

CW @ 20

20

2.1

0.026

–

Stanković
et al. (2011)

1554

Pulse @ RT

35

3

0.05

–

Hong et al.
(2010)

1590

CW @ 20

0.9

0.012

0.008

–

Van Campenhout
et al. (2008)

Microring

Molecular

Racetrack microring, hybrid

laser

bonding

waveguide

Molecular

Hybrid microring with side

bonding

coupled Si WG

a

1590

CW @ 15

175

29

0.089

–

Fang et al.
(2007a,b)

–

CW @ 10

7.5

2.5

0.2

–

Liang et al.
(2011)

Data are extracted from the power–current curves.

pattern density, which will finally affect the bonding strength, it
is desirable to take into account the design tradeoff between the
bonding strength and the gas removal effectiveness.
Alternatively, plasma-assisted D2W molecular bonding has also
been investigated for hybrid silicon lasers mainly by LETI. For
large-scale manufacturability for potential massive production,
the key enabling capability is the multiple dies to wafer bonding with high yield. Kostrzewa et al. (2006) first demonstrated
a molecular bonding of multiple InP dies to a 200 mm silicon
CMOS wafer with only 1 mm × 1 mm die size (Kostrzewa et al.,
2006). For strong hydrophilic molecular bonding, both InP and

Frontiers in Materials | Optics and Photonics

silicon wafers were covered with oxide layer. Pick-and-place technology was used in order to align the InP dies to specific spots
in silicon wafer, as well as to supply mechanical force to the
dies through pick-and-place head. Using such D2W bonding,
they demonstrated electrically pumped microdisk lasers integrated
with a silicon waveguide circuit (Van Campenhout et al., 2007).
However, in such D2W bonding, as the cleaning of the dies is performed ahead of the pick-and-place process, the bonding surface
could be contaminated, and subsequently affecting the bonding
quality and bonding yield. Furthermore, with the consideration
of the pick-and-place time of 30 s/die, it takes approximately an

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Luo et al.

III/V-on-Si bonding and hybrid lasers

hour for bonding 100 dies. Such long bonding time through
pick-and-place process for individual die would cause the bonding surface deactivation for the molecular bonding with plasma
activation.
BCB-assisted D2W adhesive bonding can address such issue
with potential capability of bonding unlimited number of dies.
Stanković et al. (2010) demonstrated such D2D adhesive bonding technology using BCB. The BCB is first spin-coated on silicon
wafer with controlled thickness of <100 nm in order to ensure
the vertical light coupling efficiency, followed with die attaching
and subsequent curing at 240°C for 1 h in a nitrogen atmosphere
at 1000 mbar. With the assistant of BCB adhesion, the stringent
requirements of contamination-free and smooth bonding surfaces
for molecular bonding are relieved significantly. Furthermore,
there is, in principle, no limitation for multiple dies bonding by the
assistance of BCB adhesive layer (Keyvaninia et al., 2012). Through
such bonding method, various hybrid lasers, including FP laser
(Stanković et al., 2011), DFB laser (Stanković et al., 2012), microring and AWG integrated multi-wavelength DBR lasers (Keyvaninia
et al., 2013a,b,c), and microdisk laser (Mechet et al., 2013) have
been demonstrated. However, although such adhesive bonding is
with good robustness and bonding strength, the thermal dissipation could be a major problem due to the low thermal conductivity
of the BCB layer. Besides, robust polymer coating process ensuring
the controllable BCB thickness is also very important.
Apart from these three major bonding methods, metal-assisted
adhesive bonding (Hong et al., 2010; Tanabe et al., 2010; Creazzo
et al., 2013) is another one that can be used for hybrid laser integration. However, due to the potential metal contamination and the
non-compatibility with the subsequent fabrication process, such
as acid etching for substrate removal, the metal-assisted adhesive bonding method might not be an optimal choice for silicon
heterogeneous optoelectronic integrated circuits.

In Table 2, we summarize and compare these four different
bonding methods.
OUTLINE OF THE MANUSCRIPT

The rest of the submission is organized as follows. In the Section
“III/V-to-Si Wafer-to-Wafer (W2W) Bonding Technology,” we
show a demonstration of the wafer-to-wafer bonding by using lowtemperature plasma-activated molecular bonding method with
oxide as the bonding interlayer. In the Section “High-Throughput
Multiple Dies-to-Wafer Bonding Technology,” we propose and
show the demonstration of an alternative bonding technology
that can perform high-throughput D2W bonding for potential
massive production of silicon hybrid lasers, which is based on
a batch process to simultaneously bonding all the dies to the
silicon wafer. In the Section “Design of III/V-on-Si Lasers,” we
provide some design guidelines of hybrid silicon laser, including the design of III/V multiple quantum wells (MQW) structures, the silicon waveguide thickness selection for hybrid laser,
and the design of the vertical coupling structures. The Section
“Demonstration of III/V-on-Si Hybrid Lasers” shows some hybrid
silicon laser demonstrations using the bonded wafers from the
proposed high-throughput D2W bonding, including FP laser,
lateral-coupled distributed-feedback (LC-DFB) laser with side
wall grating, and MLL. The Section “Summary and Future Outlook” summarizes this paper and addresses some of the future
challenges.

III/V-TO-Si WAFER-TO-WAFER BONDING TECHNOLOGY
We have started the development of wafer bonding technology
for hybrid silicon photonics integration in 2011. Considering the
complete integration with existing silicon photonic integrated circuit, which consisting various silicon passive waveguide devices,
high-speed modulator, and photodetectors, and are normally

Table 2 | The major bonding technology for hybrid integration.
Bonding methods

Process description

Fabrication tolerance

Manufacturing scalability

Comments

W2W molecular

O2 plasma-assisted

Small tolerance of contamination-free,

Difficult due to wafer size

Low utilization of both

bonding

direct bonding with

smooth, and flat bonding surfaces

mismatch

III/V and silicon wafers

12 h annealing at 300°C

Sensitive to the wafer
bowing

D2W molecular

O2 plasma-assisted

Small tolerance of contamination-free

Difficult due to contamination

Difficult to ensure high

bonding

bonding with oxide

and smooth bonding surfaces

and surface deactivation during

yield with large number

interlayer and 3 h

pick-and-place process for large

of dies bonding

annealing at 250°C

number of dies bonding

BCB-assisted

BCB adhesive bonding

Large tolerance with low requirement

Easy to be scalable with multiple

Thermal dissipation

D2W adhesive

with post curing of 1 h

on the bonding surface. Yet it requires

dies and large-sized wafers

problem due to the low

bonding

at 240°C

controllable polymer coating regarding

thermal conductivity of

the thickness and flatness

the BCB layer

Metal-assisted

Metal-assisted bonding

Large tolerance with low requirement on

Easy to be scalable with multiple

Enhanced thermal

adhesive bonding

with annealing

the bonding surface. However, potential

dies and large-sized wafers

resistant due to the

metal contamination, and process

metal utilization

incompatibility. Potential coupling
problem due to the metal absorption

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Luo et al.

with thick oxide cladding, we adopt the low-temperature plasmaactivated molecular bonding method (Pasquariello and Hjort,
2002) with oxide as the bonding interlayer. Furthermore, such
thick cladding oxide also serves as the diffusion and absorption
medium for the bonding by-products gases, thus with enhanced
bonding quality and bonding yield.
For the initial development, we deposit 1.1 µm PECVD oxide
on top of silicon wafers, followed with chemical mechanic polishing (CMP) to remove 100 nm oxide in order to ensure the
smooth bonding interface. For all the bonding process described
hereafter, we will use the similar PECVD oxide as cladding followed with CMP to smooth the bonding surface. Thus, we characterize and compare the oxide properties in terms of waferlevel uniformity and surface roughness before and after CMP.
Figures 1A,B show the wafer-level oxide thickness before and
after CMP. The non-uniformity is only ~1% after CMP, which
suggests a very flat surface. Figures 1C,D show the AFM results
before and after CMP. As deposited, the surface is relatively
rough, with RMS of ~2.5 nm, while after CMP, the surface roughness is reduced significantly with RMS of ~0.4 nm, which is
more suitable for wafer molecular bonding (Christiansen et al.,
2006).

III/V-on-Si bonding and hybrid lasers

The bonding process starts with the wafer cleaning, separately
for silicon wafer and III/V wafer. First, standard SPM clean for
10 min is performed to the silicon wafer in order to remove any
organic contaminants, followed with 5 min SC1 clean with megasonic to remove any particle on the surface. The III/V wafer is separately cleaned in the NH4 OH solution (NH4 OH:DI water = 1:15)
for 1 min. Second, O2 plasma activation in a RIE chamber is
performed for both silicon wafer and III/V wafer, subsequently
followed with DI water rinse. These two bonded wafers are then
physically contacted with each other immediately after drying and
placed inside to the EVG 520 bonder for pre-bonding under N2
for 3 min with 1000 N mechanic force. After that, post bonding
annealing at 300°C in vacuum is applied to the bonded pair for
12 h. Figure 2A shows the optical image of a 50 mm InP wafer
bonding to a 200 mm silicon wafer before unloading from the
bonder track.
The bonded wafers are first characterized by scanning acoustic
microscope (SAM) using Sonix HS3000. Figure 2B shows the typical CSAM image for the bonded wafer. We observe that larger than
98% of the 50 mm InP area is bonded to the silicon wafer, with
only limited voids, which are attributed to the particles remaining on bonding surface. Besides, the bonding quality in the wafer

FIGURE 1 | The oxide thickness (A) before and (B) after CMP. The AFM results of the wafer surface roughness (C) before and (D) after CMP.

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III/V-on-Si bonding and hybrid lasers

FIGURE 2 | (A) The bonded wafer before unloading from the EVG bonder. (B) The CSAM result. (C) The shear testing result. (D) The TEM results show the
high-quality bonding interface.

periphery is also not good enough, which is due to the ring-shaped
imperfection of the InP wafer.
The whole wafer is then diced into 5 mm × 5 mm dies for shear
testing by using a Die Shear Tester (Dage Series 4000). Figure 2C
shows the extracted bonding strength, with maximum bonding
strength of ~30 MPa in the wafer center region, and the averaged
value of 15 MPa. We believe such bonding strength is high enough
for any of the post optoelectronic fabrication process. Figure 2D
shows the TEM results of the bonded wafer, which indicate a
very tight bonding between InP and oxide, again suggesting a
high-quality bonding.
However, although such W2W bonding has been demonstrated
with high quality, there are still existing big challenges, including:
(1) insufficient III/V wafer utilization,
(2) insufficient silicon wafer utilization due to wafer size mismatch,
(3) III/V wafer global stress-induced bonding voids.
First of all, for practical application of optical interconnection, only very small portion of the silicon waveguide area needs
to be bonded with III/V material for optoelectronic fabrication
to form optical lasers. With whole III/V wafer bonding, most of
the III/V material will be subsequently etched away during post
optoelectronic fabrication. Giving such precious III/V wafers, the
insufficient utilization of the III/V material results in significantly

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increased device cost and waste, which in turn makes it ineffectual to use the silicon photonics though it is of low cost. Second,
the main stream silicon photonics has already adopted 200 mm
silicon wafers. However, due to the brittleness of the InP wafers,
it is very difficult to make large-sized wafers to match with silicon wafers. Although the largest available III/V epitaxial wafer can
go with 150 mm, the commercially available largest-sized III/V
epitaxial wafer is only 75 mm. Thus, such wafer size mismatching definitely results in the insufficient utilization of the silicon
device wafer, which in turn increased the cost. Furthermore, InP
wafers with multiple quantum well structures are normally with
high global stress, which induces the wafer bowing. Such stressinduced wafer bowing will easily trap the air between the bonding interfaces with remained voids, thus reducing the bonding
quality.

HIGH-THROUGHPUT MULTIPLE DIES-TO-WAFER BONDING
TECHNOLOGY
Based on the aforementioned W2W bonding method, we propose an alternative proprietary high-throughput multiple D2W
bonding method, which is based on temporarily bonding III/V
dies to a handle silicon wafer through pick-and-place process for
simultaneous batch processing. Such high-throughput multiple
D2W bonding method is the key enabling technique for potential
manufacturability of large-scale hybrid optoelectronic integrated
circuit (H-OEIC).

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III/V-on-Si bonding and hybrid lasers

FIGURE 3 | Illustration of the key processing steps of the multiple D2W bonding technology.

Figure 3 shows the key process steps of the proposed multiple
D2W bonding technology, which includes:
(a) the programmable reconfiguration of III/V dies onto a handle
wafer via pick-and-place process,
(b) the D2W bonding through the notch alignment between the
two 800 wafers, after batch processing of wafer cleaning and
plasma activation, and
(c) the dies releasing from the handle wafer and transferring to
silicon device wafer.
The most critical step here is the choice of the adhesion layer
for the temporary III/V dies bonding to the handle wafer, which
includes the following two trade-off considerations.
(1) The adhesion should be strong enough to stick the III/V dies
on the handle wafer without peeling off during the subsequent
III/V dies batch processing, including InGaAs cap layer wet
etching, pre-clean, wafer drying, and plasma activation, etc.
(2) The adhesion should not be excessively strong so that the
III/V dies can be successfully released and transferred to the
Si device wafer after pre-bonding.
The programmable reconfiguration of the dies onto the handle wafer is realized through pick-and-place process by predetermining the position coordinates of each die with considering
the wafer-level silicon device die distribution. Unlike the pick-andplace process in flip-chip bonding, which directly bonds the dies
to the actual wafer (Kostrzewa et al., 2006), the pick-and-place in
our proposed method only helps to distribute the dies onto a handle wafer without flipping the chips. Thus, all the dies attached to
the handle wafer can be simultaneously performed with different
process steps for wafer bonding, such as InGaAs cap layer etching,
wafer clean, and plasma activation.
The D2W bonding alignment accuracy is mainly determined
by the notch alignment, which is performed manually and induces
a relatively large misalignment of ±500 µm, compared to the

Frontiers in Materials | Optics and Photonics

misalignment of only ±5 µm from the programmable reconfiguration by pick-and-place process. However, as the alignment of the
III/V devices to the silicon waveguide device is determined through
photolithography during optoelectronic fabrication process after
wafer bonding, such misalignment can easily be compensated by
adopting relatively large-sized III/V dies.
Figure 4 schematically illustrates the detailed bonding process
flow starting from the preparations of silicon and III/V wafer. For
either blanket silicon wafer or patterned silicon wafer with photonic devices, the wafers are cladded with PECVD oxide, followed
with CMP process to smooth the bonding surface. As the hybrid
laser performance is largely dependent on the vertical coupling
efficiency, which is determined by the inter-layer oxide thickness,
it is of very importance to control the oxide thickness by CMP
process.
The preparation of the III/V dies includes the III/V wafer dicing
into certain sized dies, the preparation of the adhesion layer to the
handle wafer, and the programmable reconfiguration of the III/V
dies temporary bonding to the handle wafer through pick-andplace process. Typically, the mechanical wafer dicing will result in
edge roughness along the dicing lane, thus subsequently cause the
low quality bonding near the die periphery. Besides, such dicing
process may also introduce particles to the wafer surface cause
contamination. Thus, a sacrificial InGaAs cap layer in order to
protect the III/V bonding surface is designed in our demonstration. The applied mechanic force by the pick-and-place head also
needs to be well controlled in order to ensure the successful die
releasing from the handle wafer after pre-bonding. Due to the
direct contact of the pick-and-place head, the die surface could be
contaminated. However, owing to the sacrificial InGaAs layer, the
bonding surface can be well protected without contamination or
surface damage. We have checked and compared the surface condition of the III/V dies before and after the etching of the sacrificial
InGaAs layer. We observe the particles on the chip surface after the
wafer dicing and pick-and-place process, which is with relatively
high RMS of 0.198 nm. In comparison, after the etching of InGaAs
cap layer, the surface roughness is improved with reduced RMS of

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FIGURE 4 | The fabrication process flow of the multiple D2W bonding
technology. The process includes two different folds, i.e., the bonding
wafers preparation including the III/V dies adhesion to handle wafer for

III/V-on-Si bonding and hybrid lasers

batch process and the silicon device wafer fabrication, and D2W bonding
through dies releasing from handle wafer and transferring to silicon device
wafer.

FIGURE 5 | Demonstration of 104 III/V dies bonding to silicon wafer. (A) Photo image of the bonded wafer, (B) CSAM results, (C) shear testing results.

0.182 nm, which is far below the required RMS of <1 nm for wafer
direct bonding (Christiansen et al., 2006).
Prior the physical contact of the wafers for molecular bonding,
the silicon wafer is performed with standard SPM clean for 10 min
and SC1 clean with mega sonic for 5 min, while III/V die-attached
handle wafer is first performed with sacrificial InGaAs cap layer
etching in H3 PO4 solution for 1 min, followed with standard clean
in NH4 OH solution for 2 min. After that, O2 plasma activation is
applied to both silicon wafer and III/V dies in a RIE chamber for
1 min, followed with DI water rinsing and wafer drying. The III/V
dies and the silicon wafer are then physically contacted with each
other by notch alignment between two 800 wafers, followed with
pre-bonding in the 200 mm EVG bonder for 2 min with 1000 N
mechanical force applied. The III/V dies are released from the handle wafer after pre-bonding, and all III/V dies are now transferred
to the silicon device wafer. Finally, the bonded pairs are placed back
to the EVG bonder for post-bonding annealing at 300°C for 12 h.
As a proof-of-concept demonstration, we show here the bonding of 104 InP dies to an 800 silicon wafer. The silicon wafer is
covered with 1 µm PECVD oxide after CMP. The InP dies are
diced into 5 mm × 5 mm in size. Figure 5A shows the photo image

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of the bonded wafers with nearly all InP dies are successfully
bonded to the silicon wafer. The only missing piece is peeled off
during pick-and-place process. The CSAM shown in Figure 5B
suggests a successful bonding. The dark areas, which suggest less
strong bonding, come from the dies located in the InP wafer edge.
Figure 5C shows the sheer testing results. The maximum bonding
strength is larger than 20 MPa, with an averaged bonding strength
of ~13 MPa, which is comparable with that of W2W bonding
under the same process.
All in all, we believe that there are at least two significant
implications of the proposed multiple D2W bonding technology:
(1) The significantly increased bonding efficiency owing to the
simultaneous batch process. Through the batch process of the
III/V dies (pre-clean, plasma activation, etc.), it is possible to
bond unlimited number of dies. It also helps to avoid potential contamination by performing the pick-and-place before
cleaning process, and eliminates the time link constraint of
the bonding surface deactivation. This is the most significant processing advantage comparing to the conventional
pick-and-place method.

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(2) The scalability to whatever-sized silicon wafers. Such multiple D2W bonding technology can easily be adopted for even
larger-sized silicon wafers, such as 300 mm wafer. This is the
most critical step toward the potential manufacturability of
H-OEIC.

DESIGN OF III/V-on-Si LASERS
A hybrid III/V-on-silicon laser consists of a III/V epitaxial-layered
structure and a silicon waveguide. It is a device that emits laser
beams from silicon waveguides by electrical/optical injection to
the III/V region. In this section, we will discuss the design of
hybrid III/V-on-silicon lasers with regard to two fundamental
laser elements, namely, optical gain medium and optical waveguide
cavity.
DESIGN OF III/V MQW STRUCTURES

There are two main material systems for the fabrication of longwavelength lasers emitting at 1.55 µm, which are InGaAsP/InP
and InGaAlAs/InP systems. Both kinds of materials can be used
to fabricate hybrid lasers. InGaAlAs MQWs exhibit a larger conduction band discontinuity (E c = 0.72E g ), and smaller valence
band discontinuity compared with InGaAsP MQW. This leads
to an improved electron confinement, which can improve the
temperature characteristics of semiconductor laser diodes. Thus,
InGaAlAs/InP material system is more suitable for high speed and
uncooled operation of semiconductor laser diode. In this study,
we select this material system for the hybrid silicon lasers demonstration. The MQW region includes eight Al0.055 Ga0.292 In0.653 As
quantum wells separated by nine Al0.055 Ga0.292 In0.653 As barriers.
The gain spectrum of the MQW is calculated and the wavelength of
peak gain is designed at 1550 nm when the carrier injection density
increases from 5 × 1017 to 5 × 1018 /cm3 as shown in Figure 6A.
Figure 6B shows the measured photoluminescence (PL) spectrum for III/V epitaxial wafer at room temperature with the peak
wavelength at about 1550 nm.
DESIGN OF HYBRID LASER VERTICAL WAVEGUIDE STRUCTURE

As mentioned, the optical gain comes from overlying III/V stack
layer, which needs to be structured to efficiently inject electrons
or holes into the MQW regions. A high overlap between the optical mode and the MQW benefits to achieve a high optical gain,

III/V-on-Si bonding and hybrid lasers

which means that the optical mode needs to be well confined in
the III/V waveguide. However, on the other hand, the light has
to be confined sufficiently inside the silicon output waveguide
for the efficient light extraction. In view of this, there are mainly
two kinds of waveguide structures considering the optical power
distribution for the hybrid laser with optical mode predominantly
confined either in the silicon waveguide or in the III/V overlay. This
leads to two different optical cavity designs. In the first design, the
optical cavity comprises both III/V and silicon waveguides and the
mode is mainly guided within the Si waveguide and evanescently
coupled with the III/V waveguide. Such structure is also called as
overlapped structure with hybrid mode (Fang et al., 2006, 2007a,b,
2008a,b,c, 2009). It has the advantage of making the coupling to a
passive silicon waveguide straightforward and wavelength selective
features can easily be defined in the silicon waveguide layer using
CMOS fabrication techniques, which provides an accurate mechanism to control the emission wavelength of the laser. However, it
requires a controllable thin bonding layer (<50 nm) for efficient
optical coupling, which may increase the difficulty of bonding
process. Furthermore, due to the weak interaction between the
optical mode and gain material, it usually requires longer laser
cavity, and thus resulting in high power consumption. In the second design, the mode in the hybrid section is mainly guided by the
III/V waveguide, and the light is coupled from the III/V waveguide
to the silicon waveguide through waveguide mode transformer,
such as inverse tapers (Yariv and Sun, 2007; Sun et al., 2009a; Ben
Bakir et al., 2011). In such design, the bonding interface can be
relatively thick (typically from 30 to 150 nm) due to the released
coupling constrain for the bonding interface. The advantage is
that the optical mode experiences a high optical gain in the central
region of the laser structure. However, the challenge of this structure is the fabrication of low-loss tapered waveguides. Hereafter,
we name such design as adiabatic tapered coupling structure.
Silicon waveguide thickness selection

The selection of the silicon waveguide thickness depends on the
detailed device dimensions/structures and the fabrication process.
For indium phosphide (InP)-based gain waveguides, the effective
refractive index is typically larger than 3.2 if the waveguide width
and height are larger than 1 µm. In order to achieve this index for
silicon waveguides for effective coupling with the InP-based gain

FIGURE 6 | (A) The calculated gain spectrum under different carrier injection concentration. (B) The measured photoluminescence (PL) spectrum for III/V
epitaxial wafer.

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region, the corresponding silicon waveguide thickness needs to be
sufficiently large. Figure 7 shows the calculated effective refractive
index of the fundamental mode in silicon waveguide depending
on the waveguide thickness. It indicates that the required silicon
thickness needs to be larger than 450 nm to achieve an effective
index of 3.2 for the waveguide with 2 µm in width. Such thick
silicon layer does not match with the current mainstream silicon
photonics. However, on the other hand, it is still possible to couple
light from 220 nm silicon to InP waveguides by using very narrow
InP waveguides (~200 nm) to push down the value of effective
index, although the fabrication is difficult to form these narrow
InP waveguides by conventional photolithography.
Overlapped structure with hybrid mode

As mentioned above, there is a tradeoff between the optical mode
confinement in the III/V and silicon regions for the overlapped

FIGURE 7 | Effective refractive index of the silicon waveguide
fundamental mode as the function of silicon waveguide thicknesses.
Both top and bottom claddings are oxide (n = 1.45) and silicon index is
chosen as 3.48 at the wavelength of 1550 nm.

III/V-on-Si bonding and hybrid lasers

structure. The bonded III/V-Si structure forms the hybrid waveguide cavity. The effective refractive index of III/V active and Si
regions are critical parameters for the hybrid waveguides, which,
respectively, determine the light confinement factors in III/V and
Si region. In our design, the confinement factors over the silicon
and the quantum well regions are modified by altering the silicon
waveguide thickness and the separate confinement heterostructure (SCH) thickness in order to ensure sufficiently low-threshold
gain for lasing. While the thicker silicon waveguide pulls the optical mode into the silicon layer, the larger SCH thickness can drag
back the optical mode into the III/V region.
Figure 8A shows the calculated optical confinement factors in
MQW and Si depending on the Si waveguide width under different Si thickness, with assuming the III/V ridge width and the SCH
thickness of 6 µm and 250 nm, respectively. It shows that when the
waveguide widths of III/V and Si are fixed, the optical confinement
in Si waveguide can be increased by using a thick Si layer. With the
silicon waveguide thickness of 700 nm, large confinement of up
to 70% in silicon waveguide is achieved. However, the device performance is very sensitive to the bonding interface quality due to
the overlapping of the optical mode with the bonding interface
between the III/V and the silicon. Based on the analysis, we adopt
silicon thickness of 500 nm for the demonstration of hybrid Si
lasers.
Figure 8B shows the simulated optical confinement factor in
MQW and in silicon waveguide with different SCH thickness, with
the fixed III/V and Si waveguide widths of 4 and 2 µm, and Si
waveguide thickness of 500 nm. As the SCH thickness increases,
the optical mode confinement in III/V region increase, which in
turn significantly decreases the optical mode confinement in silicon waveguide. Inserts in Figure 8B show the simulated field
distributions with SCH thicknesses of 0.1 and 0.5 µm. It shows
obviously that for the small SCH thicknesses, the optical mode lies
primarily in the silicon region, while the optical mode is dragged
into III/V region with increased SCH thickness. The ability to control the optical mode with the SCH thickness is a key feature of this
platform. For hybrid lasers, higher optical confinement is needed
to achieve lower threshold current. Thus, we choose an optimized
SCH thickness of 0.18 µm for the hybrid lasers.

FIGURE 8 | Confinement factor of optical mode in multiple quantum wells (MQW) and Si waveguide as a function of (A) the Si waveguide width
under different Si waveguide thicknesses, and (B) the SCH thickness. Insets: the simulated field distributions of the fundamental TE mode with different
SCH thicknesses. It shows that by increasing the thickness of SCH layer up to 500 nm, the optical mode is more confined in the III/V active layer.

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For such hybrid III/V-on-silicon lasers, another challenge arises
from the control of the bonding layer thickness. Generally, a thin
bonding layer (<50 nm) is needed for efficient optical coupling
between III/V and silicon regions, while the thicker bonding layer
benefits to the bonding quality of III/V layer and the bonding
yield improvement. For direct bonding without oxide interlayer,
it is easy to achieve such thin thickness, which is usually only the
native oxide. However, this process is particularly sensitive to surface roughness and particles contamination, which would limit
the bonding quality and bonding yield. DVS-BCB bonding can be
used for the heterogeneous integration of III/V material on silicon
to improve the yield. However, it is difficult to obtain a controllable thin bonding interlayer of <50 nm. In our case, we choose
silicon oxide as interlayer between III/V and silicon, which is also
compatible with the mainstream silicon phonics, in which all the
devices are with oxide cladding.
Figure 9A shows the calculated optical confinement factor in
MQW and silicon waveguide as the function of interlayer oxide
thickness. In the simulation, we assume the fixed silicon thickness
of 500 nm, the silicon waveguide width of 3 µm, and III/V ridge
waveguide with of 6 µm. We observe from the results that the Si
confinement factor largely decreases when the interlayer thickness
increases from 10 to 100 nm. Only approximately 5% optical light
is confined in the silicon waveguide when the interlayer thickness
is 100 nm.
Additionally, the interlayer of oxide at the bonding interface
also affects the characteristics temperature of hybrid III/V-onsilicon laser due to the poor thermal conductivity of as low as

FIGURE 9 | (A) The confinement factor of the optical mode in the MQW
and silicon layers, respectively, as a function of the interlayer thickness.
(B) The simulation structure of the thermal distribution. (C) The temperature

Frontiers in Materials | Optics and Photonics

III/V-on-Si bonding and hybrid lasers

1.3 W/m/K. The modal gain of laser is dependent on the temperature of the active region. As the temperature of active region
increases, the modal gain decreases due to the increased carrier
leakage out or not reaching the active region, and/or increased
non-radiative recombination. The decrease of modal gain leads to
high threshold current and low output optical power. In order
to investigate the effect of interlayer on the thermal characteristics of the hybrid lasers, a two-dimensional model of the
device structure is conducted using COMSOL by mapping out
the heat dissipation of each layer. Figure 9B shows the simulation structure. In the simulation, the structure parameters are
as follows: III/V ridge width, Si ridge width and thickness, and
the laser cavity length are assumed to be 6 µm, 1 µm, 500 nm,
and 1000 µm, respectively. Injection current is 500 mA and the
corresponding voltage is 4 V. Figure 9C shows the calculated
working temperatures in the III/V active region with different
thicknesses of the oxide interlayer. The increase of temperature
in the III/V active region versus interlayer thickness is about
0.02 K/nm. For illustration purpose, Figure 9D shows as an
example the thermal distribution within the layered structure
for the oxide interlayer thickness of 0 nm. The thermal distributions for the other thicknesses are similar. Actually, we can
conclude from the study that the main hurdle for the heat dissipation is the SOI BOX layer, which can be seen from the results
without oxide bonding interlayer (thickness = 0 nm). Thus, for
enhanced thermal management, novel designs such as thermal
shunt (Liang et al., 2012) are required to effectively remove the
generated heat.

changes in the III/V active region with regard to the oxide interlayer
thickness. (D) The simulated thermal distribution with the interlayer
thickness is 0 nm.

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Adiabatic tapered coupling structure

For the adiabatic tapered coupling structure, the mode in the
hybrid section is mainly guided by the III/V waveguide, and the
light is coupled from the III/V waveguide to the silicon waveguide through a tapered waveguide. It shows that a tapering length
~100 µm is required for a sufficient light coupling with minimized optical loss. By using such tapered coupling, it eliminates the
tricky tradeoff between the modal gain and vertical coupling efficiency, which is inherent in the overlapped waveguide structures.
Therefore, hybrid lasers with a short cavity as pure III/V laser are
possible. Up to now, the hybrid lasers with the high performances
are achieved using such tapered coupling scheme (Levaufre et al.,
2014; Zhang et al., 2014).
In order to efficiently couple the light between the Si-III/V
hybrid waveguide and the silicon waveguide, the III/V waveguide
and silicon waveguides are tapered simultaneously in the same

FIGURE 10 | (A) Schematics of the tapering structure for the vertical
coupling between III/V waveguide and silicon waveguide. WInP : III/V
waveguide width, Wend : III/V waveguide taper width, Wd : silicon
waveguide width, LTaper1 : silicon waveguide taper length, LTaper2 : III/V

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III/V-on-Si bonding and hybrid lasers

direction. Here, we adopt a three-dimensional approximated
model based on beam propagation method (BPM) in order to
optimize the tapering structure of the silicon waveguide and III/V
waveguide for an efficient coupling. Figure 10A schematically
illustrates the design of such waveguide tapering structure. The
coupling efficiency largely depends on the tapering design, especially the III/V waveguide taper width and taper length. Here, we
simulate such dependency by varying the taper width and taper
length, while fixing the III/V waveguide width of 5 µm, the silicon
waveguide width of 1 µm, and the silicon taper length of 100 µm.
Figures 10B,C, respectively, show the simulated coupling efficiency from III/V-Si hybrid waveguide to silicon waveguide as
functions of the III/V waveguide taper width and tape length.
It suggests that the coupling efficiency from III/V-Si hybrid waveguide to Si waveguide can be as high as 85% by using an 80-µmlong III/V waveguide taper and a 100-µm-long silicon waveguide

waveguide taper length. The whole structure is cladded with oxide.
Coupling efficiency of III/V waveguide to Si waveguide as functions of
(B) the III/V waveguide taper width, and (C) the III/V waveguide taper
length.

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tape. Through optimizing the III/V waveguide taper width and
III/V waveguide taper length, the maximum coupling efficiency
can be as high as 99%. However, due to the optoelectronic fabrication limitation, we are not able to demonstrate the hybrid laser
using such adiabatic tapered coupling structure.

DEMONSTRATION OF III/V-on-Si HYBRID LASERS
Using the proposed multiple D2W bonding technology, we have
demonstrated various hybrid silicon lasers, including FP lasers,
DBR lasers, sidewall-grating lasers, racetrack-shaped microring
lasers, and MLL. In this section, we will first introduce the hybrid
silicon laser fabrication process, leveraging on IME’s CMOScompatible silicon photonic fabrication facilities and NTU’s
expertise in optoelectronics fabrication capability, followed with
showing some hybrid silicon laser demonstrations as the examples.
III/V-on-Si HYBRID LASER FABRICATION PROCESS

The III/V-on-Si hybrid laser fabrication in our demonstrations
includes two parts, namely silicon passive device fabrication using
IME’s CMOS line and multiple D2W bonding in IME’s MEMS
line, and III/V optoelectronics fabrication in NTU. Figure 11
shows the fabrication process flow. We adopt commercially available SOI wafer with 340 nm silicon layer sitting on a 2 µm buried
oxide (BOX) layer. The fabrication starts with the blanket silicon
epitaxy to ~500 nm for refractive index matching between silicon
waveguide device and InP gain medium. After the deposition of
70 nm oxide as the hard mask, the waveguide structures, including
both grating coupler and inverse taper coupler are patterned by
deep UV photolithography and transferred onto the silicon layer
by using deep RIE etching. For the grating coupler, the silicon
etching thickness is 377 nm. While for other waveguide devices,
second silicon etch is applied down to the BOX layer by covering
the surface grating coupler area with additional photo resist. Oxide
cladding of 650 nm in thickness is deposited followed by a surface

III/V-on-Si bonding and hybrid lasers

planarization step, which includes oxide etch-back with 500 nm in
depth and CMP process. Such planarization steps with CMP also
help to smooth the bonding surface with very small surface roughness for molecular bonding. The interlayer oxide thickness can be
well controlled by the CMP process, with only ~50 nm oxide left
atop the silicon waveguide in our demonstration. For enhanced
flatness and uniformity of the bonding surface, we only etch away
the silicon surrounding the devices, remaining most of the silicon
areas forming silicon plateaus.
The multiple D2W bonding is then performed after the preparation of III/V dies, followed with the process described in Section
“III/V-to-Si Wafer-to-Wafer (W2W) Bonding Technology.” As the
designed devices are all within an area of 8 mm × 8 mm, the InP
dies are all diced with 9 mm × 9 mm with the consideration of
bonding misalignment of ±500 µm for the notch alignment, thus
ensuring the full covering of all the silicon photonic devices within
the III/V die area. For a 50 mm InP wafer, there are only 16 full
dies with 9 mm × 9 mm in size. Thus, for the purpose of hybrid
silicon laser demonstration using such D2W bonding technology, we only perform 16-dies bonding to 200 mm silicon wafers,
with some of the silicon photonics device area being wasted.
Figure 12A shows the photo image of the 16-dies bonded silicon
wafer. Figure 12B shows the CSAM results. Except some particleinduced bonding defects, all the dies are bonded very well. However, we clearly observe that some of the die edge periphery regions
are not bonded well due to the wafer dicing induced damage.
Figures 12C,D, respectively, show the TEM and cross-SEM of the
bonding structures, both suggesting very reliable bonding quality.
The III/V optoelectronic fabrication starts from the InP substrate removal using HCl solution. After photolithography, the
InP mesa structures are formed by using H3 PO4 , H2 O2 , and
HCl mixed solution to etch the InGaAs contact layer and p-InP
cladding layer. The SCH layer and QW layer are also etched using
H3 PO4 :H2 O2 :H2 O solution, stopping on the n-InP cladding layer.

FIGURE 11 | The fabrication process of the hybrid silicon laser in a ridge waveguide structure.

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Then, an SiO2 insulator layer with the thickness of 300 nm is
deposited, followed with the contact opening for p-type and n-type
injection by one-time photolithography and the oxide is etched by
HF solution. After that, Ti/Au metal contacts are formed by sputtering, with wet etching to form a Ti/Au slot between the n-type
and p-type contacts using diluted HF and KI solution, respectively.
FABRY–PEROT LASERS

The CW operation of the optical laser requires good thermal
management to remove the generated heat. In the case of FP
lasers, another way is to design narrow ridge waveguide to generate less heat. In our demonstration, we design a FP laser with
ridge waveguide width of 6 µm. The laser facets are formed by
lapping down the Si substrate to around 60 µm, followed with
mechanical cleaving. The length of the FP laser is ~720 µm. The
demonstrated FP laser is able to work at room temperature with
CW operation. Figure 13A shows the measured P–I curves under
different temperatures. The threshold current at 264 K is ~45 mA,

FIGURE 12 | (A) The optical image of the 16-dies bonded silicon wafer.
(B) CSAM results. (C) TEM of the bonding structure. (D) Cross-SEM of the
bonding structures.

III/V-on-Si bonding and hybrid lasers

and increases significantly to ~100 mA at room temperature. We
attribute such fast increase of the threshold to the thick oxide
interlayer, which prohibits the heat dissipation efficiently. The
measured output power from a single facet without any reflection coating is more than 1 mW. This includes the coupling loss
due to the un-optimized testing setup for light collection, which is
estimated only with ~20% light collection efficiency. The thermal
dissipation is very important for CW lasing. Figure 13B shows
the measured lasing spectra under different temperatures. The
wavelength shift with temperature is about 0.75 nm/K.
LATERAL-COUPLED DISTRIBUTED-FEEDBACK LASERS WITH SIDE WALL
GRATING

The FP laser is fabricated by lapping down the silicon substrate and
mechanical cleaving to form the facets. From the optical communication and optical interconnect applications point of view, such
FP lasers are not practical for photonic integration. Furthermore,
how to achieve good facet is still a main challenge and a key limiting factor for high-performance hybrid lasers as reflection coating
is always required in order to optimize the cavity transmission
and reflection. In view of this, optical resonators, Bragg grating
structures that form the cavities through fabrication are the good
candidates for on-chip hybrid laser. We here show as an example
of a hybrid laser using LC-DFB structure as the laser cavity.
Figure 14A schematically shows the perspective view of the LCDFB hybrid laser with illustration of the key parameters, including
the Bragg grating period Λ, the silicon ridge width D, and the grating teeth width, W 1 . Considering the fabrication limitation, we
design third-order later Bragg grating in order to achieve singlemode operation. With regard to the silicon thickness of 500 nm,
the grating period Λ is 670 nm with filling factor of 0.5. The ridge
width D and the teeth width W1 are, respectively, designed with
2 and 1 µm. The Bragg grating is centered beneath the III/V gain
region, which is with the width of 12 µm. Both LC-DFB structure and III/V gain region are designed with the same lengths. In
order to extract the output laser light for easy characterization, the
vertical grating couplers are adopted. For the vertical grating coupler, the period is designed to be 640 nm with filling factor of 0.5,
and the silicon etch depth is 377 nm. Such grating coupler design
is purely based on theoretical calculation, without any process

FIGURE 13 | (A) The power–current characteristic curves and (B) the lasing spectra of a typical hybrid silicon FP laser measured under different temperature.

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Luo et al.

FIGURE 14 | (A) The perspective view of the hybrid LC-DFB laser
integrating with surface grating coupler, with illustration of the key
design parameters. Λ: grating period, D: the ridge width, and W1 : the
grating teeth width. (B) Top-view optical microscope image of a LC-DFB
hybrid laser integrated with vertical surface coupler. (C–E) The SEM of

verification and optimization. For this demonstration, we did not
design any mode transformer between III/V layer and silicon waveguide layer, thus expecting some transition loss. Figure 14B shows
the optical microscope image of the fabricated hybrid LC-DFB
laser with integrated vertical grating couplers. The LC-DFB structure and the III/V gain region are designed with same length of
700 µm, while the silicon device including two grating couplers is
~2750 µm. Due to the optoelectronic fabrication limitation, there
is no designed taper between III/V waveguide and silicon waveguide, thus expecting relatively high transition loss. Figures 14C–E
show the SEM images of the fabricated LC-DFB and vertical grating coupler, while Figure 14F shows the cross-sectional SEM of the
vertical structures, illustrating the Si waveguide, the III/V layer, and
the Ti/Au layer.
Figures 14G,H show the measured P–I curve and the spectrum of the LC-DFB hybrid laser under pulse operation. The
threshold current is ~120 mA, corresponding to a threshold current density of ~1.42 kA/cm2 . From the spectrum, we see clearly
single-mode operation with the peak wavelength at 1559.8 nm and
a side-mode-suppression ratio (SMSR) larger than 20 dB. This is
expected from the LC-DFB design. However, the maximum output

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III/V-on-Si bonding and hybrid lasers

the LC-DFB structures and the grating coupler. (F) The cross SEM of
the vertical layered structure. (G) The light power output and (H) the
laser spectrum of the hybrid silicon laser with sidewall Bragg grating
structure. The output power is directly measured from the surface
grating coupler.

power is only ~10 µW upon 250 mA current injection, which also
can be observed from the spectrum measurement. We attribute the
relatively low output power to the following two reasons, namely,
the accumulated optical loss, and the inefficient vertical light coupling. First of all, the optical loss, which mainly includes the surface
grating coupler coupling loss, the Bragg grating scattering loss, and
the non-radiative recombination loss from the bonding interface,
affects the light output significantly. From the reference measurement for the device only with passive silicon waveguide yet bonded
III/V layer, the accumulated total loss is >40 dB, which is mainly
due to the unoptimized surface grating coupler. Second, the oxide
interlayer in our design, which might not be able to control precisely, will affect the light coupling efficiently from III/V layer to
silicon waveguide. Furthermore, the polarization sensitivity of the
surface grating coupler can also induce additional optical loss.
Thus, the optimized grating coupler design for the light extraction
from the silicon waveguide and the vertical light transition structure for light coupling from III/V layer to silicon waveguide can
significantly increase the laser output power. Besides, by optimization of the Bragg grating period and silicon waveguide thickness,
the SMSR can also be enhanced.

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PASSIVELY MODE-LOCKED LASERS

Semiconductor MLLs are excellent candidates for generating stable
ultra-short optical pulses, which have a corresponding wide optical spectrum of phase correlated modes and high repetition rate.
Optical frequency combs emitted by MLLs can have high extinction ratios, low jitter, and low chirp, which can be utilized in a variety of applications including AWG, optical clock generation and
recovery, coherent communications systems, high-speed analogto-digital conversion (ADC), and optical time-division multiplexing (OTDM), etc. Integration of MLLs on silicon is very promising
as it combines the low-loss and low-dispersion characteristics
of silicon material with high gain III/V material, thus ensuring
improved performance. Furthermore, it will be possible for semiconductor MLLs to generate ultra-short optical pulses with low
repetition rate on the silicon platform owing to the long cavity
length. Here, we show our preliminary demonstration of a passive
MLL using the developed heterogeneous integration platform.
The optical cavity of the MLL is defined by a 1250-µmlong gain section, a saturable absorber (SA) with the length of
30-100µm, and cleaved facet at the waveguide end. The gain
section and SA are separated by a 20 µm electrical isolation region
with isolation resistance >1 kΩ. The SA is made up of the same
active material as the gain section. The difference between the SA
and gain section is that SA absorbs the light in the cavity upon
applying a reverse bias, while the gain section amplifying light
upon forward current injections.
The laser optical output is collected by a photodiode located
in front of the cleaved facet. The typical threshold current with
an unbiased 50-µm-long SA section is 88 mA. The device has a
maximum single facet CW output power of 1 mW at room temperature when the injection current is 140 mA. The series resistance
is about 8.5 Ω, while the slope efficiency is about 0.02 mW/mA.
Figure 15A shows the measured optical spectra at different injection currents. It shows that the widest optical emission is centered
at about 1605 nm with a full-width at half-maximum (FWHM) of
5.4 nm at the injection current of 110 mA measured by an optical
spectrum analyzer (OSA). Assuming the generated optical pulse is
chirp-free and the shape of the pulse is with a Sech-function, the
width of the optical pulse is calculated to 0.5 ps.

III/V-on-Si bonding and hybrid lasers

Passive mode locking of the device is obtained by forward biasing the gain section (I gain ) and reverse biasing (V sa ) or un-biasing
the SA section. The mode locking behavior of the device is characterized by measuring the radio frequency (RF) spectrum using the
spectrum analyzer (Agilent E4448A). Figure 15B shows the measured RF spectrum of the III/V-on-Si MLL at the injection current
to the gain section (I gain ) of 110 mA and reverse biasing the SA
section at −0.9 V. The resolution bandwidth (RBW) during measurement is 1 MHz. The repetition frequency is about 30.0 GHz
with signal-to-noise ratio above 30 dB. By changing I gain , it can
be tuned to more than 30 GHz, giving clear evidence of passively
mode locking of light signal. The measured RF linewidth of the
injection locked laser is about 7 MHz by Lorentzian fitting the RF
spectrum.

SUMMARY AND FUTURE OUTLOOK
KEY ACHIEVEMENTS

In summary, we reviewed in this paper the recent demonstrations of optical light source in silicon for the application of HOEIC, with major focus on hybrid silicon lasers through wafer
bonding technology. Furthermore, we proposed a proprietary
high-throughput multiple dies-to-wafer (D2W) bonding technology by temporarily bonding III/V dies to a handle silicon
wafer through pick-and-place process for subsequent simultaneous batch processing. Such high-throughput multiple D2W
bonding technology features the merits of high bonding yield
with unlimited III/V dies and scalability to whatever-size silicon wafers, thus is the key enabling technique toward potential
manufacturability of large-scale H-OEIC. As proof-of-concept
demonstration, we showed the III/V dies to silicon wafer bonding with up to 104 dies. Repeatable demonstrations of 16-III/V
dies bonding to pre-patterned 200 mm silicon wafers are performed for the fabrication of hybrid silicon lasers with various
laser cavities, including FP lasers, LC-DFB laser with side wall
grating, and MLL.
CHALLENGES AND FUTURE OUTLOOK

However, there are still many key issues need to be addressed before
the hybrid silicon laser applied to optical interconnects system.

FIGURE 15 | (A) The measured optical spectra of the III/V-on-silicon mode-locked laser at different injection currents to the gain section while the SA section is
floating. (B) Measured RF spectrum of the III/V-on-Si MLL at 110 mA injection current and saturation voltage of −0.9V. The RBW during measurement is 1 MHz.

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Luo et al.

Here, we will only discuss three of the most important issues,
including:
(1) the thermal management;
(2) the integration with other silicon photonic devices with full
wafer processing capability; and
(3) the new platform beyond silicon for high-performance
advanced hybrid lasers.
Thermal management is one of the major obstacles for achieving high-performance hybrid silicon lasers for practical applications (Sysak et al., 2011). Due to the poor thermal conductivity of
the SOI BOX layer as well as the inter oxide layer between bonding
surface, such layers would prevent the efficient heat dissipation to
the silicon substrate, thus resulting in the poor laser performances,
such as lower laser power. One of the simple ways to increase the
thermal dissipation efficiency is to design the contact electrodes
with thick and large area metal, serving as the top surface heat
sink. Another efficient way for thermal dissipation is to remove the
BOX layer in some areas and refill it with high thermal conductive
materials such as polycrystalline silicon or metals, serving as thermal shunt (Liang et al., 2012). However, although such approach
has been demonstrated with enhanced thermal management and
increased laser performance, it still requires further development
in order to further improve the performance.
The integration of such hybrid laser with the existing silicon
photonics building blocks is another key issue before it is applied
for H-OEIC. For most of the demonstrated hybrid silicon lasers,
the silicon waveguide is normally with more than 500 nm thickness in order to ensure the optical index matching with III/V
material for efficient light coupling to silicon waveguide. Such
thick waveguide design is actually not compatible with the mainstream silicon photonics, with most of the key building blocks
are demonstrated in 220 nm silicon wafers (Xu et al., 2014). Thus,
novel designs taking care of both of these design considerations
are required. Recently, Dong et al. (2014a,b) demonstrated novel
integration scheme with associated transition structure via epitaxial growth of silicon in a pre-defined trench. Such epitaxial-grown
silicon mesa also serves as the bonding interface with III/V gain
material. Thus, the rest of the device area leaves with 220 nm silicon
for other existing silicon photonic devices. Such novel demonstration sets a path toward the integration of hybrid silicon laser with
existing silicon photonics building blocks. However, for practical
integration with high-speed modulator and photodetector, which
involves even complicated integration process with multiple oxide
etch-back and CMP, it is still very challenging on how to ensure
the flatness and smoothness of the bonding surface. More sophisticated design and further demonstration with integration of such
are highly demanded.
The third issue is associated with current new demonstration
trend that utilizes extremely low-loss SiN or SiON waveguide as
the passive waveguide layer (Bovington et al., 2014; Luo et al.,
2014). As we know, for some advanced type of hybrid lasers, such
as the MLLs, extremely low optical loss is required for achieving
high performances. The state-of-the-art demonstration of the silicon waveguide is still with propagation loss of ~2 dB/cm, which
is higher comparing that of SiN waveguide of 0.1 dB/m (Bauters

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III/V-on-Si bonding and hybrid lasers

et al., 2011). Thus, III/V-SiN platform for hybrid lasers is another
interest research area, which can address the loss issue. The integration between SiN waveguide and other SOI-based devices is
also CMOS-compatible and ready for further application (Huang
et al., 2014).

ACKNOWLEDGMENTS
This work was supported by A*STAR SERC Future Data Center Technologies Thematic Strategic Research Programme under
Grant No. 112 280 4038, and A*STAR – MINDEF Science
and Technology Joint Funding Programme under Grant No.122
331 0076.

REFERENCES
Alduino, A., and Paniccia, M. (2007). Interconnects: wiring electronics with light.
Nat. Photonics 1, 153–155. doi:10.1038/nphoton.2007.17
Alexe, M., and Gösele, U. (2004). Wafer Bonding: Applications and Technology. Berlin:
Springer-Verlag.
Bauters, J. F., Heck, M. J., John, D. D., Barton, J. S., Blumenthal, D. J., Bowers, J. E.,
et al. (2011). “Ultra-low-loss (< 0.1 dB/m) planar silica waveguide technology,”
in Paper Presented at the IEEE Photonics Conference (Arlington, VA: Marriott
Crystal Gateway).
Beausoleil, R. G., Kuekes, P. J., Snider, G. S., Wang, S.-Y., and Williams, R. S.
(2008). Nanoelectronic and nanophotonic interconnect. Proc. IEEE 96, 230–247.
doi:10.1109/JPROC.2007.911057
Ben Bakir, B., Descos, A., Olivier, N., Bordel, D., Grosse, P., Augendre, E., et al. (2011).
Electrically driven hybrid Si/III-V Fabry-Pérot lasers based on adiabatic mode
transformers. Opt. Express 19, 10317–10325. doi:10.1364/OE.19.010317
Bogaerts, W., Baets, R., Dumon, P., Wiaux, V., Beckx, S., Taillaert, D., et al. (2005).
Nanophotonic waveguides in silicon-on-insulator fabricated with CMOS technology. J. Lightwave Technol. 23, 401–412. doi:10.1109/JLT.2004.834471
Bovington, J., Heck, M., and Bowers, J. (2014). Heterogeneous lasers and coupling
to Si 3 N 4 near 1060 nm. Opt. Lett. 39, 6017–6020. doi:10.1364/OL.39.006017
Boyraz, O., and Jalali, B. (2004). Demonstration of a silicon Raman laser. Opt. Express
12, 5269–5273. doi:10.1364/OPEX.12.005269
Boyraz, O., and Jalali, B. (2005). Demonstration of directly modulated silicon Raman
laser. Opt. Express 13, 796–800. doi:10.1364/OPEX.13.000796
Camacho-Aguilera, R. E., Cai, Y., Patel, N., Bessette, J. T., Romagnoli, M., Kimerling,
L. C., et al. (2012). An electrically pumped germanium laser. Opt. Express 20,
11316–11320. doi:10.1364/OE.20.011316
Cheng, S.-L., Lu, J., Shambat, G., Yu, H.-Y., Saraswat, K., Vuckovic, J., et al. (2009).
Room temperature 1.6 µm electroluminescence from Ge light emitting diode
on Si substrate. Opt. Express 17, 10019–10024. doi:10.1364/OE.17.010019
Christiansen, S. H., Singh, R., and Gosele, U. (2006). Wafer direct bonding: from
advanced substrate engineering to future applications in micro/nanoelectronics.
Proc. IEEE 94, 2060–2106. doi:10.1109/JPROC.2006.886026
Chu, T., Fujioka, N., and Ishizaka, M. (2009). Compact, lower-power-consumption
wavelength tunable laser fabricated with silicon photonic-wire waveguide microring resonators. Opt. Express 17, 14063–14068. doi:10.1364/OE.17.014063
Creazzo, T., Marchena, E., Krasulick, S. B., Yu, P. K., Van Orden, D., Spann, J. Y.,
et al. (2013). Integrated tunable CMOS laser. Opt. Express 21, 28048–28053.
doi:10.1364/OE.21.028048
Dong, P., Hu, T.-C., Liow, T.-Y., Chen,Y.-K., Xie, C., Luo, X., et al. (2014a). Novel integration technique for silicon/III-V hybrid laser. Opt. Express 22, 26854–26861.
doi:10.1364/OE.22.026854
Dong, P., Liu, X., Chandrasekhar, S., Buhl, L. L., Aroca, R., and Chen, Y.-K. (2014b).
Monolithic silicon photonic integrated circuits for compact 100 Gb/s coherent
optical receivers and transmitters. IEEE J. Sel. Top. Quantum Electron. 20, 1–8.
doi:10.1109/JSTQE.2013.2295181
Dong, P., Hu, T.-C., Zhang, L., Dinu, M., Kopf, R., Tate, A., et al. (2013). 1.9 µm
hybrid silicon/iii-v semiconductor laser. Electron. Lett. 49:664. doi:10.1049/el.
2013.0674
Dong, P., Liao, S., Feng, D., Liang, H., Zheng, D., Shafiiha, R., et al. (2009). Low V
pp, ultralow-energy, compact, high-speed silicon electro-optic modulator. Opt.
Express 17, 22484–22490. doi:10.1364/OE.17.022484

April 2015 | Volume 2 | Article 28 | 62

Luo et al.

Dumon, P., Bogaerts, W., Wiaux, V., Wouters, J., Beckx, S., Van Campenhout, J., et al.
(2004). Low-loss SOI photonic wires and ring resonators fabricated with deep
UV lithography. IEEE Photonics Technol. Lett. 16, 1328–1330. doi:10.1109/LPT.
2004.826025
Fang, A. W., Jones, R., Park, H., Cohen, O., Raday, O., Paniccia, M. J., et al. (2007a).
Integrated AlGaInAs-silicon evanescent racetrack laser and photodetector. Opt.
Express 15, 2315–2322. doi:10.1364/OE.15.002315
Fang, A. W., Park, H., Kuo, Y.-H., Jones, R., Cohen, O., Liang, D., et al. (2007b).
Hybrid silicon evanescent devices. Mater. Today 10, 28–35. doi:10.1016/S13697021(07)70177-3
Fang, A. W., Koch, B. R., Gan, K.-G., Park, H., Jones, R., Cohen, O., et al. (2008a).
A racetrack mode-locked silicon evanescent laser. Opt. Express 16, 1393–1398.
doi:10.1364/OE.16.001393
Fang, A. W., Koch, B. R., Jones, R., Lively, E., Liang, D., Kuo, Y.-H., et al. (2008b). A
distributed Bragg reflector silicon evanescent laser. IEEE Photonics Technol. Lett
20, 1667–1669. doi:10.1109/LPT.2008.2003382
Fang, A. W., Lively, E., Kuo, Y.-H., Liang, D., and Bowers, J. E. (2008c). A distributed
feedback silicon evanescent laser. Opt. Express 16, 4413–4419. doi:10.1364/OE.
16.001393
Fang, A. W., Park, H., Cohen, O., Jones, R., Paniccia, M. J., and Bowers, J. E. (2006).
Electrically pumped hybrid AlGaInAs-silicon evanescent laser. Opt. Express 14,
9203–9210. doi:10.1364/OE.14.009203
Fang, A. W., Sysak, M. N., Koch, B. R., Jones, R., Lively, E., Kuo, Y.-H., et al. (2009).
Single-wavelength silicon evanescent lasers. IEEE J. Sel. Top. Quantum Electron.
15, 535. doi:10.1364/OE.22.005448
Fang, Q., Song, J., Luo, X., Jia, L., Yu, M., Lo, G., et al. (2012). High efficiency
ring-resonator filter with NiSi heater. IEEE Photonics Technol. Lett. 24, 350–352.
doi:10.1109/LPT.2011.2177816
Fedeli, J., Di Cioccio, L., Marris-Morini, D., Vivien, L., Orobtchouk, R., Rojo-Romeo,
P., et al. (2008). Development of silicon photonics devices using microelectronic tools for the integration on top of a CMOS wafer. Adv. Opt. Tech. 2008.
doi:10.1155/2008/412518
Feng, S., Lei, T., Chen, H., Cai, H., Luo, X., and Poon, A. W. (2012). Silicon photonics: from a microresonator perspective. Laser Photonics Rev. 6, 145–177.
doi:10.1002/lpor.201100020
Fujioka, N., Chu, T., and Ishizaka, M. (2010). Compact and low power consumption hybrid integrated wavelength tunable laser module using silicon waveguide
resonators. J. Lightwave Technol. 28, 3115–3120. doi:10.1109/JLT.2010.2073445
Goodman, J. W., Leonberger, F. J., Kung, S.-Y., and Athale, R. A. (1984). Optical
interconnections for VLSI systems. Proc. IEEE 72, 850–866. doi:10.1109/PROC.
1984.12943
Grenouillet, L., Dupont, T., Philippe, P., Harduin, J., Olivier, N., Bordel, D., et al.
(2012). Hybrid integration for silicon photonics applications. Opt. Quant. Electron. 44, 527–534. doi:10.1038/nnano.2014.215
Guha, B., Kyotoku, B. B., and Lipson, M. (2010). CMOS-compatible athermal silicon
microring resonators. Opt. Express 18, 3487–3493. doi:10.1364/OE.18.003487
Guillot, G., and Pavesi, L. (2006). Optical Interconnects: The Silicon Approach. Berlin:
Springer-Verlag.
Han, H.-S., Seo, S.-Y., and Shin, J. H. (2001). Optical gain at 1.54 µm in erbiumdoped silicon nanocluster sensitized waveguide. Appl. Phys. Lett. 79, 4568–4570.
doi:10.1063/1.1419035
Hattori, H. T., Seassal, C., Touraille, E., Rojo-Romeo, P., Letartre, X., Hollinger,
G., et al. (2006). Heterogeneous integration of microdisk lasers on silicon strip
waveguides for optical interconnects. IEEE Photonics Technol. Lett. 18, 223–225.
doi:10.1109/LPT.2005.861542
Hong, T., Ran, G.-Z., Chen, T., Pan, J.-Q., Chen, W.-X., Wang, Y., et al. (2010). A
selective-area metal bonding InGaAsP–Si laser. IEEE Photonics Technol. Lett. 22,
1141–1143. doi:10.1364/OE.22.005448
Huang, Y., Song, J., Luo, X., Liow, T.-Y., and Lo, G.-Q. (2014). CMOS compatible monolithic multi-layer Si3N4-on-SOI platform for low-loss high performance silicon photonics dense integration. Opt. Express 22, 21859–21865.
doi:10.1364/OE.22.021859
Jalali, B., and Fathpour, S. (2006). Silicon photonics. J. Lightwave Technol. 24,
4600–4615. doi:10.1109/JLT.2006.885782
Justice, J., Bower, C., Meitl, M., Mooney, M. B., Gubbins, M. A., and Corbett, B.
(2012). Wafer-scale integration of group III-V lasers on silicon using transfer
printing of epitaxial layers. Nat. Photonics 6, 610–614. doi:10.1038/nphoton.
2012.204

www.frontiersin.org

III/V-on-Si bonding and hybrid lasers

Keyvaninia, S., Muneeb, M., Stanković, S., Roelkens, G., Van Thourhout, D., and
Fedeli, J. (2012). “Multiple die-to-wafer adhesive bonding for heterogeneous
integration,” in Paper Presented at the 16th European Conference on Integrated
Optics (ECIO-2012). Sitges, Barcelona, Spain.
Keyvaninia, S., Roelkens, G., Van Thourhout, D., Jany, C., Lamponi, M., Le Liepvre,
A., et al. (2013a). Demonstration of a heterogeneously integrated III-V/SOI single wavelength tunable laser. Opt. Express 21, 3784–3792. doi:10.1364/OE.21.
003784
Keyvaninia, S., Verstuyft, S., Pathak, S., Lelarge, F., Duan, G.-H., Bordel, D., et al.
(2013b). III-V-on-silicon multi-frequency lasers. Opt. Express 21, 13675–13683.
doi:10.1364/OE.21.013675
Keyvaninia, S., Verstuyft, S., Van Landschoot, L., Lelarge, F., Duan, G.-H., Messaoudene, S., et al. (2013c). Heterogeneously integrated III-V/silicon distributed
feedback lasers. Opt. Lett. 38, 5434–5437. doi:10.1364/OL.38.005434
Kissinger, G., and Kissinger, W. (1993). Void-free silicon-wafer-bond strengthening in the 200–400 C range. Sens. Actuators A 36, 149–156. doi:10.1016/09244247(93)85009-5
Kostrzewa, M., Di Cioccio, L., Zussy, M., Roussin, J., Fedeli, J., Kernevez, N., et al.
(2006). InP dies transferred onto silicon substrate for optical interconnects application. Sens. Actuators A 125, 411–414. doi:10.1016/j.sna.2005.07.023
Kurczveil, G., Heck, M. J., Peters, J. D., Garcia, J. M., Spencer, D., and Bowers, J. E.
(2011). An integrated hybrid silicon multiwavelength AWG laser. IEEE J. Sel. Top.
Quantum Electron. 17, 1521–1527. doi:10.1109/JSTQE.2011.2112639
Lasky, J. (1986). Wafer bonding for silicon-on-insulator technologies. Appl. Phys.
Lett. 48, 78–80. doi:10.1063/1.96768
Lee, A., Jiang, Q., Tang, M., Seeds, A., and Liu, H. (2012). Continuous-wave
InAs/GaAs quantum-dot laser diodes monolithically grown on Si substrate with
low threshold current densities. Opt. Express 20, 22181–22187. doi:10.1364/OE.
20.022181
Levaufre, G., Le Liepvre, A., Jany, C., Accard, A., Kaspar, P., Brenot, R., et al.
(2014). “Hybrid III-V/silicon tunable laser directly modulated at 10Gbit/s for
short reach/access networks,” in Paper Presented at the Optical Communication
(ECOC), 2014 European Conference on, Cannes France.
Liang, D., and Bowers, J. (2008). Highly efficient vertical outgassing channels for
low-temperature InP-to-silicon direct wafer bonding on the silicon-on-insulator
substrate. J. Vac. Sci Technol. B 26, 1560–1568. doi:10.1116/1.2943667
Liang, D., Fang, A. W., Chen, H.-W., Sysak, M. N., Koch, B. R., Lively, E., et al. (2009a).
Hybrid silicon evanescent approach to optical interconnects. Appl. Phys. A 95,
1045–1057. doi:10.1007/s00339-009-5118-1
Liang, D., Fiorentino, M., Okumura, T., Chang, H.-H., Spencer, D. T., Kuo, Y.H., et al. (2009b). Electrically-pumped compact hybrid silicon microring lasers
for optical interconnects. Opt. Express 17, 20355–20364. doi:10.1364/OE.17.
020355
Liang, D., Fiorentino, M., Srinivasan, S., Bowers, J. E., and Beausoleil, R. G. (2011).
Low threshold electrically-pumped hybrid silicon microring lasers. IEEE J. Sel.
Top. Quantum Electron. 17, 1528–1533. doi:10.1109/JSTQE.2010.2103552
Liang, D., Roelkens, G., Baets, R., and Bowers, J. E. (2010). Hybrid integrated platforms for silicon photonics. Materials 3, 1782–1802. doi:10.1038/nnano.2014.
215
Liang, D., Srinivasan, S., Fiorentino, M., Kurczveil, G., Bowers, J., and Beausoleil,
R. (2012). “A metal thermal shunt design for hybrid silicon microring laser,”
in Paper Presented at the Optical Interconnects Conference, 2012 (Santa Fe,
NM: IEEE).
Lim, A. E.-J., Song, J., Fang, Q., Li, C., Tu, X., Duan, N., et al. (2014). Review of silicon photonics foundry efforts. IEEE J. Sel. Top. Quantum Electron. 20, 8300112.
doi:10.1109/JSTQE.2013.2293274
Liow, T.-Y., Song, J., Tu, X., Lim, A.-J., Fang, Q., Duan, N., et al. (2013). Silicon
optical interconnect device technologies for 40 Gb/s and beyond. IEEE J.
Sel. Top. Quantum Electron. 19, 8200312–8200312. doi:10.1109/JSTQE.2012.
2218580
Lipson, M. (2005). Guiding, modulating, and emitting light on silicon-challenges
and opportunities. J. Lightwave Technol. 23, 4222. doi:10.1109/JLT.2005.858225
Liu, H.,Wang, T., Jiang, Q., Hogg, R., Tutu, F., Pozzi, F., et al. (2011). Long-wavelength
InAs/GaAs quantum-dot laser diode monolithically grown on Ge substrate. Nat.
Photonics 5, 416–419. doi:10.1364/OE.20.022181
Liu, J., Sun, X., Camacho-Aguilera, R., Kimerling, L. C., and Michel, J. (2010). Geon-Si laser operating at room temperature. Opt. Lett. 35, 679–681. doi:10.1364/
OL.35.000679

April 2015 | Volume 2 | Article 28 | 63

Luo et al.

Liu, J., Sun, X., Kimerling, L. C., and Michel, J. (2009). Direct-gap optical gain
of Ge on Si at room temperature. Opt. Lett. 34, 1738–1740. doi:10.1364/OL.34.
001738
Liu, J., Sun, X., Pan, D., Wang, X., Kimerling, L. C., Koch, T. L., et al. (2007). Tensilestrained, n-type Ge as a gain medium for monolithic laser integration on Si. Opt.
Express 15, 11272–11277. doi:10.1364/OE.15.011272
Luo, X., Song, J., Feng, S., Poon, A. W., Liow, T.-Y., Yu, M., et al. (2012). Silicon
high-order coupled-microring-based electro-optical switches for on-chip optical interconnects. IEEE Photonics Technol. Lett. 24, 821–823. doi:10.1109/LPT.
2012.2188829
Luo, X., Song, J., Zhou, H., Liow, T. Y., Yu, M., and Lo, P. G. Q. (2014). United States
Patent No. US 2014/0153600 A1.
Maszara, W. (1991). Silicon-on-insulator by wafer bonding: a review. J. Electrochem.
Soc. 138, 341–347. doi:10.1149/1.2085575
Mechet, P., Verstuyft, S., De Vries, T., Spuesens, T., Regreny, P., Van
Thourhout, D., et al. (2013). Unidirectional III-V microdisk lasers heterogeneously integrated on SOI. Opt. Express 21, 19339–19352. doi:10.1364/OE.21.
019339
Michel, J., Liu, J., and Kimerling, L. C. (2010). High-performance Ge-on-Si photodetectors. Nat. Photonics 4, 527–534. doi:10.1038/nphoton.2010.157
Miller, D. A. (2000). Optical interconnects to silicon. IEEE J. Sel. Top. Quantum
Electron. 6, 1312–1317. doi:10.1109/2944.902184
Miller, D. A. (2009). Device requirements for optical interconnects to silicon chips.
Proc. IEEE 97, 1166–1185. doi:10.1109/JPROC.2009.2014298
Ohashi, K., Nishi, K., Shimizu, T., Nakada, M., Fujikata, J., Ushida, J., et al. (2009).
On-chip optical interconnect. Proc. IEEE 97, 1186–1198. doi:10.1109/JPROC.
2009.2020331
Park, H., Fang, A., Kodama, S., and Bowers, J. (2005). Hybrid silicon evanescent laser
fabricated with a silicon waveguide and III-V offset quantum wells. Opt. Express
13, 9460–9464. doi:10.1364/OPEX.13.009460
Pasquariello, D., and Hjort, K. (2002). Plasma-assisted InP-to-Si low temperature
wafer bonding. IEEE J. Sel. Top. Quantum Electron. 8, 118–131. doi:10.1109/2944.
991407
Pavesi, L., Dal Negro, L., Mazzoleni, C., Franzo, G., and Priolo, F. (2000). Optical
gain in silicon nanocrystals. Nature 408, 440–444. doi:10.1038/35044012
Pavesi, L., and Lockwood, D. J. (2004). Silicon Photonics. Berlin: Springer-Verlag.
Poon, A. W., Luo, X., Xu, F., and Chen, H. (2009a). Cascaded microresonatorbased matrix switch for silicon on-chip optical interconnection. Proc. IEEE 97,
1216–1238. doi:10.1109/JPROC.2009.2014884
Poon, A. W., Luo, X., Zhou, L., Li, C., Lee, J. Y., Xu, F., et al. (2009b). “Microresonatorbased devices on a silicon chip: novel shaped cavities and resonance coherent
interference,” in Practical Applications of Microresonators in Optics and Photonics,
ed. A. B. Matsko (Boca Raton, FL: CRC Press), 211–263.
Reed, G. T., and Knights, A. P. (2004). Silicon Photonics: An Introduction. England:
John Wiley & Sons.
Reed, G. T., Mashanovich, G., Gardes, F., and Thomson, D. (2010). Silicon optical
modulators. Nat. Photonics 4, 518–526. doi:10.1038/nphoton.2010.219
Roelkens, G., Van Thourhout, D., and Baets, R. (2005). Ultra-thin benzocyclobutene bonding of III–V dies onto SOI substrate. Electron. Lett. 41, 561–562.
doi:10.1049/el:20050807
Roelkens, G., Van Thourhout, D., Baets, R., and Smit, M. (2006). Laser emission and photodetection in an InP/InGaAsP layer integrated on and coupled to a Silicon-on-Insulator waveguide circuit. Opt. Express 14, 8154–8159.
doi:10.1364/OE.14.008154
Rong, H., Jones, R., Liu, A., Cohen, O., Hak, D., Fang, A., et al. (2005a). A
continuous-wave Raman silicon laser. Nature 433, 725–728. doi:10.1038/
nature03273
Rong, H., Liu, A., Jones, R., Cohen, O., Hak, D., Nicolaescu, R., et al. (2005b). An
all-silicon Raman laser. Nature 433, 292–294. doi:10.1038/nature03273
Rong, H., Xu, S., Kuo, Y.-H., Sih, V., Cohen, O., Raday, O., et al. (2007). Lowthreshold continuous-wave Raman silicon laser. Nat. Photonics 1, 232–237.
doi:10.1038/nphoton.2007.29
Rotem, E., Shainline, J. M., and Xu, J. M. (2007a). Electroluminescence of nanopatterned silicon with carbon implantation and solid phase epitaxial regrowth. Opt.
Express 15, 14099–14106. doi:10.1364/OE.15.014099
Rotem, E., Shainline, J. M., and Xu, J. M. (2007b). Enhanced photoluminescence
from nanopatterned carbon-rich silicon grown by solid-phase epitaxy. Appl.
Phys. Lett. 91, 051127. doi:10.1063/1.2766843

Frontiers in Materials | Optics and Photonics

III/V-on-Si bonding and hybrid lasers

Seassal, C., Rojo-Romeo, P., Letartre, X., Viktorovitch, P., Hollinger, G., Jalaguier,
E., et al. (2001). InP microdisk lasers on silicon wafer: CW room temperature
operation at 1.6 µm. Electron. Lett. 37, 222–223. doi:10.1049/el:20010173
Shainline, J. M., and Xu, J. (2007). Silicon as an emissive optical medium. Laser
Photonics Rev. 1, 334–348. doi:10.1002/lpor.200710021
Song, J., Luo, X., Tu, X., Jia, L., Fang, Q., Liow, T.-Y., et al. (2013). On-chip quasidigital optical switch using silicon microring resonator-coupled Mach-Zehnder
interferometer. Opt. Express 21, 12767–12775. doi:10.1364/OE.21.012767
Soref, R. (2006). The past, present, and future of silicon photonics. IEEE J. Sel. Top.
Quantum Electron. 12, 1678–1687. doi:10.1021/ar900141y
Soref, R., and Lorenzo, J. (1985). Single-crystal silicon: a new material for 1.3 and
1.6 µm integrated-optical components. Electron. Lett. 21, 953–954. doi:10.1049/
el:19850673
Sparacin, D. K., Spector, S. J., and Kimerling, L. C. (2005). Silicon waveguide
sidewall smoothing by wet chemical oxidation. J. Lightwave Technol. 23, 2455.
doi:10.1109/JLT.2005.851328
Stanković, S., Jones, R., Sysak, M. N., Heck, J. M., Roelkens, G., and Van Thourhout,
D. (2011). 1310-nm hybrid III–V/Si Fabry–Pérot laser based on adhesive
bonding. IEEE Photonics Technol. Lett. 23, 1781–1783. doi:10.1109/LPT.2011.
2169397
Stanković, S., Jones, R., Sysak, M. N., Heck, J. M., Roelkens, G., and Van Thourhout,
D. (2012). Hybrid III–V/Si distributed-feedback laser based on adhesive
bonding. IEEE Photonics Technol. Lett. 24, 2155–2158. doi:10.1109/LPT.2012.
2223666
Stanković, S., Van Thourhout, D., Roelkens, G., Jones, R., Heck, J., and Sysak, M.
(2010). Die-to-die adhesive bonding for evanescently-coupled photonic devices.
ECS Trans. 33, 411–420. doi:10.1149/1.3483531
Sun, X., Liu, H.-C., and Yariv, A. (2009a). Adiabaticity criterion and the shortest adiabatic mode transformer in a coupled-waveguide system. Opt. Lett. 34, 280–282.
doi:10.1364/OL.34.000280
Sun, X., Liu, J., Kimerling, L. C., and Michel, J. (2009b). Direct gap photoluminescence of n-type tensile-strained Ge-on-Si. Appl. Phys. Lett. 95,
011911–011911–011913. doi:10.1364/OL.38.000652
Sun, X., Liu, J., Kimerling, L. C., and Michel, J. (2009c). Room-temperature direct
bandgap electroluminesence from Ge-on-Si light-emitting diodes. Opt. Lett. 34,
1198–1200. doi:10.1364/OL.34.001198
Sysak, M. N., Liang, D., Jones, R., Kurczveil, G., Piels, M., Fiorentino, M., et al. (2011).
Hybrid silicon laser technology: a thermal perspective. IEEE J. Sel. Top. Quantum
Electron. 17, 1490–1498. doi:10.1109/JSTQE.2011.2109940
Tanabe, K., Guimard, D., Bordel, D., Iwamoto, S., and Arakawa, Y. (2010). Electrically pumped 1.3 µm room-temperature InAs/GaAs quantum dot lasers on Si
substrates by metal-mediated wafer bonding and layer transfer. Opt. Express 18,
10604–10608. doi:10.1364/OE.18.010604
Tatsumi, T., Tanabe, K., Watanabe, K., Iwamoto, S., and Arakawa, Y. (2012). 1.3 µm
InAs/GaAs quantum dot lasers on Si substrates by low-resistivity, Au-free metalmediated wafer bonding. J. Appl. Phys. 112, 033107. doi:10.1063/1.4742198
Tong, Q.-Y., and Goesele, U. (1999). Semiconductor Wafer Bonding: Science and Technology. Wiley, 1998.
Tu, X., Chang, K.-F., Liow, T.-Y., Song, J., Luo, X., Jia, L., et al. (2014). Silicon
optical modulator with shield coplanar waveguide electrodes. Opt. Express 22,
23724–23731. doi:10.1364/OE.22.023724
Tu, X., Liow, T.-Y., Song, J., Luo, X., Fang, Q., Yu, M., et al. (2013). 50-Gb/s silicon
optical modulator with traveling-wave electrodes. Opt. Express 21, 12776–12782.
doi:10.1364/OE.21.012776
Urino, Y., Shimizu, T., Okano, M., Hatori, N., Ishizaka, M., Yamamoto, T., et al.
(2011). First demonstration of high density optical interconnects integrated with
lasers, optical modulators, and photodetectors on single silicon substrate. Opt.
Express 19, B159–B165. doi:10.1364/OE.19.00B159
Van Campenhout, J., Green, W. M., Assefa, S., and Vlasov, Y. A. (2009). Low-power,
2x2 silicon electro-optic switch with 110-nm bandwidth for broadband reconfigurable optical networks. Opt. Express 17, 24020–24029. doi:10.1364/OE.17.
024020
Van Campenhout, J., Liu, L., Romeo, P. R., Van Thourhout, D., Seassal, C., Regreny,
P., et al. (2008). A compact SOI-integrated multiwavelength laser source based
on cascaded InP microdisks. IEEE Photonics Technol. Lett. 20, 1345–1347.
doi:10.1109/LPT.2008.926857
Van Campenhout, J., Rojo Romeo, P., Regreny, P., Seassal, C.,Van Thourhout, D.,Verstuyft, S., et al. (2007). Electrically pumped InP-based microdisk lasers integrated

April 2015 | Volume 2 | Article 28 | 64

Luo et al.

with a nanophotonic silicon-on-insulator waveguide circuit. Opt. Express 15,
6744–6749. doi:10.1364/OE.15.006744
Vivien, L., and Pavesi, L. (2013). Handbook of Silicon Photonics. Boca Raton, FL: CRC
Press.
Xia, F., Sekaric, L., and Vlasov, Y. (2006). Ultracompact optical buffers on a silicon
chip. Nat. Photonics 1, 65–71. doi:10.1038/nphoton.2006.42
Xiao, S., Khan, M. H., Shen, H., and Qi, M. (2007). Multiple-channel silicon microresonator based filters for WDM applications. Opt. Express 15, 7489–7498.
doi:10.1364/OE.15.009386
Xu, D.-X., Schmid, J. H., Reed, G. T., Mashanovich, G. Z., Thomson, D. J.,
Nedeljkovic, M., et al. (2014). Silicon photonic integration platform-have we
found the sweet spot? IEEE J. Sel. Top. Quantum Electron. 20, 189–205.
doi:10.1109/JSTQE.2014.2299634
Yang, H., Zhao, D., Chuwongin, S., Seo, J.-H., Yang, W., Shuai, Y., et al. (2012).
Transfer-printed stacked nanomembrane lasers on silicon. Nat. Photonics 6,
615–620. doi:10.1038/nphoton.2012.160
Yariv, A., and Sun, X. (2007). Supermode Si/III-V hybrid lasers, optical amplifiers and modulators: a proposal and analysis. Opt. Express 15, 9147–9151.
doi:10.1364/OE.15.009147
Zhang, C., Srinivasan, S., Tang, Y., Heck, M. J., Davenport, M. L., and Bowers, J. E.
(2014). Low threshold and high speed short cavity distributed feedback hybrid
silicon lasers. Opt. Express 22, 10202–10209. doi:10.1364/OE.22.010202

www.frontiersin.org

III/V-on-Si bonding and hybrid lasers

Zhou, L., and Poon, A. W. (2007). Electrically reconfigurable silicon microring resonator-based filter with waveguide-coupled feedback. Opt. Express 15,
9194–9204. doi:10.1364/OE.15.009194
Conflict of Interest Statement: The authors declare that the research was conducted
in the absence of any commercial or financial relationships that could be construed
as a potential conflict of interest.
Received: 08 January 2015; paper pending published: 13 February 2015; accepted: 17
March 2015; published online: 07 April 2015.
Citation: Luo X, Cao Y, Song J, Hu X, Cheng Y, Li C, Liu C, Liow T-Y, Yu M, Wang
H, Wang QJ and Lo PG-Q (2015) High-throughput multiple dies-to-wafer bonding
technology and III/V-on-Si hybrid lasers for heterogeneous integration of optoelectronic
integrated circuits. Front. Mater. 2:28. doi: 10.3389/fmats.2015.00028
This article was submitted to Optics and Photonics, a section of the journal Frontiers
in Materials.
Copyright © 2015 Luo, Cao, Song , Hu, Cheng , Li, Liu, Liow, Yu, Wang , Wang and
Lo. This is an open-access article distributed under the terms of the Creative Commons
Attribution License (CC BY). The use, distribution or reproduction in other forums is
permitted, provided the original author(s) or licensor are credited and that the original
publication in this journal is cited, in accordance with accepted academic practice. No
use, distribution or reproduction is permitted which does not comply with these terms.

April 2015 | Volume 2 | Article 28 | 65

REVIEW ARTICLE

MATERIALS

published: 17 September 2014
doi: 10.3389/fmats.2014.00015

Group IV light sources to enable the convergence of
photonics and electronics
Shinichi Saito 1 *, Frederic Yannick Gardes 1 , Abdelrahman Zaher Al-Attili 1 , Kazuki Tani 2,3,4 , Katsuya Oda 2,3,4 ,
Yuji Suwa 2,3,4 , Tatemi Ido 2,3,4 , Yasuhiko Ishikawa 5 , Satoshi Kako 3,6 , Satoshi Iwamoto 3,6 and
Yasuhiko Arakawa 3,6
1
2
3
4
5
6

Faculty of Physical Sciences and Engineering, University of Southampton, Southampton, UK
Photonics Electronics Technology Research Association (PETRA), Tokyo, Japan
Institute for Photonics-Electronics Convergence System Technology (PECST), Tokyo, Japan
Central Research Laboratory, Hitachi Ltd., Tokyo, Japan
Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo, Japan
Institute of Industrial Science, The University of Tokyo, Tokyo, Japan

Edited by:
Jifeng Liu, Dartmouth College, USA
Reviewed by:
Androula Galiouna Nassiopoulou,
National Centre for Scientific
Research Demokritos, Greece
Raul J. Martin-Palma, Universidad
Autonoma de Madrid, Spain
Jifeng Liu, Dartmouth College, USA
*Correspondence:
Shinichi Saito, Nano Research Group,
Electronics and Computer Science,
Faculty of Physical Sciences and
Engineering, Highfield Campus,
University of Southampton,
Southampton SO17 1BJ, UK
e-mail: s.saito@soton.ac.uk

Group IV lasers are expected to revolutionize chip-to-chip optical communications in terms
of cost, scalability, yield, and compatibility to the existing infrastructure of silicon industries
for mass production. Here, we review the current state-of-the-art developments of silicon
and germanium light sources toward monolithic integration. Quantum confinement of electrons and holes in nanostructures has been the primary route for light emission from silicon,
and we can use advanced silicon technologies using top-down patterning processes to fabricate these nanostructures, including fin-type vertical multiple-quantum-wells. Moreover,
the electromagnetic environment can also be manipulated in a photonic crystal nanocavity
to enhance the efficiency of light extraction and emission by the Purcell effect. Germanium
is also widely investigated as an active material in Group IV photonics, and novel epitaxial
growth technologies are being developed to make a high quality germanium layer on a silicon substrate. To develop a practical germanium laser, various technologies are employed
for tensile-stress engineering and high electron doping to compensate the indirect valleys
in the conduction band. These challenges are aiming to contribute toward the convergence
of electronics and photonics on a silicon chip.
Keywords: silicon, photonics, CMOS, germanium, epitaxy, luminescence, quantum, strain

1.

INTRODUCTION

As the integration of transistors in a chip increases, the demands
of the interconnections are expanding, since more information
will be transferred between chips optically (Miller, 2009). The
advantage of optical interconnection over electrical wiring is
fundamentally coming from the elementary particles, photons,
used for signal transmission. We can transmit photons without an electrical connection throughout an optical fiber, since
photons do not have charge. Of course, optical loss exists, but
still the total energy consumption of the optical interconnection can be much lower than that of the electrical connection,
especially for the long-distance communications at higher data
rate, even including the energy required to convert electrons to
photons and vice versa (Miller, 2009). Si photonics is revolutionizing optical interconnections in terms of cost, power, bandwidth, and scalability (Zimmermann, 2000; Pavesi and Lockwood,
2004; Reed and Knights, 2004; Pavesi and Guillot, 2006; Reed,
2008; Deen and Basu, 2012; Fathpour and Jalali, 2012; Vivien
and Pavesi, 2013). III-V (Wale, 2008; Evans et al., 2011) and Sibased platform technologies (Reed and Knights, 2004; Gunn, 2006;
Rylyakov et al., 2011; Arakawa et al., 2013; Urino et al., 2013) are
competing for the next generation of optical interconnections.
The critical missing component for Si photonics is a monolithic light source compatible with the existing infrastructure of

Frontiers in Materials | Optics and Photonics

complementary-metal-oxide-semiconductor (CMOS) technologies for fabrication. The hybrid integration of III-V devices on
an Si substrate (Fang et al., 2006) or feeding of an optical fiber to
an Si waveguide coupled with a grating from a III-V laser diode
(Gunn, 2006) would be the near-term solution, but it is desirable to
realize monolithic light sources for the long term. Comprehensive
reviews on developing practical lasers on Si have been published by
various authors (Cullis et al., 1997; Ossicini et al., 2006; Daldosso
and Pavesi, 2009; Liang and Bowers, 2010; Steger et al., 2011; Liu
et al., 2012; Michel and Romagnoli, 2012; Boucaud et al., 2013;
Shakoor et al., 2013; Liu, 2014). Here, we review this active field
focusing on the progress of Si and germanium (Ge) light sources
fabricated by standard CMOS processes.
Photoluminescence (PL) (Canham, 1990) and electroluminescence (EL) (Koshida and Koyama, 1992) from porous-Si are
the most famous achievements to overcome the fundamental
limitations of the indirect band-gap character of Si. The maximum PL (Gelloz and Koshida, 2000) and EL (Gelloz et al.,
2005) quantum efficiency exceeded 23 and 1%, respectively. The
mechanism of light emission from porous-Si is considered to
originate from quantum confinement effects (Canham, 1990;
Koshida and Koyama, 1992; Cullis et al., 1997; Nassiopoulou,
2004; Ossicini et al., 2006; Daldosso and Pavesi, 2009) in the
self-organized nanostructure. The typical length scale to expect

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Saito et al.

quantum confinement would be comparable to the exciton Bohr
radius, which is about 5 nm for Si and 18 nm for Ge (Cullis et al.,
1997). On the other hand, the gate length fabricated by CMOS
technologies is comparable to the exciton Bohr radius so that we
can fabricate various quantum structures, including quantum dots
(Arakawa and Sakaki, 1982), nano-wires, and quantum-wells, by
lithographically controlled top-down processes. In addition, novel
cavity structures (Iwamoto and Arakawa, 2012) can be fabricated
to enhance the internal quantum efficiency by the Purcell effect
(Purcell, 1946) as well as the extraction efficiency by improved
coupling to a lens. Ge is also intensively studied, since the direct
band-gap energy is closer to the indirect transition energy than
that of Si. Highly, n-type doping and strain engineering are effective to enhance the light emissions from Ge (Liu et al., 2012; Michel
and Romagnoli, 2012; Boucaud et al., 2013; Liu, 2014), and some
of these recent advances are reviewed in this paper.

2.

STRATEGIES TO ENHANCE LIGHT EMISSION FROM
GROUP IV MATERIALS

2.1.

THEORETICAL STUDY OF LIGHT EMISSION FROM SILICON

Both Si and Ge are known to be poor light emitters because of their
indirect band-gap structures. Even so, there are some methods for
making direct transitions to occur in these materials. These possibilities were examined theoretically by first-principles calculations
based on density functional theory using plane-wave-based ultrasoft pseudo-potentials (Vanderbilt, 1990; Laasonen et al., 1993).
Generalized gradient approximation (Perdew et al., 1996) is used
for the calculation of Si, and hybrid functional (Perdew et al., 1996)
is used for Ge. The optical matrix elements are calculated with the
aid of core-repair terms (Kageshima and Shiraishi, 1997).
The lowest conduction band (LCB) of bulk Si has a minimum
near the X -point, and six electron valleys exist near X -points.
Two valleys among the six are projected onto 0-point in twodimensional momentum (k)-space when an Si quantum-well
(QW) with (001) surfaces is fabricated; this is called a valleyprojection. Because the top of the valence band is also projected
onto 0, direct transitions are possible in an Si(001) QW.
Optical gain of Si(001) QWs is shown in Figure 1 as a function
of the thickness (Suwa and Saito, 2009). Here, losses due to transitions within conduction bands and those within valence bands
are not taken into account. The thinner QW shows the larger gain,
since the surface of the QW plays an important role in this direct
transition and it dominates if the QW is thin. Figure 1 also shows
that the surface structure of the QW affects the efficiency of light
emission strongly.
Experimentally, optical gain from the Si quantum dots (Pavesi
et al., 2000) embedded in an insulating matrix (Pavesi et al., 2000;
Nassiopoulou, 2004; Ossicini et al., 2006; Pavesi and Guillot, 2006)
has been reported. It was confirmed that the interface states associated with oxygen atoms were important to explain the positive
optical gain (Pavesi et al., 2000; Nassiopoulou, 2004; Ossicini et al.,
2006; Pavesi and Guillot, 2006). It will be interesting to make these
structures by top-down CMOS processes.
2.2.

THEORETICAL STUDY OF LIGHT EMISSION FROM GERMANIUM

Ge has two important differences from Si. One is that Ge has the
minimum of the LCB at the L-point (L-valley), while Si has it

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Group IV light sources

FIGURE 1 | Optical gain of Si(001) thin films calculated from direct
transitions across the energy gap only.

near the X -point. The other is that Ge has a local minimum of
the LCB at the 0-point (0-valley), while Si does not. An L-valley
is projected onto the 0-point in the two-dimensional k-space for
Ge(111) QW. For Ge(001) QW, no L-valley is projected onto the
0-point. The small 0-valley, which is not occupied unless a large
number of electrons are injected, is always projected onto the
0-point independently of the direction of the QW. While there
are two approaches to obtain efficient light emission from Ge
by direct transitions, using L-valleys of a Ge(111) QW or using
the 0-valley of bulk Ge, we think the latter is more promising. This is due to the fact that the calculated optical matrix
element for the 0-valley is very large compared to that for the
L-valleys.
To enhance light emission from bulk Ge, applying tensile strain
is known to be effective (Liu et al., 2010). Tensile strain makes
the energy difference between the 0 and L-valleys small, and that
makes electron injection into the 0-valley easier. Also heavy n-type
doping is known to be effective, because electrons can be injected
into the 0-valley if the L-valleys are already occupied by doped
electrons.
In order to predict required strength of strain and amount
of doping, we calculated optical gains of bulk Ge with and
without strain. Figure 2 shows calculated optical gain as functions of injected electron density and hole densities. Here, the
applied strain is assumed to be 0.25% biaxial tensile strain parallel
to (001) surface and optical losses due to free carrier absorptions (Wang et al., 2013) are taken into account. This result
shows that even bulk Ge without strain can have a positive
optical gain, but number of electrons required for that is very
large (1020 cm−3 ). Despite the relatively small amount of the
strain (0.25%), the impact on the gain is clear. Owing to this
enhancement, only half the electron density (5 × 1019 cm−3 ) is
needed to have positive gain. In experiment, applying 0.25%
strain is rather easy, and making higher strain will be possible, as we see in the following sections. Therefore, Ge lasers
will be realized when an appropriate strain and carrier injection
are achieved.

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3.

ELECTRO-LUMINESCENCE FROM SILICON
QUANTUM-WELL

As we reviewed in Section 1, it is well established that efficient
recombination is observed in Si nanostructures by quantum confinement effects (Canham, 1990; Koshida and Koyama, 1992;
Cullis et al., 1997; Ossicini et al., 2006; Daldosso and Pavesi, 2009).
The nanostructures include quantum dots (Arakawa and Sakaki,
1982), nano-wires (Canham, 1990; Koshida and Koyama, 1992),
quantum-well (QW) (Saito et al., 2006a,b, 2008, 2009; Saito, 2011),
and fins (Saito et al., 2011a,b). One of the difficulties in developing an efficient light-emitting diode (LED) made of Si comes from
the trade-off between quantum confinement and carrier injection.
The surface of these Si nanostructures is easily oxidized to SiO2 ,
and the band offsets between Si and SiO2 are too high to expect
efficient current injection except for tunneling. In order to overcome this trade-off, lateral carrier injection into the Si QW was
proposed (Saito et al., 2006a,b, 2008, 2009; Hoang et al., 2007;
Noborisaka et al., 2011; Saito, 2011). As shown in Figures 3A–C,
the Si QW LEDs were fabricated by local thinning of a silicon-oninsulator (SOI) substrate, and the Si QW was directly connected to
the thick Si diffusion electrodes (Saito et al., 2006a,b). Both electrons and holes are laterally injected to the Si QW in these planar

FIGURE 2 | Optical gain of germanium as a function of injected
electron density and hole density is shown. Those without strain and
with 0.25% biaxial tensile strain parallel to (001) surface are shown.

Group IV light sources

p-i-n diodes (Saito et al., 2006a,b, 2008, 2009; Noborisaka et al.,
2011; Saito, 2011). Another advantage of these device structures
is the fabrication of the Si QW through the LOCal-Oxidation of
Si (LOCOS) process. The LOCOS process was originally developed for isolation of CMOS transistors (Sze and Lee, 2012; Taur
and Ning, 2013). It was also used to evaluate the carrier mobility
in the ultra-thin Si QW (Uchida and Takagi, 2003). Oxidation is
one of the most precisely controlled processes in CMOS technologies, and we can routinely oxidize a large Si wafer (typically 8–1000
in diameter) within the local variation of <0.1 nm. Besides, the
interface between Si and SiO2 is excellent with low interface trap
density (<1011 cm−2 ) (Sze and Lee, 2012; Taur and Ning, 2013).
The excellent interfacial quality and strong quantum confinement
in Si nanostructures are critical to ensure high quantum efficiency
(Gelloz et al., 2005). As shown in Figure 3E, EL is observed exclusively from the thin Si QW and EL from thick Si electrodes is
negligible (Saito et al., 2006a). This supports the mechanism of EL
based on quantum confinement (Ossicini et al., 2006; Suwa and
Saito, 2009). The high carrier density in the thin Si QW also contributes to enhance the emissions (Saito et al., 2006a). By applying
the back gate to the Si substrate, we can modulate the intensities of
light emission, and the device can be called as an Si light-emitting
transistor (Saito et al., 2006b).
The next step toward the practical light source for Si photonics
is to couple the light from Si to a cavity and a waveguide (WG).
An Si-based WG cannot be used for emission from Si QW due
to the absorption. An Si3 N4 WG was fabricated on top of the Si
QW by conventional lithography and dry etching (Saito et al., 2008,
2009). To enhance the optical confinement in the WG of the Si Resonant Cavity LED (RCLED), part of the supporting substrate was
removed by using double sided aligner and anisotropic wet etching (Saito et al., 2008, 2009), as shown in Figure 3B. Evanescent
coupling between the propagating optical mode and Si QW was
expected, and the enhanced EL from the edge of the waveguide was
observed (Figure 3F). More recently, SOI substrates with superior
uniformities with thick Buried-OXide (BOX) (>2 µm) became
available, and by using these wafers, strong optical confinement
within the Si3 N4 WG was ensured without removing the supporting Si substrate (Saito, 2011), as shown in Figure 3C. In fact, the
near-field image of the propagating optical mode was taken at the
edge of the WG (Figure 3G).

FIGURE 3 | Development of an Si light source. (A) Si QW LED, (B) Si RCLED, (C) Si QW LED with thick BOX, (D) Si FinLED, and (E–H) EL images from these
devices. (E,F) are plan views. (G,H) are near-field images at the edge of WG.

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Saito et al.

FIGURE 4 | EL from Si FinLED taken from edge of WG. (A) Spectra and
(B) integrated intensity.

The obvious disadvantage of using the planar Si QW is the
small confinement factor of the optical mode in the Si QW due
to the thin single QW layer. It is not straightforward to make Si
Multiple QWs (MQWs) (Fukatsu et al., 1992), if the surface of the
Si QW is covered with the amorphous SiO2 . As an alternative to
the stacking of the Si MQWs, the Si FinLED has been proposed
(Saito et al., 2011b), as shown in Figure 3D. Si fin is a vertical QW
located perpendicular to an Si substrate, and it was proposed for
a self-aligned double-gate CMOS field-effect-transistor, called a
FinFET (Hisamoto et al., 2000). FinFETs are already used for mass
production and more than one billion of FinFETs are integrated
in the most recent MPU (INTEL, 20131 ; ITRS, 20122 ). Therefore,
we can fabricate thousands of Si fins as MQWs at the same time
simply by conventional photolithography and dry etching (Saito
et al., 2011b). By applying forward bias to the Si FinLED, we can
observe edge emission from the Si3 N4 WG (Figure 3H). The EL
spectra from the edge of the Si FinLED are shown in Figure 4A.
The enhanced peaks from the edge of the stop band were observed
due to the distributed-feedback structure of the periodic fins
(Saito et al., 2011b). The non-linear increase of the EL intensity
against the current is considered to come from stimulated emission (Figure 4B), but the estimated gain of <1 cm−1 was too low to
overcome the threshold for a laser operation (Saito et al., 2011b).

4.
4.1.

APPLICATION OF PHOTONIC NANOSTRUCTURES TO
GROUP IV MATERIALS
CONTROL OF LIGHT EMISSION BY PHOTONIC CRYSTALS

The light emission properties of materials depend not only
on material characteristics such as the dipole moment and the

1 http://www.intel.com/content/www/us/en/history/museum-transistors-totransformations-brochure.html
2 http://www.itrs.net

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Group IV light sources

refractive index but also on the electromagnetic environment surrounding the material. In the previous sections, engineering group
IV materials themselves such as quantum confinement, doping,
and strain engineering have been discussed. Here, we discuss
another approach, i.e., tailoring the electromagnetic environment
by photonic nanostructures for improving light emission properties. The total efficiency of light-emitting devices can be expressed
as a product of three factors: light emission efficiency ηemission ,
extraction efficiency ηextraction , and collection efficiency ηcollection .
ηemission denotes how efficiently injected carriers recombine by
emitting photons. ηextraction takes into account the fact that only
a part of emitted photons can be extracted from the material.
ηcollection expresses how much extracted photons can be collected
by the first lens of the setup. All of them can be improved by
photonic nanostructures. Photonic crystal (PhC) (Jannopoulos
and Winn, 1995), which has a wavelength-scale periodic variation
of refractive index, is an important photonic nanostructure for
this application (see discussions in Iwamoto and Arakawa, 2012).
Figure 5A shows a scanning electron microscope (SEM) image of a
two-dimensional (2D) PhC slab, which is the most widely studied
PhC structure. The structure can be fabricated by forming air holes
in a thin semiconductor plate using conventional lithography and
etching processes. In the structure, owing to the periodic modulation in refractive index, in-plane light propagation is governed by
the photonic band structure. Strikingly, propagation is forbidden
in photonic bandgaps (PBGs). Photonic band structures and PBGs
can play roles to improve mainly ηextraction and ηcollection . Another
important structure is the PhC nanocavity (Figure 5B), which is
created by omitting air holes from the regular array. Photons are
confined in in-plane and out-of-plane directions due to the PBG
effect and total internal reflection, respectively. PhC nanocavities
have a high quality factor Q and small mode volume V c (~1 cubic
wavelength or less). These two quantities are key parameters to
enhance the spontaneous emission rate through the Purcell effect
(Purcell, 1946) and improve ηemission . Particularly, for light emitters with broad linewidth such as bulk Si, V c has a stronger impact
(Ujihara, 1995). Such high-Q PhC nanocavities can uncover the
quantum nature of light-matter interaction. Cavity quantum electrodynamics in a high-Q PhC nanocavity coupled with a single
semiconductor quantum dot is a hot topic in the field (see, for
example, Arakawa et al., 2012). Purcell enhancement factors of as
large as 12 (Lo Savio et al., 2011) and 30 (Sumikura et al., 2014)
were reported, which would be limited by the emission linewidth
and the Q factor, respectively.
4.2.

ENHANCED LIGHT EMISSION FROM SILICON PHOTONIC CRYSTAL
STRUCTURES

PhC structures without cavities have been firstly applied to control the light emission from crystalline Si. In 2003, Zelsmann et al.
(2003) reported enhanced PL extraction from a 2D PhC slab fabricated into the top Si layer of a SOI substrate at low temperature
(Zelsmann et al., 2003). Similar enhancements at room temperature have been observed from arrays of Si nanoboxes (Cluzel et al.,
2006a) and rods (Cluzel et al., 2006b) formed on SOI substrates.
Strong light emission was observed at wavelengths corresponding
to photonic band edges at the 0 point. Increasing the number of
band edge within the emission spectrum of Si can lead to higher

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luminescence intensity. This is experimentally verified by increasing the lattice constant of PhC so that normalized frequencies
corresponding to the Si emission wavelengths are increased (Fujita
et al., 2008). Si light-emitting diodes (LEDs) with PhC patterns
have also been demonstrated (Nakayama et al., 2010a; Iwamoto
and Arakawa, 2012). The device schematically shown in Figure 6A
was fabricated using a SOI substrate. Firstly, a lateral p-i-n junction
was formed into the top 200-nm-thick Si layer by area-selective
implantations of boron and phosphorous ions. Then, a PhC structure was patterned. To keep mechanical stability and better thermal
conductivity, the buried-oxide (BOX) layer was not removed. An
SEM image of the central part of a device is shown in Figure 6A.
The i-region is 5 µm in length and 250 µm in width. EL spectra
from devices with different PhC periods and from a device without
PhC are shown in the inset of Figure 6B. EL emission increased
as the period a increased. Figure 6B shows the integrated intensities from these devices as a function of injected current. The
integrated intensity from the device with a = 750 nm is ~14 times
stronger than that from an unpatterned LED. This enhancement
is mainly caused by the improvement of ηextraction and ηcollection
due to the photonic band structures as discussed above. ηemission
is also expected to be enhanced in PhC nanocavities. Figure 7
shows room-temperature µ-PL spectra measured at the center of

FIGURE 5 | SEM images of a regular PhC structure with a triangular
lattice (A) and a L3-type PhC nanocavity, in which three air holes along
a 0-K direction are omitted (B).

Group IV light sources

an L3-type PhC nanocavity compared to a non-patterned region
(see the inset). The L3 PhC nanocavity was also fabricated into
an SOI substrate. In this sample, the BOX layer was etched out
in order to confine the photons strongly in the vertical direction.
The PL intensity from the cavity was much larger than that from
the non-patterned region. In addition, sharp peaks are observed
only in the spectrum from the cavity. These peaks originate from
the cavity resonant modes. For this particular sample with the air
hole radius r = 0.37a, large enhancement of PL over 300 times was
obtained for a cavity mode at 1,191 nm. As discussed in Section
1, this enhancement can be attributed to three factors. Detailed
analysis including numerical simulation indicated that ηemission is
improved by ~5 times (Iwamoto et al., 2007). The enhancement
factors in ηemission ranging ~5−10 have been reported for Si interband transition (Fujita et al., 2008) and for light emission from
optically active defects in Si (Lo Savio et al., 2011). The temperature dependence of cavity mode emission (Hauke et al., 2010;
Lo Savio et al., 2011) and the dependence of PL on cavity mode
volume V c (Nakayama et al., 2012) suggest that the Purcell effect
plays a role in this enhancement. The enhancement in ηemission
reported so far is still too small for practical applications. However, this research would provide important insights for further
development of light-emitting devices using group IV materials.
Indeed, these pioneering works have stimulated theoretical investigations, which discuss the possibility of lasing oscillation in Si
(Escalante and Martínez, 2012, 2013). Recent advances in this field
are developments of Si LEDs with PhC nanocavities (Nakayama
et al., 2011; Shakoor et al., 2013). Shakoor et al. (2013) recently
reported Si LEDs using L3-type nanocavity structure, in which
optically active defects created by hydrogen bombardment are used
as light emission centers. They carefully designed the cavity structure to improving ηcollection and obtained sharp light emission at
around 1.5 µm with a power density of 0.4 mW/cm2 . The straininduced dislocations (Ng et al., 2001; Kittler et al., 2013) will also
be compatible to PhC nanocavities, since the emission energies
are smaller than the band gap of Si. The combination of PhC
nanocavities and defect engineering is very promising, and a wall
plug efficiency of 0.7 × 10−8 was reported (Shakoor et al., 2013).

FIGURE 6 | (A) Schematic representation of a silicon PhC LED is shown. The SEM image shows the center area of a device. (B) Integrated EL intensities for
silicon PhC LED with various periods a and for an SOI LED with a flat surface. The inset shows corresponding EL spectra at 10 mA.

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FIGURE 7 | Room-temperature µ-PL spectra measured at the center of
a PhC nanocavity and at a non-patterned area. The spectrum for the
latter is magnified by ten times for better viewing.

4.3.

APPLICATION OF PHOTONIC CRYSTAL STRUCTURES TO OTHER
EMITTERS IN GROUP IV MATERIALS

Erbium ions have been investigated as one of the promising
light emitters in Si. PhC nanocavities have been also applied to
enhance the light emission from Er ions (Wang et al., 2012; Savio
et al., 2013). Narrowing the cavity linewidth in Er-doped silicon
nitride PhC nanocavities has been also demonstrated under optical pumping condition (Gong et al., 2010). As discussed in the
previous sections, Ge is, at present, the most important material
for future light-emitting devices in Si photonics. PhC (Nakayama
et al., 2010b) and PhC nanocavities (Kurdi et al., 2008; Ngo et al.,
2008) have been applied to increase the light emission from bulk
Ge. Applying advanced strain/doping engineering technologies to
photonic nanostructures would open a new route for boosting the
light emission efficiency of Ge.

5.

GENERATION OF TENSILE STRAIN IN Ge LAYERS
EPITAXIALLY GROWN ON Si SUBSTRATE

In epitaxial growth of Ge on an Si substrate, a compressive strain
in Ge, derived from the 4.2% lattice mismatch with Si, should be
relaxed after growth beyond the critical thickness, while it has been
reported by one of the authors that, during the cooling from the
growth temperature to room temperature, a biaxial tensile strain
as large as 0.2% is built-in due to the thermal expansion mismatch (Ishikawa et al., 2003, 2005; Cannon et al., 2004; Liu et al.,
2005). It is known that the strain in semiconductors causes shifts
in band edge energies, e.g., de Walle, 1989, modifying the gap energies, i.e., properties of optical transitions. The 0.2% tensile strain
in Ge reduces the direct bandgap energy from 0.80 to ~0.77 eV,
and as a result, the optical absorption edge (or the longer limit of
detection wavelength) shifts from 1.55 to >1.60 µm, causing the
increase of optical absorption coefficient at 1.55 µm (Ishikawa
et al., 2003, 2005; Cannon et al., 2004; Liu et al., 2005). This
property is effective for the detection of near-infrared (NIR) light

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Group IV light sources

used in the optical fiber communications (1.3–1.6 µm). A further attractive feature of the tensile strain in Ge is the reduction
of energy difference in the conduction band between the direct
0 valley and indirect L-valley (e.g., Fischetti and Laux, 1996;
Wada et al., 2006; Camacho-Aguilera et al., 2012; Nama et al.,
2013; Süess et al., 2013). This feature stimulates researchers to
obtain efficient NIR light emission from tensile-strained Ge due
to the enhanced direct transition around the 0 point (e.g., Liu
et al., 2007; Lim et al., 2009). In this section, the grown-in tensile strain in Ge on Si, generated due to the thermal expansion
mismatch, is described. Figure 8A shows typical ω − 2θ x-ray
diffraction (XRD) curves taken for 0.6-µm-thick Ge grown on
a 525-µm-thick Si(001) substrate with the Cu Kα radiation as
the x-ray source (0.15406 nm in wavelength). The samples were
grown by ultrahigh-vacuum chemical vapor deposition with a
source gas of GeH4 (9%) diluted in Ar. The growth temperature was 600°C, while a lower temperature of 370°C was used
at the initial stage of Ge growth (~50 nm) in order to prevent
the islanding, leading to Ge layers uniform in thickness (Luan
et al., 1999; Ishikawa and Wada, 2010). After the growth, hightemperature annealing was carried out for one of the samples at
800°C for 20 min. Such annealing is often performed in order to
reduce the threading-dislocation density (Luan et al., 1999). In our
case, the density was reduced from 1 × 109 to 1 − 2 × 108 cm−2 . In
Figure 8A, the peaks due to the (004) diffraction are clearly seen at
around 2θ ~ 66° for both of the as-grown and annealed samples.
It is important that the peaks were located at larger diffraction
angles than that for unstrained Ge, indicating the reduction of
out-of-plane lattice constant, i.e., the increase of in-plane lattice
constant due to the generation of tensile strain. According to the
peak positions, the in-plane biaxial tensile strain was estimated
to be 0.11 and 0.22% for the as-grown and annealed samples,
respectively.
As mentioned above, such a tensile strain is generated in Ge
due to the mismatch of thermal expansion coefficient with Si.
As schematically shown in Figure 8B, the compressive strain in
Ge due to the 4.2% lattice mismatch should be relaxed at the
growth/annealing temperature, while the shrinkage in the Ge
lattice during the cooling should be prevented by the thick Si substrate, since Si has a smaller thermal expansion coefficient than
that of Ge. This means that a tensile (compressive) stress/strain is
generated in Ge (Si), as in the bottom of Figure 8B. Taking into
account the balance of forces together with the balance of moments
in the stacked structure of Ge and Si, the tensile (compressive)
strain in Ge (Si) is theoretically expressed as:
1
∈|| (Ge) =
R
∈|| (Si) = & −

1
R





Y1 t13 + Y2 t23
+
6Y1 t1 (t1 + t2 )

Y1 t13 + Y2 t23
−
6Y2 t2 (t1 + t2 )





t1
− z1
2

t2
− z2
2


(1)


,

(2)

where, α i , Yi , ti , and zi represent the thermal expansion coefficient,
the Young’s modulus, the layer thickness, and the location in the
layer measured from the bottom of the layer for the i-th layer (1
for Ge and 2 for Si), respectively. The radius of curvature R is

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Group IV light sources

FIGURE 8 | (A) ω − 2θ XRD curves for 0.6-µm-thick Ge on Si (001) substrate, (B) schematic illustration showing the generation of tensile stress/strain in Ge, and
(C) theoretical curves and experimental data for biaxial tensile strain in Ge.

represented from

the Ge layer as well as deposition of dielectric films embedding a
strain could intentionally modify the strain in the Ge.

R TRT
6 (t1 + t2 ) Y1 Y2 t1 t2 TGR/AN
(α1 − α2 ) dT
1


=
, (3)
2
3
R
3(t1 + t2 ) Y1 Y2 t1 t2 + Y1 t1 + Y2 t23 (Y1 t1 + Y2 t2 )
where T GR/AN and T RT represent the growth/annealing temperature before cooling and the room temperature (after the cooling),
respectively. Since the first term is dominant in the right side of
equation (1), the strains are almost independent of z i , the location
within the layer. Therefore, equations (1) and (2) are simplified to:

∈|| (Ge) ∼

1 Y1 t13 + Y2 t23
R 6Y1 t1 (t1 + t2 )

∈|| (Si) ∼ −

1 Y1 t13 + Y2 t23
.
R 6Y2 t2 (t1 + t2 )

(4)

(5)

The lines in Figure 8C represent the strains calculated for the Ge
thickness of 0.6 µm and the Si thickness of 525 µm. Note that
almost identical results can be obtained when the thickness of Si
substrate t 2 is much larger (more than ~100 times) than the Ge
thickness t 1 . The parameters used in the calculation can be found
in Ishikawa et al. (2005). It is found that a tensile strain on the
order of 0.1% is generated in Ge at room temperature, while the
compressive strain in Si is negligible. It is also found that higher
growth/annealing temperature generates larger tensile strain after
the cooling. These properties are qualitatively in good agreement
with the XRD results in Figure 8A. However, quantitatively, the
tensile strain observed by XRD was smaller than the theoretical
one. This is probably ascribed to the residual compressive strain
in Ge at the growth/annealing temperature (Ishikawa et al., 2005).
From the viewpoint of optoelectronic integration of Ge devices
on an Si platform, Si-on-insulator (SOI) wafers have been widely
used. For Ge layers grown on SOI wafers, a similar amount of
tensile strain should be generated, since the elastic deformation,
derived from the thermal expansion mismatch, is governed by the
thick Si substrate, rather than the buried SiO2 and the top Si layers
with the thicknesses on the order of 1 µm or below. Patterning of

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6.

DIRECT GERMANIUM EPITAXIAL GROWTH PROCESS ON
SILICON

The Ge was epitaxially grown by using a cold-wall rapid thermal chemical vapor deposition system. Germane (GeH4 ) was
used as a source gas, which was supplied with H2 carrier gas.
As the starting point of improving the crystallinity and controlling the lattice strain, Ge layers with good surface morphology
were grown at 420°C under relatively high pressure of 7,000 Pa.
Then, the Ge layers were annealed in the same H2 atmosphere
to improve the crystallinity. Figure 9 shows a reciprocal space
map (RSM) of XRD (XRD-RSM) from the 130-nm-thick Ge layer
directly grown on the Si substrate before and after H2 annealing. An intense Si (-1-13) peak was observed, which represented
the diffraction from the Si substrate under the Ge layer. Since the
XRD-RSM was measured by using semiconductor array detectors, errors in the counts occur if the diffraction intensity is
very high; therefore, the streak line observed around the Si (-113) peak does not represent any actual diffraction. Since a Ge
(-1-13) diffraction peak was observed from the Ge layer without annealing (Figure 9A), it could be confirmed that a single
crystalline Ge layer was obtained by using low-temperature epitaxial growth. The displacement of the diffraction peak shows that
the as-grown Ge layer still contained a compressive strain just
after the low-temperature epitaxial growth at 420°C due to the
larger lattice constant of Ge compared to that of the Si substrate.
It has been reported that cyclic annealing at a relatively higher
temperature can reduce the threading-dislocation density (Luan
et al., 1999) in Ge layers. This has led to studies on the effect
of annealing on the crystallinity and lattice strain of Ge layers.
After low-temperature epitaxial growth of Ge layers at 420°C, the
temperature was increased to the annealing temperature in the
same H2 atmosphere as that during the epitaxial growth, and the
Ge layers were then annealed at various temperatures for 10 min.
XRD-RSMs of Ge layers annealed at a temperature (T GR/AN ) of
700°C after the low-temperature epitaxial growth are shown in
Figure 9B. The Ge (-1-13) diffraction peaks became much steeper
and the peak intensity increased when the annealing temperature

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FIGURE 9 | XRD-RSM of (-1-13) diffraction from Ge layer grown on Si
substrate, (A) after low-temperature epitaxial growth and (B) after
post-annealing.

FIGURE 10 | Photoluminescence spectra from Ge layers annealed with
different temperatures are shown. Peak wavelength of
photoluminescence from Ge layers red-shifted as annealing temperature
increased, consistent with temperature induced tensile strain. Inset shows
lattice strain of Ge layers grown on Si substrate along <001> and <110>
crystal orientations as a function of annealing temperature. Dotted line
indicates lattice strain calculated with difference between thermal
expansion coefficients of Si and Ge.

was increased, indicating that the crystallinity of the Ge layers was
increased by the post-annealing.
The inset of Figure 10 shows the lattice strain in the Ge
layers in the <001> and <110> crystal orientations as a function of the annealing temperature. We used standard Si wafers

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Group IV light sources

for this experiment, so that <001> is perpendicular to the surface of the Ge film, while <110> is in the plane of Ge film.
The lattice strain in the <110> crystal orientation increased as
T GR/AN increased, and the strain in the <001> crystal orientation showed an opposite dependence. Although the Ge layer
contained a compressive strain in the <110> crystal orientation at T GR/AN = 420°C, i.e., without annealing, this strain started
decreasing when T GR/AN was increased, and the Ge was completely
un-strainedat T GR/AN = 530°C. Furthermore, the sign of the lattice strain changed from compressive to tensile after annealing at
T GR/AN > 530°C, and the tensile strain at T GR/AN = 700°C reached
0.19%. This result is consistent with previous studies (Cannon
et al., 2004). Normally, a grown layer with a larger lattice constant
compared to a substrate contains a compressive strain within the
growth plane. However, since the Ge layers grown on the Si substrate were almost completely relaxed even after low-temperature
growth, the Ge lattice could be dislocated at the Ge/Si interface by
post-annealing, and the lattice strain of the Ge layer was relaxed
during annealing at the relatively higher temperature with the
volumes of Ge and Si determined by the thermal expansion coefficients (Singh, 1968; Okada and Tokumaru, 1984). After annealing,
the volume of the Ge layer and the Si substrate both shrunk as
the temperature decreased, and there was barely any change to the
lattice alignment at low-temperature. The volume of the Si substrate returned to its original value because it was thick enough.
However, the volume of the Ge layer could not return due to its
larger thermal expansion coefficients. Therefore, the tensile lattice
strain remained only in the Ge layers after cooling (Cerdeira et al.,
1972). The ideal lattice strain in <110> crystal orientation was also
plotted in the inset of Figure 10, which was calculated with only
the difference of the thermal expansion coefficients between Si
and Ge, so these values indicate the maximum lattice strain. Since
there are large discrepancies between calculation and measured
values, it seems that relaxation ratio has a large effect on the lattice
strain even at the lower temperatures. PL spectra from the postannealed Ge layers with various annealing temperatures are shown
in Figure 10. Although Ge is an indirect bandgap material and the
L-valley has the lowest energy level in the conduction band, we
were able to observe recombination between electrons and holes
at the 0-valley as luminescence at a wavelength of 1,550 nm, even
from the bulk Ge (dashed line in Figure 10). A comparison with
the post-annealed Ge layers shows that although the spectrum was
very weak and broad for the as-grown Ge layer, an obvious peak
could be observed from annealed samples at T GR/AN > 530°C.
Moreover, the PL intensity increased and the peak shape became
sharper as the annealing temperature was increased. The PL spectrum is strongly affected by crystallinity, because non-radiative
recombination was significantly increased with defects such as
dislocation and stacking faults. Therefore, these results suggest
that the crystallinity of the Ge layers was improved by the postannealing. The peak was observed at a shorter wavelength from
the Ge layer annealed at 500°C compared with that from bulk Ge,
and a red shift of the PL peaks occurred after post-annealing at
a higher temperature. In addition, the peak wavelength from the
unstrained Ge was 1,550 nm, which is almost the same value as that
of the bulk Ge. These results show that the bandgap energy at the
0-point was varied by the lattice strain in the Ge layers (Cerdeira

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Saito et al.

et al., 1972; de Walle and Martin, 1986; de Walle, 1989), which
is consistent with the XRD measurements. These results indicate
that, in the range of this study, the most favorable PL characteristic
can be obtained from the Ge layer after post-annealing at higher
temperatures.

7.

GERMANIUM LIQUID-PHASE EPITAXY AND DEVICES
FOR PHOTONIC APPLICATION

Liquid-phase epitaxy (LPE) is a technique that was invented in
the 1960s (Nelson, 1963) and developed in the 1970s (Wieder
et al., 1977) for the fabrication of detectors, solar cells, LEDs
(Saul and Roccasecca, 1973), and laser diodes (Panish et al., 1970).
Originally used for III-V crystal growth, it has been adapted for
SiGe-on-insulator (SGOI) and Ge-on-Insulator (GOI) growth by
various groups (Liu et al., 2004; Tweet et al., 2005; Feng et al., 2008;
Hashimoto et al., 2009; Miyao et al., 2009; Ohta et al., 2011) and is
also referred to as rapid melt growth (RMG). The GOI technique
was pioneered by Liu et al. (2004) for Ge-on-insulator fabrication.
In this technique, a thin insulating layer is deposited on an Si substrate and patterned to open up seed windows. The target material,
in this case Ge, is deposited using a non-selective method and patterned to form the desired features. This is then encapsulated using
an insulating layer and heated up in a rapid-thermal-annealer
(RTA) in order to melt the Ge. The micro-crucible holds the melt in
place until the liquid epitaxial growth is complete. Upon cooling,
liquid-phase epitaxial growth starts from the seed and propagates
to the extremities of the strip structure. For the realization of single crystal Ge, epitaxial growth must proceed faster than unseeded
random nucleation, so that the crystal regrowth starting from the
seed is uninterrupted. Misfit dislocations arising at the SiGe interface in the seed area are necked down to the seed window as shown
in Figure 11. The RMG is limited to the growth of structures of the
order of around 3 µm in width and with a length of above 100 µm.
The limitation is largely due to the surface tension of the insulator
causing the Ge to form ball shapes while in the liquid phase.
RMG is very attractive for the heterogeneous integration of Gebased devices on insulator for electronics and photonics and has
been demonstrated for Gate all around P-MOSFET (Feng et al.,
2008), P-Channel FinFET (Feng et al., 2007), waveguide integrated
Ge/Si heterojunction photodiodes (Tseng et al., 2013), or Ge Gate
PhotoMOSFET (Going et al., 2014). These devices demonstrate

Group IV light sources

the possibility of using RMG to obtain high quality Ge crystalline
layers to create a bridge between electronic components and photonic components. This vision is clearly demonstrated by Going
et al. (2014) in a Ge Gate PhotoMOSFET (Carroll et al., 2012)
where a Ge-gated NMOS phototransistor is integrated on an Si
photonics platform on SOI substrate. The resulting device, with
1-µm channel length, and 8-µm channel width, demonstrates a
responsivity of over 18 A/W at 1550 nm with 583 nW of incident
light. By increasing the incident power to 912 µW, the device operates at 2.5 GHz. Ge RMG or LPE on Si is therefore a promising
technology for the fabrication of heterogeneous devices requiring
high quality Ge layers such as MOSFETs, near-infrared detectors
but also Ge-based lasers that are still to be demonstrated using this
specific process technique. In fact, a highly tensile strain of 0.4%
has successfully been applied to a Ge film grown by RMG process
(Matsue et al., 2014), which is quite promising for light emission.

8.

TIME-RESOLVED PHOTOLUMINESCENCE STUDY OF
GERMANIUM ON SILICON

The use of n-type tensile-strained Ge grown on Si substrates is one
promising way to realize an efficient light source for Si photonics through the enhanced direct recombination from the 0 valley.
However, the large lattice mismatch between Ge and Si inherently
causes misfit dislocations at the interface, and threading dislocations during the growth. Besides, epitaxially grown Ge is usually a
thin layer, so that both the interface and the surface become important. Therefore, investigation of the excess carrier lifetime is crucial
for the realization of efficient light-emitting devices. Recently,
the excess carrier dynamics of thin Ge film grown on either Si
or SOI substrates have been investigated by time-resolved photoluminescence (Kako et al., 2012), microwave photoconductive
decay (Sheng et al., 2013), and pump-probe transmission (Geiger
et al., 2014) methods. Here, we present the time-resolved photoluminescence study of both non-doped and n-type Ge samples
grown on Si.
The Ge samples were epitaxially grown on (100) Si substrates by
using a cold-wall rapid thermal chemical vapor deposition system
(Oda et al., 2014). There were two primary growth steps. The first
step was the growth of an intrinsic Ge thin layer (≈100 nm) at low
temperature followed by an annealing process. The second step
was the regrowth of Ge on the first layer with another annealing

FIGURE 11 | Transmission-electron-microscope (TEM) image of a high quality single crystalline Ge-on-insulator obtained using RMG. It can clearly be
seen that the misfit dislocations from the lattice mismatch are confined to the seed region and that the crystalline Germanium lateral overgrowth is free from
defects.

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Group IV light sources

process. In situ n-type doping was carried out during the second
growth step by supplying phosphine. The Ge becomes biaxially
strained (≈0.15%) due to the difference of the thermal expansion
coefficients between Si and Ge. Time-resolved photoluminescence
measurements were performed using a time-correlated singlephoton counting method employing a superconducting singlephoton detector (SSPD) with a time resolution of about 50 ps. A
Ti:Sapphire pulsed laser was used as the excitation source (wavelength 710 nm, repetition rate 80 MHz, and pulse-duration 100 fs).
The laser beam was focused on the sample surface using an objective lens. The photoluminescence from the samples was collected
by the same objective and focused on to an optical fiber connected
to the SSPD. Photoluminescence ranging from 1.2 to 1.8 µm was
detected.
Figure 12A shows a time-resolved photoluminescence decay
curve measured from a nominally undoped Ge sample (thickness
500 nm). In order to limit the effects of lateral diffusion, the laser
spot size was set to ≈10 µm. The decay is a single exponential with
a lifetime of 1 ns, which corresponds to the excess carrier lifetime
of 2 ns. Germanium has an indirect bandgap, and as such, its excess
carrier dynamics are determined by non-radiative recombination
processes, such as Shockley-Read Hall (SRH) recombination and
surface recombination processes. The photoluminescence decay
lifetime, τ PL , of undoped Ge is then related to the excess carrier
lifetime, τ ex , as 2τ PL = τ ex . The lifetime of excess carriers τ ex of
an indirect semiconductor film depends on the thickness and can
be represented by Sproul (1994) and Gaubas and Vanhellemont
(2006) as:
1
1
=
+
τex
τB

1
d
2S

+

d2
π 2D

,

(6)

where τ B is the bulk lifetime, S is the surface recombination velocity, D is the ambipolar diffusion constant, and d is the layer

thickness. The excess carrier lifetimes obtained for undoped Ge
layers with different thicknesses (filled black circles) are shown in
the inset of Figure 12A together with the black curve, which is a
fit to the data using equation (6) with parameters of τ B = 3.5 ns,
S = 5.5 × 103 cm/s, and D = 30 cm2 /s (The ambipolar diffusion
constant Da could be estimated by changing the spot size and measuring the photoluminescence decay time). Both SRH bulk recombination and the surface recombination processes determine the
excess carrier dynamics in our undoped Ge samples.
Figure 12B shows time-resolved photoluminescence decay
curves measured at two different excitation power densities from
an n-type Ge sample (thickness 500 nm, doping concentration
7 × 1019 cm−3 ). The measured decay depends on both the excitation power density and time (in contrast to those measured
from undoped samples, which are independent of the excitation power). The instantaneous lifetime (that measured at a
particular point during the decay) depends on the photoluminescence intensity, and thus the excess carrier density. Based
on the SRH non-radiative recombination model, the lifetime of
excess carriers depends on their density (Linnros, 1998). This
dependence can be simplified to τ hl = τ n + τ p (τ ll = τ p for ntype doping) in the two extreme conditions where the carrier
density is high (low) when compared to the doping concentration (τ n and τ p are the inverse capture rates of the electrons
and holes, respectively). The photoluminescence lifetime can be
expressed as 2τ PL = τ hl (high excess carrier density) and τ PL = τ ll
(low excess carrier density). Therefore, from our measurements,
we estimate τ ll = 0.14 ns, τ hl = 0.8 ns based on SRH theory. The
estimated τ hl value is shorter than those found from the undoped
samples. This difference might be attributed to an increased
dislocation density introduced by the doping, but the estimation of τ hl could be underestimation because the Auger process
becomes important for doped samples (Gaubas and Vanhellemont, 2006). Further investigation is needed in order to obtain
a better understanding.

9.

FIGURE 12 | (A) Time-resolved PL curve for an undoped Ge sample. The
inset shows the measured excess carrier lifetimes for two Ge thicknesses
with simulated lifetimes using the equations shown in the text.
(B) Time-resolved PL curves of an n-type sample for two excitation powers,
150 kW/cm2 (black line) and 15 kW/cm2 (red line).

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ELECTRO-LUMINESCENCE FROM GERMANIUM

Realization of monolithic light sources compatible with the existing Si photonics platform is one of the most difficult challenges.
Ge has attracted much attention as for possible future monolithic
light sources owing to its emission wavelengths of ~1.6 µm suitable
for an Si-based WG, in addition to the CMOS compatibility and
the pseudo-direct band-gap character (Menéndez and Kouvetakis,
2004; Liu et al., 2007, 2012; Liang and Bowers, 2010; Michel et al.,
2010; Boucaud et al., 2013; Liu, 2014). Recently, laser operation
from Ge pumped optically (Liu et al., 2010) and electrically (Cheng
et al., 2007; Camacho-Aguilera et al., 2012) has been reported.
However, there is no report so far to reproduce their results. The
optical gain from Ge is also achieved by the tensile-stress engineering (de Kersauson et al., 2011). The precise nature of the optical
gain in Ge is still controversial (Carroll et al., 2012), but the high
crystalline quality of Ge is one of the most critical factor to avoid
non-radiative recombinations at dislocations. It is confirmed by
several groups (Michel et al., 2010; Liu et al., 2012; Boucaud et al.,
2013; Liu, 2014) that the primary challenges for engineering Ge
as an active layer are: (i) crystallinity, (ii) high n-type doping, (iii)
tensile strain, as confirmed theoretically (Suwa and Saito, 2010,

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Saito et al.

2011; Virgilio et al., 2013a,b). Here, we review some of the Ge light
sources developed on SOI substrates.
9.1.

DEVICE STRUCTURE AND FABRICATION PROCESS

As we discussed in section for Si light sources, lateral carrier injection is a natural choice for electrical pumping, since fabrication
processes are based on planar CMOS technologies. We show several candidates for Ge light sources suitable for lateral carrier
injection in Figure 13.
Figures 13A,E,I show schematic views and a transmissionelectron-microscope (TEM) image of a Ge FinLED (Saito et al.,
2011a), which uses Ge fins as MQWs embedded in Si3 N4 WG.
Ge fins were fabricated by the oxidation condensation technique
(Tezuka et al., 2009) applied to SiGe fins (Saito et al., 2011a). Relatively, high crystallinity is expected in Ge fins, since the lattice
mismatch between Si and Ge would be relaxed by stretching the
fins during the oxidation (Saito et al., 2011a). In fact, the low dark
current density of 1.86 × 10−5 A/cm−2 at a reverse bias of 1 − V
and the strong breakdown current density of >1 MA/cm−2 were
confirmed (Saito et al., 2011a).
In order to enhance the overlap between an optical mode
and fins, Ge fins with (111) orientation at the sidewall were also
developed (Tani et al., 2012), as shown in Figures 13B,F,J. To
improve the patterning accuracy, Si (111) fins were fabricated
by anisotropic wet etching, and n-Ge was re-grown after the
condensation oxidation of SiGe fins (Tani et al., 2011).
Further increase of the coupling is realized by using a bulk
Ge WG (Liu et al., 2007; Camacho-Aguilera et al., 2012; Tani et al.,
2013a,b), as shown in Figures 13C,G,K for schematic views and the
scanning electron microscope (SEM) image, rather than using Ge
QW or Ge fins. The p- and n-type diffusion regions were formed
in the 40 nm-thick SOI layer, and the Ge waveguide with 500nm width and 500-µm length was directly grown on the SOI
diode. The SOI thickness was designed to minimize the optical

Group IV light sources

loss due to free carrier absorption in the diffusion electrodes. The
Ge waveguide was doped with 1 × 1019 cm−3 of phosphorus, and
the surface of the Ge waveguide was then passivated with GeO2
formed by low-temperature oxidation to reduce interfacial traps
(Tani et al., 2012, 2013a). Then, metal electrodes were made on
both diffusion regions.
To enhance light emission efficiency from Ge by tensile stress,
several techniques have been developed, e.g., the use of the thermal
expansion of relaxed Ge grown on Si (Ishikawa et al., 2003), the
growth on buffer layers with larger lattice parameter (Huo et al.,
2011), the mechanical deformation using membrane structures
(Kurdi et al., 2010), the stress concentration in a membrane structure (Nama et al., 2013), and using external stressors (Ortolland
et al., 2009; Ghrib et al., 2013). Considering the process compatibility to the lateral carrier injection, the Si3 N4 film with the tensile
stress of 250-MPa was employed (Tani et al., 2013a), as shown in
Figures 13D,H,L.

9.2.

IMPACT OF STRESS ENGINEERING FOR LATERAL GERMANIUM
ON SILICON DIODE

Figure 14A shows EL spectra of the Ge waveguide with 500-nm
width and 500-µm length taken from the top of the substrate
under continuous current injection of 60-mA. EL peak wavelength of the device with an SiN stressor is slightly longer than
that without the SiN stressor due to the tensile strain-induced
band-gap shrinkage, although the exact band-gap energy cannot be quantitatively estimated due to the additional peak shifts
caused by heating under high currents. Moreover, as shown in
Figure 14B, the peak intensity of the EL of the device with
SiN stressor is 1.65 times larger than that without SiN stressors.
Figure 14C shows two-dimensional stress mapping calculated by
a finite element modeling of the Ge waveguide on the Si substrate covered by Si3 N4 stressor. The tensile stress of 100 MPa

FIGURE 13 | Development of a Ge light source. (A) i -Ge FinLED, (B) n-Ge FinLED, (C) n-Ge-WG-on-Si LED without SiN, and (D) n-Ge-WG-on-Si LED with SiN.
(A–D) Cross section, (E–H) plan views, and (I–L) microscope images.

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Saito et al.

Group IV light sources

FIGURE 14 | Strain engineering for n-Ge-WG-on-Si LED. (A) Spectra and (B) integrated intensity from experiments. (C) Stress mapping simulation.

is localized on the side wall of the Ge waveguide, while the inplane compressive stress of 40 MPa exists on the top part of the
Ge waveguide. The increase of the light emission efficiency was
22% caused by the tensile stress, after subtracting of the additional increase of 35% caused by the light extraction efficiency
due to the reduced reflectance at the surface of the Ge waveguide
by the 500 nm-thick Si3 N4 layer (Tani et al., 2013a). Therefore, the
stress engineering by Si3 N4 is an appropriate option to improve
the performance of Ge light sources. Recently, there are significant
advances in stress engineering by manipulating free-standing Ge
structures (Jain et al., 2012; Boztug et al., 2013; Süess et al., 2013;
Sukhdeo et al., 2014), and enhanced direct recombination has been
achieved.

10.

CONCLUSION AND FUTURE OUTLOOK

In this paper, we reviewed the recent progress on the developments
of silicon and germanium light sources. There are many process
options to fabricate silicon- and germanium-based nanostructures
by using modern silicon technologies. For active materials, planar silicon single-quantum-well (Saito et al., 2006a,b, 2008, 2009;
Hoang et al., 2007; Noborisaka et al., 2011; Saito, 2011) or multiplequantum-wells made of silicon or germanium fins (Saito et al.,
2011a,b) can be used. To enhance the recombination rates and
the extraction efficiencies, photonic crystal structures have been
introduced (Fujita et al., 2008; Nakayama et al., 2010a; Iwamoto
and Arakawa, 2012). The further increase of the efficiency can
be achieved by introducing tensile strain and n-type doping of
the germanium (Ishikawa et al., 2003; Menéndez and Kouvetakis,
2004; Liu et al., 2007, 2012; Kurdi et al., 2010; Michel et al., 2010;
Huo et al., 2011; Boucaud et al., 2013; Ghrib et al., 2013; Nama
et al., 2013; Tani et al., 2013a; Liu, 2014).
Considering the success of the laser operation using the bulk
germanium waveguides (Liu et al., 2010; Camacho-Aguilera et al.,
2012), the next step will be to reduce the threshold current for
pumping. It is critical to develop a process technology to fabricate
a high crystalline quality germanium quantum-well compatible
with the silicon photonics platform. If practical silicon or germanium laser diodes are available in the future, these group IV
lasers will realize the convergence of electronics and photonics on
a silicon chip.

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ACKNOWLEDGMENTS
We would like to thank research collaborators, engineers, and
line managers in Hitachi, the University of Tokyo, and University of Southampton for supporting this project. We are
also grateful to Prof. H. N. Rutt for his careful reading of
the manuscript and constructive comments. Funding: parts of
the studied discussed here was supported by Japan Society for
the Promotion of Science (JSPS) through its “Funding Program for World-Leading Innovation R&D on Science and Technology (FIRST Program),” the Project for Developing Innovation Systems, and Kakenhi 216860312, MEXT, Japan. This work
is also supported by EU, FP7, Marie-Curie, Carrier Integration Grant (CIG), PCIG13-GA-2013-618116, and University of
Southampton, Zepler Institute, Research Collaboration Stimulus Fund.

REFERENCES
Arakawa, Y., Iwamoto, S., Nomura, M., Tandaechanurat, A., and Ota, Y. (2012).
Cavity quantum electrodynamics and lasing oscillation in single quantum dotphotonic crystal nanocavity coupled systems. IEEE J. Selec. Top. Quant. Elec. 18,
1818–1829. doi:10.1109/JSTQE.2012.2199088
Arakawa, Y., Nakamura, T., and Urino, Y. (2013). Silicon photonics for next generation system integration platform. Commun. Mag. IEEE 51, 72–77. doi:10.1109/
MCOM.2013.6476868
Arakawa, Y., and Sakaki, H. (1982). Multidimensional quantum well laser and temperature dependence of its threshold current. Appl. Phys. Lett. 40, 939–941.
doi:10.1063/1.92959
Boucaud, P., Kurdi, M. E., Ghrib, A., Prost, M., de Kersauson, M., Sauvage, S.,
et al. (2013). Recent advances in germanium emission. Photon. Res. 1, 102–109.
doi:10.1364/PRJ.1.000102
Boztug, C., Sanchez-Perez, J. R., Yin, J., Lagally, M. G., and Paiella, R. (2013).
Graiting-coupled mid-infrared light emission from tensile strained germanium
nanomembranes. Appl. Phys. Lett. 103, 201114. doi:10.1002/smll.201201090
Camacho-Aguilera, R. E., Cai, Y., Patel, N., Bessette, J. T., Romagnoli, M., Kimerling,
L. C., et al. (2012). An electrically pumped germanium laser. Opt. Express 20,
11316–11320. doi:10.1364/OE.20.011316
Canham, L. T. (1990). Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers. Appl. Phys. Lett. 57, 1046–1048.
doi:10.1063/1.103561
Cannon, D. D., Liu, J., Ishikawa, Y., Wada, K., Danielson, D. T., Jongthammanurak, S., et al. (2004). Tensile strained epitaxial Ge film on Si(100) substrate with
potential application to L-band telecommunications. Appl. Phys. Lett. 84, 906.
doi:10.1063/1.1645677
Carroll, L., Friedli, P., Neunschwander, S., Sigg, H., Cecchi, S., Isa, F., et al. (2012).
Direct-gap gain and optical absorption in germanium correlated to the density

September 2014 | Volume 1 | Article 15 | 77

Saito et al.

of photoexcited carriers, doping, and strain. Phys. Rev. Lett. 109, 057402.
doi:10.1103/PhysRevLett.109.057402
Cerdeira, F., Buchenauer, C. J., Pollak, F. H., and Cardona, M. (1972). Stress-induced
shifts of first-order Raman frequencies of diamond- and zinc-blende-type semiconductors. Phys. Rev. B 5, 580–593. doi:10.1103/PhysRevB.5.580
Cheng, T. H., Kuo, P. S., Lee, C. T., Liao, M., Hung, T. A., and Liu, C. W. (2007).
“Electrically pumped Ge laser at room temperature,” in IEEE Int. Conf. Electron
Devices Meeting (IEDM), Washington, 659–662.
Cluzel, B., Pauc, N., Calvo, V., Charvolin, T., and Hadji, E. (2006a). Nanobox array
for silicon-on-insulator luminescence enhancement at room temperature. Appl.
Phys. Lett. 88, 133120. doi:10.1063/1.2191089
Cluzel, B., Calvo, V., Charvolin, T., Picard, E., Noé, P., and Hadji, E. (2006b). Singlemode room-temperature emission with a silicon rod lattice. Appl. Phys. Lett. 89,
201111. doi:10.1063/1.2364876
Cullis, A. G., Canham, L. T., and Calcott, P. D. J. (1997). The structural and luminescence properties of porous silicon. J. Appl. Phys. 82, 909–965. doi:10.1063/1.
366536
Daldosso, N., and Pavesi, L. (2009). Nanosilicon photonics. Laser Photon. Rev. 3,
508–534. doi:10.1002/lpor.200810045
de Kersauson, M., Kurdi, M. E., David, S., Checoury, X., Fishman, G., Sauvage, S.,
et al. (2011). Optical gain in single tensile-strained germanium photonic wire.
Opt. Express 19, 17925–17934. doi:10.1364/OE.19.017925
de Walle, C. G. V. (1989). Band lineups and deformation potentials in the modelsolid theory. Phys. Rev. B 39, 1871. doi:10.1103/PhysRevB.39.1871
de Walle, C. G. V., and Martin, R. M. (1986). Theoretical calculations of heterojunction discontinuities in the Si/Ge system. Phys. Rev. B 34, 5621–5634.
doi:10.1103/PhysRevB.34.5621
Deen, M. J., and Basu, P. K. (2012). Silicon Photonics Fundamentals and Devices. West
Sussex: Wiley.
Escalante, J. M., and Martínez, A. (2012). Theoretical study about the gain in indirect bandgap semiconductor optical cavities. Physica B Condens. Matter 407,
2044–2049. doi:10.1016/j.physb.2012.02.002
Escalante, J. M., and Martínez, A. (2013). Optical gain by simultaneous photon
and phonon confinement in indirect bandgap semiconductor acousto-optical
cavities. Opt. Quant. Electron. 45, 1045–1056. doi:10.1007/s11082-013-9715-z
Evans, P., Fisher, M., Malendevich, R., James, A., Studenkov, P., Goldfarb, G., et al.
(2011). “Multi-channel coherent PM-QPSK InP transmitter photonic integrated
circuit (PIC) operating at 112 gb/s per wavelength,” in Optical Fiber Communication Conference (OFC) (Los Angeles: (IEEE) PDPC, OSA).
Fang, A. W., Park, H., Cohen, O., Jones, R., Paniccia, M. J., and Bowers, J. E. (2006).
Electrically pumped hybrid AlGaInAs-silicon evanescent laser. Opt. Express 14,
9203–9210. doi:10.1364/OE.14.009203
Fathpour, S., and Jalali, B. (eds) (2012). Silicon Photonics for Telecommunications and
Biomedicine. Boca Raton: CRC Press.
Feng, J., Thareja, G., Kobayashi, M., Chen, S., Poon, A., Bai, Y., et al. (2008). Highperformance gate-all-around GeOI p-MOSFETs fabricated by rapid melt growth
using plasma nitridation and ALD Al2 O3 gate dielectric and self-aligned NiGe
contacts. IEEE Electron Device Lett. 29, 805–807. doi:10.1109/LED.2008.2000613
Feng, J., Woo, R., Chen, S., Liu, Y., Griffin, P. B., and Plummer, J. D. (2007). P-channel
germanium FinFET based on rapid melt growth. IEEE Electron Device Lett. 28,
637–639. doi:10.1109/LED.2007.899329
Fischetti, M. V., and Laux, S. E. (1996). Band structure, deformation potentials,
and carrier mobility in strained Si, Ge, and SiGe alloys. J. Appl. Phys. 80, 2234.
doi:10.1063/1.363052
Fujita, M., Tanaka, Y., and Noda, S. (2008). Light emission from silicon in
photonic crystal nanocavity. IEEE J. Selec. Top. Quant. Elec. 14, 1090–1097.
doi:10.1109/JSTQE.2008.918941
Fukatsu, S., Usami, N., Chinzei, T., Shiraki, Y., Nishida, A., and Nakagawa, K. (1992).
Electroluminescence from strained SiGe/Si quantum well structures grown by
solid source si molecular beam epitaxy. Jpn. J. Appl. Phys. 31, L1015–L1017.
doi:10.1143/JJAP.31.L1015
Gaubas, E., and Vanhellemont, J. (2006). Dependence of carrier lifetime in germanium on resistivity and carrier injection level. Appl. Phys. Lett. 89, 142106.
doi:10.1063/1.2358967
Geiger, R., Frigerio, J., Süess, M. J., Chrastina, D., Isella, G., Spolenak, R., et al.
(2014). Excess carrier lifetimes in Ge layers on Si. Appl. Phys. Lett. 104, 062106.
doi:10.1063/1.4865237

Frontiers in Materials | Optics and Photonics

Group IV light sources

Gelloz, B., Kojima, A., and Koshida, N. (2005). Highly efficient and stable luminescence of nanocrystalline porous silicon treated by high-pressure water vapor
annealing. Appl. Phys. Lett. 87, 031107. doi:10.1063/1.2001136
Gelloz, B., and Koshida, N. (2000). Electroluminescence with high and stable quantum efficiency and low threshold voltage from anodically oxidized thin porous
silicon diode. J. Appl. Phys. 88, 4319–4324. doi:10.1063/1.1290458
Ghrib, A., Kurdi, M. E., de Kersauson, M., Prost, M., Sauvage, S., Checoury, X., et al.
(2013). Tensile-strained germanium microdisks. Appl. Phys. Lett. 102, 221112.
doi:10.1063/1.4809832
Going, R. W., Loo, J., Liu, T.-J. K., and Wu, M. C. (2014). Germanium gate PhotoMOSFET integrated to silicon photonics. IEEE J. Selec. Top. Quant. Elec. 20,
8201607. doi:10.1109/JSTQE.2013.2294470
Gong, Y., Makarova, M., Selçuk Yerci, R. L., Stevens, M. J., Baek, B., Nam, S. W.,
et al. (2010). Linewidth narrowing and Purcell enhancement in photonic crystal
cavities on an Er-doped silicon nitride platform. Opt. Express 18, 2601–2612.
doi:10.1364/OE.18.002601
Gunn, C. (2006). Cmos photonics for high-speed interconnects. Micro IEEE 26,
58–66. doi:10.1109/MM.2006.32
Hashimoto, T., Yoshimoto, C., Hosoi, T., Shimura, T., and Watanabe, H. (2009). Fabrication of local Ge-on-insulator structures by lateral liquid-phase epitaxy: effect
of controlling interface energy between Ge and insulators on lateral epitaxial
growth. Appl. Phys. Express 2, 066502. doi:10.1143/APEX.2.066502
Hauke, N., Zabel, T., Müller, K., Kaniber, M., Laucht, A., Bougeard, D., et al.
(2010). Enhanced photoluminescence emission from two-dimensional silicon
photonic crystal nanocavities. New J. Phys. 98, 053005. doi:10.1088/1367-2630/
12/5/053005
Hisamoto, D., Lee, W., Kedzierski, J., Takeuchi, H., Asano, K., Kuo, C., et al. (2000).
FinFET-a self-aligned double-gate MOSFET scalable to 20 nm. IEEE Trans. Electron Devices 47, 2320–2325. doi:10.1109/16.887014
Hoang, T., LeMinh, P., Holleman, J., and Schmitz, J. (2007). Strong efficiency
improvement of SOI-LEDs through carrier confinement. IEEE Electron Device
Lett. 28, 383–385. doi:10.1109/LED.2007.895415
Huo, Y., Lin, H., Chen, R., Makarova, M., Rong, Y., Li, M., et al. (2011). Strong
enhancement of direct transition photoluminescence with highly tensilestrained Ge grown by molecular beam epitaxy. Appl. Phys. Lett. 98, 011111.
doi:10.1063/1.3534785
Ishikawa, Y., and Wada, K. (2010). Germanium for silicon photonics. Thin Solid
Films 518, S83–S87. doi:10.1016/j.tsf.2009.10.062
Ishikawa, Y., Wada, K., Cannon, D. D., Liu, J., Luan, H.-C., and Kimerling, L. C.
(2003). Strain-induced band gap shrinkage in Ge grown on Si substrate. Appl.
Phys. Lett. 82, 2044. doi:10.1063/1.1564868
Ishikawa, Y., Wada, K., Liu, J., Cannon, D. D., Luan, H.-C., Michel, J., et al. (2005).
Strain-induced enhancement of near-infrared absorption in Ge epitaxial layers
grown on Si substrate. J. Appl. Phys. 98, 013501. doi:10.1063/1.1943507
Iwamoto, S., and Arakawa, Y. (2012). Enhancement of light emission from silicon
by utilizing photonic nanostructures. IEICE Trans. Electron E95-C, 206–212.
doi:10.1587/transele.E95.C.206
Iwamoto, S., Arakawa, Y., and Gomyo, A. (2007). Observation of enhanced photoluminescence from silicon photonic crystal nanocavity at room temperature. Appl.
Phys. Lett. 91, 211104. doi:10.1063/1.2816892
Jain, J. R., Hryciw, A., Baer, T. M., Miller, D. A. B., Brongersma, M. L., and Howe, R.
T. (2012). A micromachining-based technology for enhancing germanium lightemission via tensile strain. Nat. Photonics 6, 398–405. doi:10.1038/nphoton.2012.
111
Jannopoulos, J. D., and Winn, R. D. M. J. N. (1995). Photonic Crystals. Princeton,
NJ: Princeton University Press.
Kageshima, H., and Shiraishi, K. (1997). Momentum-matrix-element calculation
using pseudopotentials. Phys. Rev. B 56, 14985. doi:10.1103/PhysRevB.56.14985
Kako, S., Okumura, T., Oda, K., Suwa, Y., Saito, S., Ido, T., et al. (2012). “Timeresolved photoluminescence study of highly n-doped germanium grown on silicon,” in The 9th International Conference on Group IV Photonics (GFP), Vol. 7,
San Diego, 340–342.
Kittler, M., Reiche, M., and Arguirov, T. (2013). “1.55 µm light emitter based on
dislocation d1-emission in silicon,” in Microelectronics Technology and Devices
(SBMicro), Symposium on IEEE, Curitiba.
Koshida, N., and Koyama, H. (1992). Visible electroluminescence from porous silicon. Appl. Phys. Lett. 60, 347–349. doi:10.1063/1.106652

September 2014 | Volume 1 | Article 15 | 78

Saito et al.

Kurdi, M. E., Bertin, H., Martincic, E., de Kersauson, M., Fishman, G., Sauvage,
S., et al. (2010). Control of direct band gap emission of bulk germanium by
mechanical tensile strain. Appl. Phys. Lett. 96, 041909. doi:10.1063/1.3297883
Kurdi, M. E., Davida, S., Checourya, X., Fishmana, G., Boucauda, P., Kermarrecb,
O., et al. (2008). Two-dimensional photonic crystals with pure germanium-oninsulator. Opt. Commun. 281, 846–850. doi:10.1016/j.optcom.2007.10.008
Laasonen, K., Pasquarello, A., Car, R., Lee, C., and Vanderbilt, D. (1993). CarParrinello molecular dynamics with Vanderbilt ultrasoft pseudopotentials. Phys.
Rev. B 47, 10142. doi:10.1103/PhysRevB.47.10142
Liang, D., and Bowers, J. E. (2010). Recent progress in lasers on silicon recent progress
in lasers on silicon recent progress in lasers on silicon recent progress in lasers
on silicon. Nat. Photonics 4, 511–517. doi:10.1088/0034-4885/76/3/034501
Lim, P. H., Park, S., Ishikawa, Y., and Wada, K. (2009). Enhanced direct bandgap
emission in germanium by micromechanical strain engineering. Opt. Express
17, 16358–16365. doi:10.1364/OE.17.016358
Linnros, J. (1998). Carrier lifetime measurements using free carrier absorption transients. I. Principle and injection dependence. J. Appl. Phys. 84, 275.
doi:10.1063/1.368024
Liu, J. (2014). Monolithically integrated Ge-on-Si active photonics. Photonics 1,
162–197. doi:10.3390/photonics1030162
Liu, J., Camacho-Aguilera, R., Bessette, J. T., Sun, X., Wang, X., Cai, Y., et al. (2012).
Ge-on-Si optoelectronics. Thin Solid Films 520, 3354–3360. doi:10.1016/j.tsf.
2011.10.121
Liu, J., Cannon, D. D., Wada, K., Ishikawa, Y., Jongthammanurak, S., Danielson,
D. T., et al. (2005). Tensile strained Ge p-i-n photodetectors on Si platform for
C and L band telecommunications. Appl. Phys. Lett. 87, 011110. doi:10.1063/1.
2037200
Liu, J., Sun, X., Camacho-Aguilera, R., Kimerling, L. C., and Michel, J. (2010). Geon-Si laser operating at room temperature. Opt. Lett. 35, 679–681. doi:10.1364/
OL.35.000679
Liu, J., Sun, X., Pan, D., Wang, X., Kimerling, L. C., Koch, T. L., et al. (2007). Tensilestrained, n-type Ge as a gain medium for monolithic laser integration on Si. Opt.
Express 15, 11272–11277. doi:10.1364/OE.15.011272
Liu, Y., Deal, M. D., and Plummer, J. D. (2004). High-quality single-crystal Ge on
insulator by liquid-phase epitaxy on Si substrates. Appl. Phys. Lett. 84, 2563.
doi:10.1063/1.1691175
Lo Savio, R., Portalupi, S. L., Gerace, D., Shakoor, A., Krauss, T. F., O’Faolain, L., et al.
(2011). Room-temperature emission at telecom wavelengths from silicon photonic crystal nanocavities. Appl. Phys. Lett. 98, 201106. doi:10.1063/1.3591174
Luan, H.-C., Lim, D. R., Lee, K. K., Chen, K. M., Sandland, J. G., Wada, K., et al.
(1999). High-quality Ge epilayers on Si with low threading-dislocation densities.
Appl. Phys. Lett. 75, 2909. doi:10.1063/1.125187
Matsue, M., Yasutake, Y., Fukatsu, S., Hosoi, T., Shimura, T., and Watanabe, H.
(2014). Strain-induced direct band gap shrinkage in local Ge-on-insulator structures fabricated by lateral liquid-phase epitaxy. Appl. Phys. Lett. 104, 031106.
doi:10.1063/1.4862890
Menéndez, J., and Kouvetakis, J. (2004). Type-I Ge/Ge1−x−v Six Snv strainedlayer heterostructure with a direct Ge bandgap. Appl. Phys. Lett. 85, 1175.
doi:10.1063/1.1784032
Michel, J., Liu, J., and Kimerling, L. C. (2010). High-performance ge-on-si photodetectors. Nat. Photonics 4, 527–534. doi:10.1038/nphoton.2010.157
Michel, J., and Romagnoli, M. (2012). Germanium as the Unifying Material for Silicon
Photonics. Bellingham: SPIE. doi:10.1117/2.1201206.004285
Miller, D. A. B. (2009). Device requirements for optical interconnections to silicon
chips. Proc. IEEE 97, 1166–1185. doi:10.1109/JPROC.2009.2014298
Miyao, M., Tanaka, T., Toko, K., and Tanaka, M. (2009). Giant Ge-on-insulator formation by Si-Ge mixing-triggered liquid-phase epitaxy. Appl. Phys. Express 2,
045503. doi:10.1143/APEX.2.045503
Nakayama, S., Ishida, S., Iwamoto, S., and Arakawa, Y. (2012). Effect of cavity mode
volume on photoluminescence from silicon photonic crystal nanocavities. Appl.
Phys. Lett. 98, 171102.
Nakayama, S., Iwamoto, S., Ishida, S., and Arakawa, Y. (2010a). “Demonstration of
a silicon photonic crystal slab led with efficient electroluminescence,” in International Conference on Solid State Devices and Materials (SSDM 2010) D-4-3,
Tokyo.
Nakayama, S., Iwamoto, S., Ishida, S., Bordel, D., Augedre, E., Calvelier, L., et al.
(2010b). Enhancement pf photoluminescence from germanium by utilizing airbridge-type photonic crytsal slab. Physica E 42, 2556–2559. doi:10.1016/j.physe.
2010.05.026

www.frontiersin.org

Group IV light sources

Nakayama, S., Iwamoto, S., Kako, S., Ishida, S., and Arakawa, Y. (2011). “Demonstration of silicon nanocavity led with enhanced luminescence,” in International
Conference on Solid State Devices and Materials, Nagoya, I–8–2.
Nama, D., Sukhdeo, D. S., Kang, J.-H., Petykiewicz, J., Lee, J. H., Jung, W. S.,
et al. (2013). Strain-induced pseudoheterostructure nanowires confining carriers at room temperature with nanoscale-tunable band profiles. Nano Lett. 13,
3118–3123. doi:10.1021/nl401042n
Nassiopoulou, A. G. (2004). Silicon Nanocrystals in SiO 2 Thin Layers,Vol. 9. Valencia:
American Scientific Publishers, 793–813.
Nelson, H. (1963). Epitaxial growth from the liquid state and its application to the
fabrication of tunnel and laser diodes. RCA Rev. 24, 603–615.
Ng, W. L., Lourenço, M. A., Gwilliam, R. M., Ledain, S., Shao, G., and Homewood,
K. P. (2001). An efficient room-temperature silicon-based light-emitting diode.
Nature 410, 192–194. doi:10.1038/35069092
Ngo, T.-P., Kurdi, M. E., Checoury, X., Boucaud, P., Damlencourt, J. F., Kermarrec, O., et al. (2008). Two-dimensional photonic crystals with germanium on
insulator obtained by a condensation method. Appl. Phys. Lett. 93, 241112.
doi:10.1063/1.3054332
Noborisaka, J., Nishiguchi, K., Ono, Y., Kageshima, K., and Fujiwara, A. (2011).
Strong stark effect in electroluminescence from phosphorous-doped silicon-oninsulator metal-oxide-semiconductor field-effect transistors. Appl. Phys. Lett. 98,
033503. doi:10.1063/1.3543849
Oda, K., Tani, K., Saito, S.-I., and Ido, T. (2014). Improvement of crystallinity by
post-annealing and regrowth of Ge layers on Si substrates. Thin Solid Films 550,
509–514. doi:10.1016/j.tsf.2013.10.136
Ohta, Y., Tanaka, T., Toko, K., Sadoh, T., and Miyao, M. (2011). Growth-directiondependent characteristics of Ge-on-insulator by Si-Ge mixing triggered melting
growth. Solid State Electron. 60, 18–21. doi:10.1016/j.sse.2011.01.039
Okada, Y., and Tokumaru, Y. (1984). Precise determination of lattice parameter and
thermal expansion coefficient of silicon between 300 and 1500 K. J. Appl. Phys.
56, 314. doi:10.1063/1.333965
Ortolland, C., Okuno,Y.,Veheyen, P., Kerner, C., Stapelmann, C., Aoulaiche, M., et al.
(2009). Stress memorization technique-fundamental understanding and lowcost integration for advanced CMOS technology using a nonselective process.
IEEE Trans. Electron Devices 56, 1690–1697. doi:10.1109/TED.2009.2024021
Ossicini, S., Pavesi, L., and Priolo, F. (2006). Light Emitting Silicon for Microphotonics.
Berlin: Springer.
Panish, M. B., Hayashi, I., and Sumski, S. (1970). Double heterostructure injection
lasers with room temperature thresholds as low as 2300 a/cm2 . Appl. Phys. Lett.
16, 326. doi:10.1063/1.1653213
Pavesi, L., and Guillot, G. (eds) (2006). Optical Interconnects The Silicon Approach.
Berlin: Springer.
Pavesi, L., and Lockwood, D. J. (eds) (2004). Silicon Photonics. Berlin: Springer.
Pavesi, L., Negro, L. D., Mazzoleni, C., Franzò, G., and Priolo, F. (2000). Optical gain
in silicon nanocrystals. Nature 408, 440–444.
Perdew, J. P., Burke, K., and Wang, Y. (1996). Generalized gradient approximation
for the exchange-correlation hole of a many-electron system. Phys. Rev. B 54,
16533. doi:10.1103/PhysRevB.54.16533
Purcell, E. M. (1946). Spontaneous emission probabilities at radio frequencies. Phys.
Rev 69, 681.
Reed, G. (2008). Silicon Photonics the State of the Art. West Sussex: Wiley.
Reed, G. T., and Knights, A. P. (2004). Silicon Photonics. West Sussex: Wiley.
Rylyakov, A., Schow, C., Lee, B., Green, W., Campenhout, J. V., Yang, M., et al. (2011).
“A 3.9ns 8.9mW 4×4 silicon photonic switch hybrid integrated with CMOS
driver,” in Int. Solid-State Circuits Conference (ISSCC) (San Francisco: IEEE),
222–224.
Saito, S. (2011). “Silicon and germanium quantum well light-emitting diode,” in
IEEE 8th Int. Conf. Group IV Photonics, London, 166–168.
Saito, S., Hisamoto, D., Shimizu, H., Hamamura, H., Tsuchiya, R., Matsui, Y., et al.
(2006a). Electro-luminescence from ultra-thin silicon. Jpn. J. Appl. Phys. 45,
L679–L682. doi:10.1143/JJAP.45.L679
Saito, S., Hisamoto, D., Shimizu, H., Hamamura, H., Tsuchiya, R., Matsui, Y., et al.
(2006b). Silicon light-emitting transistor for on-chip optical interconnection.
Appl. Phys. Lett. 89, 163504. doi:10.1063/1.2360783
Saito, S., Oda, K., Takahama, T., Tani, K., and Mine, T. (2011a). Germanium fin
light-emitting diode. Appl. Phys. Lett. 99, 241105. doi:10.1364/OE.22.005927
Saito, S., Takahama, T., Tani, K., Takahashi, M., Mine, T., Suwa, Y., et al. (2011b).
Stimulated emission of near-infrared radiation in silicon fin light-emitting diode.
Appl. Phys. Lett. 98, 261104. doi:10.1063/1.3605255

September 2014 | Volume 1 | Article 15 | 79

Saito et al.

Saito, S., Sakuma, N., Suwa, Y., Arimoto, H., Hisamoto, D., Uchiyama, H., et al.
(2008). “Observation of optical gain in ultra-thin silicon resonant cavity lightemitting diode,” in IEEE Int. Conf. Electron Devices Meeting (IEDM), San Francisco, 19.5.
Saito, S., Suwa, Y., Arimoto, H., Sakuma, N., Hisamoto, D., Uchiyama, H., et al.
(2009). Stimulated emission of near-infrared radiation by current injection
into silicon (100) quantum well. Appl. Phys. Lett. 95, 241101. doi:10.1063/1.
3273367
Saul, R. H., and Roccasecca, D. D. (1973). Vapor-doped multislice LPE for efficient
GaP green LED’s. J. Electrochem. Soc. 120, 1128–1131. doi:10.1149/1.2403644
Savio, R. L., Miritello, M., Shakoor, A., Cardile, P., Welna, K., Andreani, L. C., et al.
(2013). Enhanced 1.54 µm emission in Y-Er disilicate thin films on silicon photonic crystal cavities. Opt. Express 21, 10278–10288. doi:10.1364/OE.21.010278
Shakoor, A., Lo Savio, R., Cardile, P., Portalupi, S. L., Gerace, D., Welna, K.,
et al. (2013). Room temperature all-silicon photonic crystal nanocavity light
emitting diode at sub-bandgap wavelengths. Laser Photon. Rev. 7, 114–121.
doi:10.1002/lpor.201200043
Sheng, J. J., Leonhardt, D., Han, S. M., Johnston, S. W., Cederberg, J. G., and Carroll,
M. S. (2013). Empirical correlation for minority carrier lifetime to defect density
profile in germanium on silicon grown by nanoscale interfacial engineering. J.
Vac. Sci. Technol. B Microelectron. 31, 051201. doi:10.1116/1.4816488
Singh, H. P. (1968). Determination of thermal expansion of germanium,
rhodium and iridium by x-rays. Acta Crystallogr. A 24, 469–471. doi:10.1107/
S056773946800094X
Sproul, A. B. (1994). Dimensionless solution of the equation describing the effect
of surface recombination on carrier decay in semiconductors. J. Appl. Phys. 76,
2851. doi:10.1063/1.357521
Steger, M., Yang, A., Sekiguchi, T., Saeedi, K., Thewalt, M. L. W., Henry, M. O.,
et al. (2011). Photoluminescence of deep defects involving transition metals i Si: new insights from highly enriched 28 Si. J. Appl. Phys. 110, 081301.
doi:10.1063/1.3651774
Süess, M. J., Geiger, R., Minamisawa, R. A., Schiefler, G., Frigerio, J., Chrastina, D.,
et al. (2013). Analysis of enhanced light emission from highly strained germanium microbridges. Nat. Photonics 7, 466–472. doi:10.1038/nphoton.2013.67
Sukhdeo, D. S., Nam, D., Kang, J. H., Brongersma, M. L., and Saraswat, K. C. (2014).
Direct bandgap germanium-on-silicon inferred from 5.7% <100> uniaxial tensile strain. Photon. Res. 2, A8–A13. doi:10.1364/PRJ.2.0000A8
Sumikura, H., Kuramochi, E., Taniyama, H., and Notomi, M. (2014). Ultrafast spontaneous emission of copper-doped silicon enhanced by an optical nanocavity.
Sci. Rep. 4, 5040. doi:10.1038/srep05040
Suwa, Y., and Saito, S. (2009). Intrinsic optical gain of ultrathin silicon quantum
wells from first-principles calculations. Phys. Rev. B 79, 233308. doi:10.1103/
PhysRevB.79.233308
Suwa, Y., and Saito, S. (2010). “First-principles study of light emisson from germanium guantum-well,” in IEEE 22nd Int. Semiconductor Laser Conference (ISLC),
Kyoto, 131–132.
Suwa, Y., and Saito, S. (2011). “First-principles study of light emission from silicon and germanium due to direct transitions,” in IEEE 8th Int. Conf. Group IV
Photonics, London, 222–224.
Sze, S. M., and Lee, M. K. (2012). Semiconductor Devices: Physics and Technology,
3rd Edn. Singapore: John Wiley and Sons.
Tani, K., Oda, K., Kasai, J., Okumura, T., Mine, T., Saito, S., et al. (2013a). “Germanium waveguides on lateral silicon-on-insulator diodes for monolithic light
emitters and photo detectors,” in Group IV Photonics (GFP), IEEE 10th International Conference (Seoul: IEEE), 134–135.
Tani, K., Oda, K., Okumura, T., Takezaki, T., Kasai, J., Mine, T., et al. (2013b).
“Enhanced electroluminescence from germanium waveguides by local tensile
strain with silicon nitride stressors,” in International Conference on Solid State
Devices and Materials (SSDM) (Fukuoka: SSDM), K–6–3.
Tani, K., Saito, S., Oda, K., Miura, M., Mine, T., Sugawara, T., et al. (2011). “Ge(111)fin light-emitting diodes,” in IEEE 8th Int. Conf. Group IV Photonics, London,
217–219.
Tani, K., Saito, S., Oda, S., Okumura, T., Mine, T., and Ido, T. (2012). “Lateral carrier injection to germanium for monolithic light sources,” in Group IV Photonics
(GFP), IEEE 9th Int. Conf, San Diego, 328–330.
Taur, Y., and Ning, T. H. (2013). Fundamentals of Modern VLSI Devices, 2nd Edn.
Cambridge: Cambridge University Press.
Tezuka, T., Toyoda, E., Irisawa, T., Hirashita, N., Moriyama, Y., Sugiyama, N.,
et al. (2009). Structural analyses of strained SiGe wires formed by hydrogen

Frontiers in Materials | Optics and Photonics

Group IV light sources

thermal etching and Ge-condensation processes. Appl. Phys. Lett. 94, 081910.
doi:10.1063/1.3086884
Tseng, C.-K., Chen, W.-T., Chen, K.-H., Liu, H.-D., Kang, Y., Na, N., et al. (2013).
A self-assembled microbonded germanium/silicon heterojunction photodiode
for 25 Gb/s high-speed optical interconnects. Sci. Rep. 3, 3225. doi:10.1038/
srep03225
Tweet, D. J., Lee, J. J., Maa, J.-S., and Hsu, S. T. (2005). Characterization and reduction of twist in Ge on insulator produced by localized liquid phase epitaxy. Appl.
Phys. Lett. 87, 141908. doi:10.1063/1.2077860
Uchida, K., and Takagi, S. (2003). Carrier scattering induced by thickness fluctuation
of silicon-on-insulator film in ultrathin-body metal–oxide–semiconductor fieldeffect transistors. Appl. Phys. Lett. 82, 2916. doi:10.1063/1.1571227
Ujihara, K. (1995). “Effects of atomic broadening on spontaneous emission in an
optical microcavity,” in Spontaneous Emission and Laser Oscillation in Microcavities, eds H. Yokoyama and K. Ujihara (Boca Raton, FL: CRC Press), 81–107.
Urino, Y., Horikawa, T., Nakamura, T., and Arakawa, Y. (2013). “High density optical
interconnects integrated with lasers, optical modulators and photodetectors on
a single silicon chip,” in Optical Fiber Communication Conference (OFC), (Anaheim: OSA), OM2J.
Vanderbilt, D. (1990). Soft self-consistent pseudopotentials in a generalized eigenvalue formalism. Phys. Rev. B 41, 7892(R). doi:10.1103/PhysRevB.41.7892
Virgilio, M., Manganelli, C. L., Grosso, G., Pizzi, G., and Capellini, G. (2013a).
Radiative recombination and optical gain spectra in biaxially strained n-type
germanium. Phys. Rev. B 87, 235313. doi:10.1103/PhysRevB.87.235313
Virgilio, M., Manganelli, C. L., Grosso, G., Schroeder, T., and Capellini, G. (2013b).
Photoluminescence, recombination rate, and gain spectra in optically excited
n-type and tensile strained germanium layers. J. Appl. Phys. 114, 243102.
doi:10.1063/1.4849855
Vivien, L., and Pavesi, L. (eds) (2013). Handbook of Silicon Photonics. Taylor and
Francis.
Wada, K., Liu, J., Jongthammanurak, S., Cannon, D. D., Danielson, D. T., Ahn, D.,
et al. (2006). Si Microphotonics for Optical Interconnection. Berlin: Springer Verlag.
Wale, M. (2008). “Photonic integration – an industrial perspective,” in European
Photonic Integration Forum (Brussels: European Photonics Integration Forum).
Wang, X., Li, H., Camacho-Aguilera, R., Cai, Y., Kimerling, L. C., Michel, J., et al.
(2013). Infrared absorption of n-type tensile-strained Ge-on-Si. Opt. Lett. 38,
652–654. doi:10.1364/OL.38.000652
Wang, Y., Zhang, J., Wu, Y., An, J., Li, J., Wang, H., et al. (2012). Light emission
enhancement from Er-doped silicon photonic crystal double-heterostructure
microcavity. IEEE Photon. Technol. Lett. 24, 1041–1135. doi:10.1109/LPT.2011.
2173183
Wieder, H. H., Clawson, A. R., and McWilliams, G. E. (1977). Inx Ga1−x Asv P1−v /InP
heterojunction photodiodes. Appl. Phys. Lett. 31, 468. doi:10.1063/1.89718
Zelsmann, M., Picard, E., Charvolin, T., Hadji, E., Heitzmann, M., Dalzotto, B.,
et al. (2003). Seventy-fold enhancement of light extraction from a defectless
photonic crystal made on silicon-on-insulator. Appl. Phys. Lett. 83, 2542–2544.
doi:10.1063/1.1614832
Zimmermann, H. (2000). Integrated Silicon Opto-Electronics. Berlin: Springer.
Conflict of Interest Statement: The authors declare that the research was conducted
in the absence of any commercial or financial relationships that could be construed
as a potential conflict of interest.
Received: 12 June 2014; paper pending published: 28 July 2014; accepted: 29 August
2014; published online: 17 September 2014.
Citation: Saito S, Gardes FY, Al-Attili AZ, Tani K, Oda K, Suwa Y, Ido T,
Ishikawa Y, Kako S, Iwamoto S and Arakawa Y (2014) Group IV light sources
to enable the convergence of photonics and electronics. Front. Mater. 1:15. doi:
10.3389/fmats.2014.00015
This article was submitted to Optics and Photonics, a section of the journal Frontiers
in Materials.
Copyright © 2014 Saito, Gardes, Al-Attili, Tani, Oda, Suwa, Ido, Ishikawa, Kako,
Iwamoto and Arakawa. This is an open-access article distributed under the terms of the
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Review
published: 15 July 2015
doi: 10.3389/fmats.2015.00052

Group iv direct band gap photonics:
methods, challenges, and opportunities
Richard Geiger, Thomas Zabel and Hans Sigg*
Laboratory for Micro- and Nanotechnology, Paul Scherrer Institut, Villigen, Switzerland

Edited by:
Koji Yamada,
National Institute of Advanced
Industrial Science and Technology,
Japan
Reviewed by:
Shinichi Saito,
University of Southampton, UK
Krishna C. Saraswat,
Stanford University, USA
*Correspondence:
Hans Sigg,
Laboratory for Micro- and
Nanotechnology, Paul Scherrer
Institut, Villigen PSI, CH 5232,
Switzerland
hans.sigg@psi.ch
Specialty section:
This article was submitted to Optics
and Photonics, a section of the
journal Frontiers in Materials
Received: 13 May 2015
Accepted: 29 June 2015
Published: 15 July 2015
Citation:
Geiger R, Zabel T and Sigg H (2015)
Group IV direct band gap photonics:
methods, challenges,
and opportunities.
Front. Mater. 2:52.
doi: 10.3389/fmats.2015.00052

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The concept of direct band gap group IV materials may offer a paradigm change for
Si-photonics concerning the monolithic implementation of light emitters: the idea is to
integrate fully compatible group IV materials with equally favorable optical properties
as the chemically incompatible group III–V-based systems. The concept involves either
mechanically applied strain on Ge or alloying of Ge with Sn, which permits to drastically
improve the radiative efficiency of Ge. The favorable optical properties result from a
modified band structure transformed from an indirect to a direct one. The first demonstration of such a direct band gap laser has recently been accomplished in GeSn. This
demonstration proves the capability of this new concept, which may permit a qualitative
as well as a quantitative expansion of Si-photonics in not only traditional but also new
areas of applications. This review aims to discuss the challenges along this path in terms
of fabrication, characterization, and fundamental understanding, and will elaborate on
evoking opportunities of this new class of group IV-based laser materials.
Keywords: Si photonics, germanium, strain, GeSn, direct band gap, laser

Introduction
The Si-based optical platform is rapidly changing the landscape of photonics by offering powerful
solutions, for example, for data links (Miller, 2010) and sensing (Passaro et al., 2012) to name only
two out of many. This development has taken place in spite of the fact that Si itself is a poor emitter of
light. This is without a doubt due to the fact that Si technology as used in very large-scale integration
(VLSI) and complementary metal-oxide-semiconductor (CMOS) technology is extremely mature
and advanced. This fact seemingly compensates for the shortfalls in concepts for Si to generate light.
Nowadays, group III–V materials are implemented to integrate active light sources onto the Si
platform by using involved coupling schemes and/or heterogeneous integration (Fang et al., 2013).
However, because these materials are chemically intolerant to Si, their integration bears a lot of
burdens, which raises the fabrication costs. Strongly preferred are materials that are compatible to Si,
tolerated by the technology (preferentially CMOS), and capable of producing light similar in efficiency
to traditional group III–V semiconductor systems.
In direct band gap systems, light generation is based on radiative recombination of electrons and
holes, both with practically the same momentum as schematically shown in Figure 1A. In unstrained,
i.e., “regular” bulk Ge, however, the excited electrons will preferentially occupy the lower conduction
band energy states of the L-valley. In Ge, the momentum of the electrons does, thus, not match those
of the holes, which occupy the degenerated heavy- and light-mass valence bands at the Γ-point (c.f.
Figure 1B). The appearing momentum mismatch requires a phonon for the recombination. But note
that except the position of the indirect L-valley, which is in Ge 140 meV below the Γ valley minimum,
the band alignments near the Γ-point in system A (say InGaAs, one of the most prominent group
III–V systems used for lasing) and B are very similar. To achieve the favorable direct recombination

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Geiger et al.

A
CONDUCTION
BAND

Group IV direct band gap photonics

B

direct
valley

Γ

C

direct
valley
Offset
0.14 eV

direct
valley

indirect
valley
L

Γ

indirect
valley
L

Γ

Offset
> kT
momentum
transfer

VALENCE
BANDS

heavy hole
light
hole

heavy hole

heavy hole
light
hole

light
hole

momentum space

FIGURE 1 | Band structure in momentum space (A) direct band gap
semiconductor. Electron/hole recombination occurs at the Γ-point.
(B) Unstrained Ge. Electrons occupy the conduction band states of the

L-valleys. Radiative recombination is hindered by the momentum mismatch.
(C) Due to strain (or alloying with Sn), the band gap of Ge (GeSn) shrinks and
the population of electrons at the Γ-point increases.

condition also for Ge, we need to find a way to inject electrons
into that conduction band valley with its energetic minimum at
the Γ-point. This is realized in the most straightforward fashion
when all unwanted electron levels are energetically shifted above
the Γ-states, which is equivalent to transfer the system from a
fundamentally indirect to a fundamentally direct one.
We will discuss the two methods that make this conversion
possible. One involves the application of tensile strain, while the
second approach relies on alloying Ge with Sn. Thus, the obtained
band alignments are depicted in Figure 1C. With either one of the
methods, the Γ-valley can be reduced below the indirect one at L
enabling efficient carrier injection into the Γ-valley. Moreover, the
VB degeneracy is lifted depending on the strain state and its loading, biaxial or uniaxial, c.f. Section “Modeling” for more details.
In Figure 2, we show the state-of-the-art of the strain and alloying approach toward the realization of a direct band gap group IV
material. For our discussion, we selected those approaches that are
potentially compatible with CMOS fabrication and are suited for
optical applications. Very thin membranes (Sánchez-Pérez et al.,
2011), nanowires clamped in bulky mechanical strain apparatus
(Greil et al., 2012), Ge bulk layers on III–V substrates (Huo et al.,
2011), etc., are not considered here because they are unpractical
for integration on Si. Not considered either is light emission from
Si-based quantum wells and defects; for a recent review, see Saito
et al. (2014). In our compilation, Figure 2, we benchmark the two
strain loadings (uniaxial and biaxial) and Sn alloy composition
against the achieved relative band offset, ΔE/E0, where an offset
ΔE of 100% is equal to E0 ~ 140 meV for the case of unstrained Ge.
An offset parameter of 0 meV (0%) corresponds, thus, to Γ- and
L-valleys having their band edges at the same energy.
The black arrow on the left hand side of the second line in
Figure 2 marks the case of highly n-doped Ge (Liu et al., 2010),
where a maximum of 0.25% biaxial strain is accomplished. This
value of 0.25% is the one typically obtained from direct epitaxy of
Ge on Si. It arises due to the difference between the thermal expansion coefficients of Si and Ge (Michel et al., 2010). High n-doping

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uniaxial
[100]
(%)

0

biaxial
(001)
(%)

0

GexSn1-x
(Sn%)

0
100

1

2

3

0.5

1.0

2
80

4
1.5

4
60

5

6
40

2.0
8

20

10
0 %

E/Eoffset (%)
FIGURE 2 | Relative offset of the Γ- and L-conduction band minima
realized in Ge either by uniaxial tensile strain along [100] direction
[dark blue: (Capellini et al., 2013), light blue: (Nam et al., 2013;
Sukhdeo et al., 2014), olive: (Süess et al., 2013; Geiger et al., 2014c)],
biaxial tensile strain on (001) oriented substrate [black: (Liu et al.,
2010); violet: (Ghrib et al., 2013, 2015)] or by alloying with Sn [yellow:
(Chen et al., 2013a); orange (Gupta et al., 2013b); red: (Wirths et al.,
2015)]. The shown offset versus Sn concentration relates to the unstrained
case. About 100% (0%) offset refers to 140 meV (vanishing energy offset).
Hence, the dash-dotted line marks the transition from an indirect to a direct
band gap semiconductor.

is introduced to fill the parasitic indirect states (Xiaochen et al.,
2010). Such doping does not transform the material into a direct
gap system, but it appeared that under optical excitation and
electrical pumping the light emission shows an intensity threshold
as well as linewidth narrowing (Liu et al., 2010; Camacho-Aguilera
et al., 2012). These results became widely known as the optically
and electrically pumped Ge-laser. However, since the time when
these announcements were made in 2010 and 2012, only one other
demonstration of these effects has been reported so far (Koerner
et al., 2015). This very recent and only result concerns a Ge diode
structure with an unstrained active region doped at 3 × 1019 cm−3.
The obtained emission spectra are similar to the one from the
original work from the MIT group. However, as we will show

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Group IV direct band gap photonics

below, these spectra significantly differ in several aspects – the
intensity, the linewidth, and the Fabry–Perot (FP) multi-mode
behavior – from those obtained with the here discussed direct band
gap lasers. Moreover, as will be discussed in Section “Lifetime,
Gain, and Loss,” these Ge-lasing observations are contradicted
by gain experiments (Carroll et al., 2012) as well as by theoretical
analysis performed by several groups (Liu et  al., 2007; Chow,
2012; Dutt et al., 2012; Peschka et al., 2015) when the lasing current density threshold is calculated using realistic non-radiative
lifetimes (Geiger et al., 2014a). As the experimental foundation
for understanding this peculiar threshold behavior of low strained
(and in one case even unstrained) Ge is ambiguous, we will focus
here on reports concerning direct band gap group IV systems,
which includes the first unmistakable proof of interband lasing in
a group IV system. Without doubt, with the advent of direct band
gap systems showing unambiguous lasing, an excellent opportunity
is created, which will help to unravel in the very near future above
raised questions regarding the lasing in highly n-doped Ge.
Coming back to Figure 2: the arrows colored in violet depict
1.0 and 1.5% biaxially tensilely strained structures that have been
achieved via deposition of Si–Nitride (SiN) stressor layers (Ghrib
et al., 2013, 2015). This strain is equivalent to a band offset of ~70
and 30 meV, which corresponds to 50 and 30% of the unstrained
band offset value, respectively. So far, the highest strain values
are obtained in suspended microbridges under uniaxial loading
as is shown on the top stroke. There, the ~0.25% biaxial prestrain
is enhanced and transformed into uniaxial strain. The arrows in
olive (Süess et al., 2013; Geiger et al., 2014c) and blue (Nam et al.,
2013; Sukhdeo et al., 2014) mark recent achievements from the two
leading groups. The latest result (Sukhdeo et al., 2014) indicates
that the bridge technology can indeed provide direct band gap
strained Ge. SiN stressor layers on suspended microbridges or
FP cavities deliver far less strain and offset reductions (Capellini
et al., 2013, 2014). As shown by the red arrows on the third stroke,
alloying Ge with Sn also provides optical group IV material with
a fundamental direct band gap. The transition from fundamental
indirect to direct occurs at a Sn concentration of ~9% for relaxed
GeSn. Depending on the strain loading, i.e., tensile or compressive,
the crossover shifts to a higher or lower Sn concentration. Hence,
a 20-nm thick GeSn layer with 8% Sn sandwiched between Ge
claddings and processed into microdisks is not as close to the
direct transition as a relaxed layer with 6% Sn because of the −1%
biaxial compressive loading (Chen et al., 2013a). The GeSn alloy
above the crossover in Figure 2 exhibits 0.7% in-plane strain at a Sn
concentration of 13%. This system shows lasing at low temperature
(Wirths et al., 2015). We will present this recent result and will,
thereby, clarify the characteristic of the experimental observation
of lasing.
The availability of direct band gap group IV semiconductors as
compiled in Figure 2, together with the rise of promising results,
in particular, the demonstration of lasing in the GeSn system, has
motivated the writing of this review. It is meant to present the
current understanding evoked from the research undertaken at
many places worldwide. Although some of the following descriptions are exemplified for only one of the systems (strain or Sn
alloying), we will argue that the physics of this two direct gap
systems can be understood by analogy. By merely emphasizing the

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similarities of the physics and the characterization methods used
for investigations, we hope to provide a comprehensive overview
that will support and interest many scientists to enter this highly
relevant field of research.
In Section “Direct Band Gap Group IV Materials,” the band
structure in Ge is given in dependence of strain. We then summarize the fabrication steps for strain engineering and Sn alloying.
In Section “Characterization Methods: Optical Properties,” several
optical characterization methods are introduced, such as pump and
probe spectroscopy developed for this very purpose at the infrared
beamline of the Swiss light source (SLS). Gain and loss studies
performed on Ge layers as well as carrier lifetime measurements are
shown in Section “Lifetime, Gain, and Loss.” These results impact
the discussion on lasing in n-doped Ge, which is briefly repeated
to exemplify the capability of these experimental methods. The
analysis of temperature-dependent photoluminescence (PL) is
found to deliver a quantitative measure for the directness of GeSn
layers, shown in Section “Photoluminescence – Direct Band Gap,”
and narrow emission spectra together with an intensity versus excitation-threshold represent the first observation of lasing in a direct
band gap group IV system, shown in Section “Optically Pumped
Laser.” Investigation challenges, such as the quantitative analysis of
the Auger recombination and the carrier transport, are appointed
in Section “Challenges” together with other fundamental devicerelated issues, such as cavity design, band gap renormalization, and
thermal budgets for alloys. We speculate about the opportunities
for Si photonics offered by an efficient monolithically integrated
laser source in Section “Opportunities,” and furthermore discuss
the prospect of a Ge and/or GeSn electro-optical data processing
platform. We conclude in Section “Conclusion and Outlook” and
give a short outlook.

Direct Band Gap Group IV Materials
Modeling
Band Structure

The effect of tensile strain on Ge’s band edges shown in Figure 3
illustrates the path of the transitions’ energies going from an indirect to a direct band gap system. The energies for interband- (solid
lines) and intervalence-band transitions (broken lines) between
the respective conduction- and valence-band edges are calculated
via deformation potential theory as implemented in the nextnano®
modeling software (Birner et al., 2007). Due to the fact that the
Γ-valley energy reduces faster than the one of the L-valley, Ge
transforms into a direct band gap semiconductor at ~4.7% uniaxial
strain along [100] when the direct transition (black line) decreases
below the energy of the indirect recombination (green line). For
Ge under biaxial tensile strain or GeSn alloys, the band edges
behave similarly with an indirect-to-direct band gap crossover at
~1.6–2.0% strain (El Kurdi et al., 2010; Virgilio et al., 2013; Wen
and Bellotti, 2015) or and at a Sn-content of ~9% (Low et al., 2012;
Gupta et al., 2013b; Wirths et al., 2015) for a fully relaxed layer.
In the valence band, strain lifts the degeneracy of light hole and
heavy hole bands and introduces a mixing such that this distinction becomes meaningless, especially under high strain. For low
strain, VB1 and VB2 in Figure 3 are mostly “heavy hole”- and

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Group IV direct band gap photonics

is nearly reached at a low injection of 1 × 1018 cm−3 as depicted in
Figure 4A. Applying a moderate doping of 1 × 1019 cm−3 results in a
gain of >500 cm−1, which is sufficient to overcome typical resonator
losses. When the doping level is increased to 2 × 1019 cm−3, the
gain approximately triples to ~1500 cm−1 according to our model.
This suggests that n-doping is a very effective method to promote
high gain for correspondingly low excitation in direct band gap
systems. This is due to the fact that as long as the offset is not much
larger than kT (Sukhdeo et al., 2014), electrons will nevertheless
spread into the L-band from where they cannot contribute to gain.
Transparency can also be achieved for an undoped and still
indirect, strained Ge system under higher excitation. We obtain
transparency at an injection of 1 × 1019 cm−3 for a system with a
remaining offset of 25% (35 meV) and an n-doping level below
1 × 1018cm−3 (see Figure 4B). When the direct band gap is reached
(0% offset), the net gain amounts to 1000  cm−1, which can be
increased up to 2000 cm−1 at a doping of 2 × 1019 cm−3. In contrast
to the indirect band gap Ge, a reduction in temperature helps to
increase the gain as soon as the Γ valley constitutes the lowest
conduction band energy due to condensation of the carriers into
the direct gap states. For example, an intrinsic direct band gap Ge
system with 25 meV band offset exhibits a net gain of the order of
4500 cm−1 at a temperature of 20 K and an injection of 1 × 1019 cm−3
compared to 1700 cm−1 at RT.
When comparing gain predictions in literature, we experience larger differences than between predictions of energy levels
and their relative positions. The reason for this stems from
the uncertainty in the loss. For weakly strained and relaxed
Ge, experimental values are available as discussed in Section
“Lifetime, Gain, and Loss.” Hence, the overall agreement of
the predictions is largely coherent. For example, calculations
consistently predict gain for Ge with a large offset (80%) only
for the case of very high doping of >5 × 1019 cm−3. For strained
and alloyed systems, however, the interband energies approach
the one of the intervalence band transitions. The energies may
even cross, as shown in Figure 3. Hence, loss processes related
to these transitions will become critical. Furthermore, the gain
as predicted by a Green’s functional approach (Wen and Bellotti,
2015) tend to be smaller than the commonly used joint density
of state formalism as applied for Figure 4.

FIGURE 3 | Transition energies between the direct and indirect
conduction band valleys and the two top valence bands and within
the valence band of uniaxially, tensilely stressed Ge. For a strain of
~4.7%, the conduction band minimum at the Γ-point reaches a lower energy
than the indirect L-valleys. The upper x-axis denotes the offset between the
Γ- and L-band edges in relative units.

“light hole”-like, respectively. VB3 refers to the split-off band. The
energetic order of the heavy and light hole bands is reverted when
moving from the uniaxial to the biaxial case.
Most of the theoretical work concerning Ge light emission utilizes k·p theory including 6 bands (Aldaghri et al., 2012; Chang and
Cheng, 2013; Virgilio et al., 2013), 8 bands (Zhu et al., 2010; Wirths
et al., 2013b), or 30 bands (El Kurdi et al., 2010). The latter is not
restricted to the Brillouin-zone center but describes the full energy
dispersion. In other works, the empirical pseudopotential method
(Dutt et  al., 2013; Wen and Bellotti, 2015), density functional
theory (Tahini et al., 2012), and the tight-binding model (Dutt
et al., 2012) are employed. The agreement between the models is
generally found to be satisfactory.

Fabrication
Microbridges

Gain

In Figure 4, we show gain calculations for uniaxially stressed Ge
in dependence of n-type doping and conduction band offset (c.f.
the scale of the upper x-axis in Figure 3). The band structure was
computed with an 8-band k·p approach (Birner et al., 2007). The
gain was calculated via Fermi’s golden rule, assuming cylindrical symmetry for the valence bands to simplify the calculation
of the joint density of states (JDOS), c.f. Virgilio et  al. (2013).
More details of the calculation can be found in Süess et al. (2013),
supplementary information. The peak gain at room temperature
(RT) is plotted after subtraction of the loss following the experimentally determined electron- and hole-absorption cross-sections
from Carroll et al. (2012) (Süess et al., 2013). The black, broken
line indicates when transparency is reached. As an example, for a
system at the crossover to a direct band gap system, transparency

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Strain engineering is nowadays a standard tool in microelectronics to improve device performance, where the lattice mismatch between Si and Ge is used to generate strain via epitaxy.
However, the pseudomorphic deposition of Ge on Si leads to
compressive strain, which deteriorates the light emission efficiency and is, furthermore, limited to small layer thicknesses.
Therefore, the main method used to introduce strain is the
application of external stressor layers, such as silicon nitride
(SiN), which is compatible with CMOS processing. Some work
following this approach includes the deposition of stressors on
the back side of Ge membranes (Nam et al., 2011, 2012), on
micropillars (Velha et  al., 2013), or on selectively grown Ge
(Oda et al., 2013).

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Group IV direct band gap photonics

FIGURE 4 | Maximum net gain at room temperature for uniaxially
stressed Ge in dependence of n-type doping and conduction band
offset. An 8-band k·p model was employed to calculate the band structure

parameters. The map is calculated for a carrier injection of (A) 1 × 1018 cm−3
and (B) 1 × 1019 cm−3. The black, broken line depicts the transparency
condition when the gain equals the losses.

An advantage of using external stressor layers is the simplicity
to combine the strain transfer with standard cavity structures like
FP waveguides (Capellini et al., 2014). However, the achieved strain
is so far limited to a predominantly uniaxial strain of 1.5%. In
other efforts, SiN layers were deposited on Ge microdisks, resulting
in a biaxial strain of 1.0% (Ghrib et al., 2013) and 1.5% (Ghrib
et al., 2015). However, these stressor layer approaches suffer from
a large strain inhomogeneity across the Ge layer, and elaborated
all-around stressor techniques using wafer transfer and bonding.
These results are included in Figure 2.
Following a different route, it was shown that high levels of
tensile strain can be locally induced without the use of any external
stressor layers (Minamisawa et  al., 2012; Süess et  al., 2013). In
the approach by Süess et  al., the starting substrate is the commonly used tensilely strained Ge layer with a biaxial strain of
~0.2%. Subsequently, the layer is patterned into a microbridge
with a narrow central cross-section (the “constriction”) and larger
outer cross-sections (the “pads”) as shown in Figures 5A,B. As
last processing step, the structure is underetched by selectively
removing the underlying buried oxide with hydrofluoric acid, c.f.
Figure 5C. Releasing the structure leads to a relaxation of the strain
in the pads, which in turn increases the strain in the constriction.
Due to Hooke’s law and force balance, strain accumulated in the
constriction will depend on the ratio of pad and constriction
widths as well as the ratio between their lengths (Minamisawa
et al., 2012; Süess et al., 2013). Hence, following this principle,
any strain can be generated in the constriction by solely varying
the geometrical parameters independent of the actual dimensions of the structure. In contrast to external stressors where the
achievable strain is limited by the efficiency of strain transfer,
this strain enhancement is only limited by the material strength.
Figure 5A shows enhancement factors of more than 20× realized

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from 0.15% biaxial strain in Ge on Si (blue squares) or Ge on
silicon-on-insulator (SOI) (green circles and red triangles) for
bridges with varying geometrical dimensions. The agreement of
the experimental values with the ones predicted by finite element
modeling (red triangles) is excellent. For Ge on SOI, the highest
strain achieved in 6  μm  ×  2  μm constrictions is 3.1%. When
starting from 200 nm thick germanium-on-insulator substrates
(GOI), which feature a significantly reduced dislocation density
(Akatsu et al., 2006; Hartmann et al., 2010), a strain of 5.7% was
observed in a 5.0 μm × 0.2 μm constriction (Sukhdeo et al., 2014).
According to Figures 2 and 3, such a strain is by far sufficient to
transform Ge into a direct band gap material showing the prospect
of the strain-enhancement technique given a starting material
with high-crystal quality.

GeSn Alloying

The epitaxial growth of GeSn alloys poses several challenges, such
as a large lattice mismatch between α-Sn and Si (17%) or Ge (15%),
and a low solid-solubility of <1%. Therefore, the fabrication of
high quality and smooth epilayers was a demanding task for many
years and the development of new growth processes to deposit
GeSn under non-equilibrium conditions at low temperatures was
required. Whereas the first attempts to grow GeSn alloys were
based on molecular beam epitaxy (MBE) in 1980s and 1990s
(Pukite et al., 1989; Harwit et al., 1990; Wegscheider et al., 1990;
Fitzgerald et al., 1991; He and Atwater, 1997), device-grade GeSn
epilayers could be synthesized since the early 2000s when the
first chemical vapor deposition (CVD) processes were developed
(Bauer et al., 2003).
Nowadays, several groups established growth processes for
GeSn utilizing either MBE (Bratland et al., 2003; Chen et al.,
2011a; Bhargava et  al., 2013; Oehme et  al., 2013) or CVD

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Group IV direct band gap photonics

A
0
25

1

2

3

4

4

4
3.5

Ge/Si: analytical
Ge/SOI: analytical
Ge/SOI: FEM

20

ε (%)

3

3

15

2

10

Model strain (%)

Model enhancement

B

Exp. strain (%)

2.5
2
1.5

1

5

1
0.5

0

0
0

C

5

10

15

20

25

0

Exp. enhancement

Electron-beam
lithography

RIE
etching

FIGURE 5 | Suspended microbridges from thermally pre-strained Ge.
(A) Experimental and modeled strain for Ge microbridges fabricated on Si and
SOI. The analytical strain-enhancement model is given in Süess et al. (2013). The
enhancement of 22× corresponds to 3.1% uniaxial strain (Süess et al., 2013).

(B) Strain profile of a suspended bridge structure as obtained by finite element
modeling (FEM). Due to the relaxation in the pads, the strain in the central
constriction is enhanced. (C) Process flow for the fabrication of suspended
microbridges.

(Vincent et al., 2011; Chen et al., 2013b; Wirths et al., 2013a;
Xu et  al., 2013; Du et  al., 2014) for a variety of applications,
e.g., photodiodes, photodetectors, or MOSFETs. Here, due to
the reduced lattice mismatch compared to Si, Ge is preferred as
virtual substrate (VS) in order to ensure layers of high monocrystalline quality. Regarding the epitaxial growth of direct band
gap GeSn alloys, nearly strain relaxed or even tensilely strained
layers are highly desired, since for compressively strained GeSn
layers, i.e., GeSn coherently grown on Ge VS, higher Sn contents
are necessary for the indirect to direct transition (Gupta et al.,
2013b). Owing to an advantageous relaxation mechanism for
GeSn layers on Ge VS, dislocations seem to mostly protrude into
the Ge VS rather than into the GeSn layer, which is beneficial
for optical properties as the density of non-radiative recombination centers is reduced (Takeuchi et al., 2006; Senaratne et al.,
2014; Wirths et  al., 2015). Although relaxation takes place, a
certain level of compressive biaxial strain (typically between
−0.6 to −0.8%) remains nevertheless, which, as already said
in connection with Figure 2, shifts the indirect-to-direct band
gap crossover to higher Sn concentrations with respect to fully
relaxed GeSn. Therefore, several approaches are being followed to
reduce the compressive strain, such as growth on lattice-matched
InGaAs VS (Chen et al., 2011a), which is not acceptable within
a CMOS processing line, or deposition of ever thicker layers to
enforce further strain relaxation (Senaratne et al., 2014; Wirths
et al., 2015). Gupta et al. (2013b) introduced a robust etching
approach enabling to selectively dry etch the Ge VS underneath

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Wet
etching

the epitaxial GeSn layers. The authors envision their method
to enable the fabrication of direct band gap GeSn micro disks.
Figures 6A–C show transmission electron microscopy images
of a GeSn layer with 13% Sn grown via reduced pressure CVD on
a Ge VS (Wirths et al., 2015). The advantageous relaxation mechanism mentioned above can be seen here with dislocation-loops
(blue arrows) emitted into the Ge VS. Despite the high-Sn content,
the thickness of the GeSn layer could be increased up to 560 nm
without deteriorating the high-crystalline quality. Owing to the
large thickness, a relaxation of 60% could be achieved such that only
a mild compressive strain of −0.6% was present. As will be shown
in Section “Photoluminescence – Direct Band Gap,” this epilayer
was proven to be a direct band gap group IV semiconductor that
provides net gain and, hence, shows lasing under optical pumping.
We conclude this section on the fabrication of GeSn alloys by
summarizing the list of beneficial assets GeSn epitaxy brings to the
current Si technology facilitating future developments and integration. Apart from the prospect to fabricate a fundamental direct
band gap group IV material, GeSn alloys are attractive because of
(i) low-temperature deposition on Si(001) compatible with existing
CMOS processes; (ii) strain relaxation with reasonably low threading dislocation density; (iii) available option for selective growth
on silicon, which is attractive for photonic integration; (iv) GeSn/
SiGeSn heterojunction layers to generate carrier confinement in
quantum wells; (v) and therefore, tunability of the lattice constant
offering opportunities to combine the alloys with the strained
membrane method.

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FIGURE 6 | Cross-sectional transmission electron microscopy (TEM)
image of a Ge0.87Sn0.13 alloy. (A) Expanded view showing the high-crystalline
quality of the GeSn epilayer. The defects are located near the interface to the
Ge virtual substrate. (B) Dislocation-loops (blue arrows) emitted below the

GeSn/Ge-interface (orange arrows) penetrating only into the Ge virtual
substrate. (C) High-resolution TEM image of the interface used for Burgers
vector calculations. Lomer dislocations with b = a/2[110] are observed (Wirths
et al., 2015).

Characterization Methods: Optical
Properties

interpretation of the Ge-lasing observations. We renarrate this
discussion at the end of this chapter.
Figure  7A shows the mid-infrared reflection of a Ge layer
grown on Si plotted as the ratio of pumped (RP) and unpumped
(RU) reflection signal. The different colors depict different optical excitation strengths between 1 and 160  MW  cm−2. All of
the spectra were taken for a pump–probe delay time of 250 ps.
The  distinct minimum observed in the spectra is attributed
to  the carriers’ plasma frequency. For an increasing excitation
power, the minimum shifts to higher energy and becomes at the
same time more pronounced. As the plasma frequency shifts in
first order proportional to the square root of the total amount
of charge carriers in the system, such reflection measurements
facilitate a convenient method for the quantitative determination
of the carrier density. Thus, the extracted carrier concentration in
dependence of the optical pump power for delay times of 0 and
250 ps is shown in the inset of Figure 7A. Moreover, by analyzing
the carrier density at a fixed pump power for varying delay times,
the reflection spectra can be used to extract the carrier decay times.
In the case shown here, the carrier density drops to ~4 × 1019 cm−3
within 250 ps for all generated carrier concentrations larger than
4 × 1019 cm−3. This behavior indicates an increasingly faster decay
time at high-carrier concentrations, which is attributed to Auger
recombination (Carroll et al., 2012).
While the analysis of mid-infrared reflection spectra enables
to directly access charge carrier concentration and decay time,
the latter can also be extracted from near-infrared transmission

Lifetime, Gain, and Loss

When describing a material with regards to its suitability as an
efficient laser source, key properties that decide upon adequacy
are the gain and loss, i.e., the material’s ability to amplify light, as
well as the non-radiative lifetime, which determines the internal
quantum efficiency as well as the achievable steady-state carrier
density. These characteristics can be extracted in a direct way
using broadband, time-resolved pump–probe transmission, and
reflection spectroscopy. Possible ways for performing such experiments could be via tunable lasers or supercontinuum sources.
However, particularly synchrotron-based infrared pump–probe
spectroscopy has been shown to offer advantageous conditions for
measuring the carrier density, their lifetime as well as gain and loss
due to its extended bandwidth and suitable pulse lengths (Carroll
et al., 2012; Geiger et al., 2014a,b). At the infrared beamline of
the SLS, 100 ps long pulses of infrared light are supplied from the
synchrotron and serve as broadband probe pulses, whereas the
excess charge carriers are optically excited by a 100 ps Nd:YAG
laser at 1064 nm (Carroll et al., 2011). The delay time between
pump and probe pulses can be varied electronically, which offers
the possibility to follow the dynamics of a system over a long time
period by probing at different times after excitation. In the following, we review some of the pump–probe measurements performed
at the SLS and give the most important results that challenge the

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FIGURE 7 | Time-resolved infrared reflection and transmission
spectroscopy: (A) mid-infrared reflection spectra of Ge on Si expressed
as the ratio of pumped (RP) and unpumped reflection (RU) for varying
excitation power at a pump–probe delay of 250 ps. The resonance in the
spectra is attributed to the carrier plasma frequency, which enables to extract

the total amount of charge carriers. The inset shows the carrier concentration for
0 and 250 ps delay time in dependence of the excitation power. (B) Normalincidence pump–probe transmission spectra for Ge on SOI for varying delay
times. Strong Fabry–Perot oscillations are observed from the thin film
interference. Analyzing the peak-shifts facilitates the extraction of the decay time.

measurements. In Figure  7B, normal-incidence transmission
spectra of intrinsic Ge are plotted, while the delay time between
pump and probe is varied. As SOI is used as substrate, distinct
FP oscillations are observed due to standing wave interferences
between the Ge/air and Si/SiO2 interfaces. For short delay times,
the transmission is significantly reduced due to absorption. Above
the direct band gap of ~0.8  eV, there is an increase compared
to the unpumped transmission due to gain or bleaching. By following the shifts of the minima or maxima, the dynamics of the
refractive index is obtained, which enables the extraction of the
carriers’ decay time. Compared to the decay time analysis from
the mid-infrared reflection, the sensitivity to detect small carrier
densities is higher in such a measurement because the refractive
index – and, hence, the oscillation extrema – follows the carrier
densities linearly, which enables to follow the decay processes
within an extended time window.
In Figure 8, the time-dependent FP peak shifts are shown for
differently prepared Ge layers. The shifts were normalized to unity
at t = 0 ns and the decay fitted to an exponential curve (Geiger
et al., 2014a). The defective Ge/Si interface was identified as the
main non-radiative loss channel, as (i) Ge selectively grown via
ultrahigh vacuum CVD (selGe in Figure 8) and a full epilayer
grown via low-energy plasma-enhanced CVD (iGe in Figure 8)
feature the same surface recombination velocity (SRV) – i.e., the
carrier lifetime normalized to the layer thickness – of ~800 m s−1,
(ii) a built-in field introduced by modulation doping (nGe/iGe)
increases the lifetime compared to iGe by keeping electrons away
from the interface, and (iii) the longest lifetime was observed for
an overgrown GOI wafer, where the defective Ge/Si interface is
removed (SRV = 490 m s−1). These results demonstrate the importance of engineering the material- and, in the case of Ge on Si,
especially the interface quality to obtain a high-internal quantum

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FIGURE 8 | Normalized peak shifts taken from normal-incidence
transmission spectra (as, e.g., in Figure 6B) for a series of differently
prepared Ge layers: iGe, nGe, selGe refers to intrinsic, n-doped, and
selectively grown Ge, respectively. GOI refers to Ge on insulator. The
non- radiative lifetime is obtained through an exponential fit to the data
(Geiger et al., 2014a).

efficiency and, thus, a low-threshold laser. Furthermore, similar
pump–probe transmission studies on strained microbridges
showed that neither strain, at least up to ~2%, nor processing
affects the lifetime (Geiger et al., 2014b), indicating that a highcrystal quality can be maintained using the microbridge strainenhancement technology.

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For the analysis of gain and loss, the transmission spectra
should be recorded under the Brewster angle such that the
obstructing FP resonances do not occur, c.f. Figure 9A for the
case of unstrained Ge on Si for different pump–probe delay
times. Δt = 0 ps refers to the maximum overlap between pump
and probe and, hence, to the highest carrier density. The thick
lines in blue and red show modeled transmission spectra for the
unpumped and pumped case. Under excitation, a strong absorption occurs with a linear dependence on energy. At the direct
band gap, the absorption gets reduced due to gain, but the gain
is too small to generate a negative absorption and, hence, light
amplification. This situation holds true for all other delay times,
i.e., carrier concentrations, as well.
In Figure 9B, the situation for Δt = 0 ps is shown again in terms
of the absorption coefficient with the modeled functions being
plotted separately for (i) direct gap absorption before pumping
(blue, thick line), (ii) direct gap absorption under excitation (red,
thick line), and (iii) the featureless pump-induced absorption
decreasing linearly in energy (red, thin line). Even though a gain
of ~850 cm−1 is observed as displayed by a negative absorption, the
loss from that spectrally distributed absorption at the same energy
is >6000 cm−1 making light amplification impossible. To show the
contrast to an established laser material featuring a direct band gap,
the same absorption properties are plotted in Figure 9C for the
case of InGaAs. Here, the pump-induced losses are independent
on energy and amount to ~1000 cm−1, which is compensated by
a direct gap gain of ~1700 cm−1 such that a net gain of 700 cm−1
is revealed. We should mention here that the theoretical analysis
of Carroll et  al. (2012) has been questioned (Dutt et  al., 2012)
concerning the strength of the gain (red line, Figure 9B) but not
the experiments, which clearly show that the loss is by far larger
than the gain.

A

From the preceding analysis, it is clear that a solid understanding and consideration of the loss processes is required for an
accurate description of gain in Ge. For illustration, the absorption cross-sections for three Ge samples (Ge#1: Nd = 0, εxx = 0;
Ge#2: Nd = 2.5 × 1019 cm−3, εxx = 0; Ge#3: Nd = 0, εxx = 0.25%) are
plotted in Figure 10 in dependence of the total carrier density
NT = Nd + NP, where Nd refers to the doping concentration and NP
to the pump-induced carrier density. As a comparison, the crosssections of three InGaAs layers (InGaAs#1: Nd  =  0, InGaAs#2:
Nd = 5.3 × 1018 cm−3; InGaAs#3: Nd = 2.1 × 1019 cm−3) are plotted
as well. Therefore, Figure 10 reveals that the absorption scales predominantly with NP indicating that the absorption cross-section
from holes σh is much larger than the cross-section for electrons
σe. Indeed, describing the absorption via a linearly dependent
cross-section as α = σeNe + σhNh (where subscripts e and h refer
to electrons and holes, respectively) offers a good representation
of the experimental data with σh/σe > 10. The absorption crosssection for holes is significantly larger than for electrons, because
in addition to the non-momentum conserving intraband or
Drude-type free carrier absorption, the holes can undergo vertical
intervalence band transitions (Newman and Tyler, 1957), which
are hereby identified as the main loss channel in Ge. A similar
conclusion concerning the cross-section ratio can be deduced from
the InGaAs data shown in Figure 10 in agreement with common
knowledge for direct band gap lasing materials (Adams et al., 1980;
Childs et al., 1986). Furthermore, the absolute values of the hole
cross-section are in a similar range but larger for InGaAs than for
Ge, c.f. Figure 10. However, the absorption is much higher in Ge
due to a much larger total carrier density needed to achieve a gain
like in the InGaAs sample.
Finally, we would like to relate the above presented data of gain
and loss as well as of lifetimes in Ge layers on Si to the observation

B

C

FIGURE 9 | Analysis of pump and probe measurements.
(A) Transmission spectra for Ge on Si measured under Brewster angle for
different delay times. (B) Modeled direct gap absorption unpumped
(blue, thick line) and pumped (red, thick line) as well as pump-induced

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absorption (red, thin line) obtained from the spectra in (A) at 0 time delay.
(C) Similar extraction of direct gap gain and losses for undoped InGaAs
showing light amplification, as the gain surpasses the pump-induced losses
(Carroll et al., 2012).

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5000

Linear Fits

PL, as has been demonstrated recently in Wirths et al. (2015). In
Figure 11B, the temperature-dependent PL intensity for a set of
samples with Sn content from 8 to 13% is shown. The data have
been normalized to unity at 300 K. For the sample with the lowest
Sn content, a rapid drop in intensity on lowering the temperature is
observed, whereas for the three other samples a steady increase in
intensity can be seen with the intensity increase being dependent
on the Sn concentration. Qualitatively, the increase from sample
to sample can be explained by the reduced conduction band offset
with increasing Sn, whereas cooling down leads to a condensation
of the carriers into the lowest energy states such that the direct
gap emission either vanishes for indirect band gap materials as
sample Ge0.92Sn0.08 because the Γ valley is not populated anymore or
increases strongly when the electrons condense at the minimum of
the Γ valley. To quantify the band offsets for the set of GeSn samples,
the emission efficiency is calculated via a similar JDOS model as the
one used for calculating the gain in Figure 4. Therein, the offset
ΔE between Γ- and L-valleys as well as the injected charge carrier
density Ni represent the free fitting parameters. Furthermore, the
temperature dependence of the lifetime is assumed to be identical for all samples and follows the Shockley–Read–Hall (SRH)
recombination characteristics (Shockley and Read, 1952; Schubert,
2006) that describes non-radiative recombination via trap states.
Using this model, an excellent agreement with the experimental
data is obtained (Wirths et al., 2015). The GeSn sample with 13%
Sn content is, hence, identified as a true direct band gap group IV
semiconductor with its Γ-valley being 25 meV below the indirect
L-valleys. From the second fit parameter, a RT carrier density Ni
of 4 × 1017 cm−3 is deduced consistent with a carrier lifetime of
0.35 ns, which corresponds to a SRV of 570 m s−1. This value is
close to the ones reported for elemental Ge on Si (Geiger et al.,
2014a), c.f. Figure 8, which is an indication of the high-crystalline
quality of the investigated GeSn epilayers.
The temperature dependence of the non-radiative carrier
lifetime is obtained as:

InGaAs#1
InGaAs#2
InGaAs#3

4000
3000
2000
Ge#1
Ge#2
Ge#3

1000
0
0.0

0.4

0.8

1.2

1.6
20

0.0

0.6
-3

NT x 10 ( cm )
FIGURE 10 | Absorption coefficients for differently doped and strained
Ge and InGaAs layers. The data are well described with a linear absorption
cross-section model.

of lasing in highly n-doped and weakly strained Ge (Liu et al., 2010;
Camacho-Aguilera et al., 2012), which was recently repeated at the
University of Stuttgart with an unstrained, highly n-doped light
emitting diode (LED) (Koerner et  al., 2015). First, as is shown
above by the gain and loss experiment (Carroll et al., 2011), the
loss in Ge layers strongly exceeds the gain at all the investigated
carrier densities up to 1020 cm−3 and in all investigated cases, i.e.,
Ge with and without weak strain and/or n-doping. Hence, Carroll’s
results are apparently in conflict with the observation of lasing
(Liu et al., 2010) in similar but not identical material. Second, the
non-radiative lifetimes, which have been determined for such Ge
layers only recently by Geiger et al. (2014a), c.f Figure 8, shine
a new light on previous and recent gain and threshold current
density calculations (Dutt et al., 2012; Peschka et al., 2015). Using
the obtained carrier lifetime of the order of 1–2  ns for threshold current estimates, the calculated threshold of the order of
100 kA cm−2 – obtained by assuming a lifetime of 100 ns – needs
to be rescaled by a factor of 50–100. Surely, such a current density is
above the material’s limit and also exceeds the observed threshold
values by ~2 orders of magnitude. Therefore, not only the gain/loss
experiments but also the theory (when fed with properly valued
parameters) shows that more research is needed to understand
the MIT results. The recent paper by the Stuttgart group (Koerner
et al., 2015) may give the directions for further thinking: a “lasing”
threshold was reached only shortly before their devices failed,
hinting at a carrier breakthrough. The heat pulse related to the
breakthrough may have caused the peaked emission signal.



(1)

where τ0 describes the lifetime at low temperature, and τSRH
describes the decay due to the capture of charge carriers by mid-gap
states, i.e., τSRH = A × (1 + cosh(ET/kT)). τAuger describes the Auger
recombination time, which can be neglected here due to the lowcarrier densities. Furthermore, ET is the difference between the trap
level energy and the intrinsic Fermi-level, k is the Boltzmann constant, and A is to normalize τ to 0.35 ns at 300 K as obtained from
the temperature-dependent PL. For ΔE = 19 meV and τ0 = 2.1 ns,
a good agreement between the extracted lifetimes and the lifetime
model is obtained (Wirths et al., 2015). For temperatures >50 K,
there is a drastic decrease in carrier lifetime from ~2 ns to 350 ps
for higher temperatures. As the temperature dependence of this
process is well described via the SRH model, the lifetime decay is
attributed to the capture of carriers via mid-gap states originating
from defects (Wirths et al., 2015). These defects could potentially
be related to defects located at the GeSn/Ge-interface (Geiger
et al., 2013; Wirths et al., 2015), but further studies are needed
to unambiguously identify the origin of this deterioration and,
subsequently, improve the material quality.

Photoluminescence – Direct Band Gap

Photoluminescence spectroscopy offers a convenient tool for probing the changes of the electronic band structure induced via strain
or Sn alloying. As shown in Figure 11A, the reduced offset between
Γ- and L-valleys manifests in an increased emission intensity of
the PL signal (Süess et al., 2013). A similar effect has also been
observed by Chen et al. (2011a).
A quantitative analysis of the relative alignment between Γand L can be obtained from the temperature dependence of the

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t = (1 / t0 + 1 / tSRH + 1 / t Auger )−1

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A

B

FIGURE 11 | Photoluminescence investigation of strained Ge and
GeSn alloys. (A) Room-temperature PL spectra for Ge samples with
increasing uniaxial strain up to 3.1%. The inset shows the good agreement
between the experimental and modeled band edges. (B) Temperaturedependent integrated PL intensity normalized to unity at 300 K for a series
of GeSn layers with Sn-content ranging from 8 to 13%. An increase in

Sn-concentration leads to a more pronounced increase in intensity.
The offset between Γ- and L-valleys is extracted from JDOS modeling
(solid lines), which reveals Ge0.87Sn0.13 to have a fundamental, direct band
gap. The inset shows the experimentally extracted non-radiative lifetime
modeled with a Shockley–Read–Hall-like temperature dependence
(red line).

Optically Pumped Laser

temperature is equivalent to the temperature range where the
lifetime was found to drop substantially from ~2 ns to 350 ps.
Hence, it is tempting to attribute the limitation of lasing to
temperatures <100 K to the carrier capture by defect-induced
mid-gap states, as has appealed from the analysis shown in
Figure 11B, inset. However, carrier transfer to the L-valleys and
carrier out diffusion into the Ge may be a determining factor,
as well.
Despite the breakthrough of presenting for the first time a direct
band gap group IV material that is lasing under optical pumping,
there still remain open questions. For example, with an excitation
power of 325 kW cm−2, a non-radiative lifetime of 2 ns, as shown
in Figure 11, and a typical absorbance of 1 × 104 cm−1 at 1064 nm,
a steady-state carrier density of ~3.5  ×  1019  cm−3 is estimated.
With this number, the gain at low temperature from our model
is found to be >5000 cm−1. And, interpolating from Figure 4, at
excitation density of 0.6 × 1018 cm−3 we would expect for a system
with positive offset of about 15%, a material gain of ~300 cm−1
at RT. We assign this large discrepancy from what is observed at
low temperature and what a RT calculation predicts to resonant
intervalence band absorption. As mentioned above, due to the
lack of experimental data for direct gap Ge or GeSn, the energy
dependence of the loss as measured by pump–probe experiments
for Ge (Carroll et al., 2012) has been used for Figure 4. Its proper
inclusion possibly adds significant contribution to the loss (Wen
and Bellotti, 2015).
In order to improve such gain calculations, which critically
depend on the knowledge of the band structure, mappings of the
entire valence, and conduction band in reciprocal space would certainly be highly valuable. This could be possible via angle-resolved

According to the modeling results shown in Section “Modeling,”
a direct band gap Ge-based system should feature a net gain and,
hence, enable light amplification at low excitation. In the previous analysis of low-temperature PL on GeSn alloys in Section
“Photoluminescence – Direct Band Gap,” a fundamental direct
band gap could be identified for a Sn content of 13% in a strainrelaxed layer with 0.7% compressive biaxial strain. To show lasing,
a 560-nm epilayer of such Ge0.87Sn0.13 material was grown providing
an overlap of 60% for the fundamental transverse electric mode in
a 5 μm wide FP cavity (Wirths et al., 2015). For this layer, modal
gain could be observed at 20 K via the variable-stripe-length (VSL)
method under pulsed optical excitation at 1064 nm with a differential gain of ≈0.4 cm kW−1 and a threshold excitation density of
≈325 kW cm−2 (c.f. Figure 12). Above threshold, the gain increases
linearly with excitation and can easily pass 100 cm−1. The stripe
length-dependent PL analysis is a widely applied technique to
measure net modal gain, but it does not allow to resolve the gain
and loss as by pump and probe spectroscopy. More evidentially, a
gain statement, such as provided by Figure 12, becomes respected
only after showing lasing.
Indeed, when pumping a FP cavity over its full length, a
strongly enhanced emission and narrowing of the line spectra
is observed as soon as the modal gain surpasses the cavity losses.
This behavior is shown in Figure 13 where the edge-emission
spectra from a 1-mm long FP cavity at 20 K are plotted for varying optical excitation powers. The curves are offset for clarity.
The threshold obtained from the lasing experiments matches well
with the one obtained from the VSL method. For an excitation
density of 1 MW cm−2, lasing could be observed up to 90 K. This

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A

B

FIGURE 12 | Gain extraction via the variable-stripe-length method
(VSL). (A) Edge-emitted intensity of Ge0.87Sn0.13 at 20 K in dependence of the
pumped waveguide length for varying excitation densities. The modal gain is

extracted by an exponential fit to the data. (B) Differential gain extracted from
the spectra in (A) indicating a linear dependence on excitation density with a
threshold at 325 kW cm−2.

In fact, similar lifetimes for Ge as obtained from synchrotron
measurements (Geiger et al., 2014a) were found (Nam et al., 2014).

Challenges
In the previous section, we reviewed experiments and results
related to the dependence of the optical properties on strain and
alloying of Ge with Sn. Furthermore, we summarized investigations concerning the first lasing of a direct band gap group IV
semiconductor and expounded on the temperature dependence
of the PL as a powerful tool to determine the directness of a
group IV material. We illustrated optical methods based on pump
and probe spectroscopy using synchrotron light to determine the
carrier lifetime, gain, and loss under optical pumping related to
the injected carrier density.
Future experiments along these lines on both, the strained Ge
system and GeSn alloys at various strain and Sn concentration,
respectively, will allow to establish the fundamentals of lasing in
direct band gap group IV systems. The impact of doping on gain,
loss, and carrier lifetime should also be addressed in dependence
of the directness of the respective system to verify the picture
elucidated by Figure 4 of Section “Gain.”
As an example, intervalence band absorption, Auger recombination, and the electrical injection are some of the many
fundamental aspects of group IV direct band gap lasing pending
to be understood and quantified.

FIGURE 13 | Lasing emission spectra measured from the facet of a
5-μm wide and 1 mm long FP waveguide cavity under optical
pumping at 20 K. A clear threshold behavior can be observed in the spectra
with respect to output intensity and linewidth, c.f. inset to the right and left
hand side, respectively.

photoelectron spectroscopy (ARPES) at high energy (Gray et al.,
2011) or the soft x-ray regime (Strocov et al., 2014). Other promising experimental techniques not covered because of lack of space
include lifetime measurements via time-resolved PL measurements (He and Atwater, 1997; Nam et al., 2014; Saito et al., 2014).

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Intervalence Band Absorption

One of the most essential parameters determining the efficiency
of a laser is associated to the parasitic absorption due to the
injected holes (Adams et al., 1980; Childs et al., 1986). As shown

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(μe = 3900 cm2/Vs and μh = 1900 cm2/Vs, respectively) (Golikova
et  al., 1962; Jacoboni et  al., 1981). For optical devices, this is
appealing because it results in diffusion lengths of several
100 μm, more than sufficient for, e.g., typical detector absorber
sizes and laser cavity lengths. However, at the same time, Ge
suffers from its low band gap, which causes large leakage currents
in Ge pn junctions (Metzger et al., 2001; Satta et al., 2006), thus,
requiring extensive work on surface passivation due to the lack
of a native oxide.
The already beneficial mobility properties can be further
improved by employing tensile strain as has been shown for both,
uniaxially and biaxially strained Ge (Schetzina and McKelvey,
1969; Chu et al., 2009; Chen et al., 2011b). Here, we would like to
highlight that all these studies have been performed on indirect
band gap Ge where the increase in electron mobility is mediated
by a reduction of the effective mass in the L valley. Even without
strain, the Γ-valley already offers an ~8 times smaller effective
mass and correspondingly higher electron mobility. Similarly,
an increase in hole mobility is expected due to the lifting of the
valence band degeneracy (Beattie and Landsberg, 1959; Fischetti
and Laux, 1996). This is advantageous as a high mobility strongly
reduces the resistivity of the device allowing an efficient injection
and extraction of charge carriers.
GeSn emerged as a material of interest in electronics only
recently; therefore, less transport data are available. However,
theoretical studies predict very large electron mobilities as well
as hole mobilities of the order of 4500  cm2/Vs for direct band
gap GeSn (Sau and Cohen, 2007). The first reported experimental
mobility study has been done on low Sn content (<6%) indirect
band gap GeSn layers yielding a Hall mobility of the order of
~200–300 cm2/Vs (Nakatsuka et al., 2010). Slightly better results
have been obtained thereafter investigating p-MOSFETs hole
channel mobility (Gupta et al., 2013a; Wang et al., 2013).
In summary, we see that a vast amount of knowledge concerning
the mobility exists leaving a good base for further studies. Moreover,
many electrical devices and the corresponding fabrication techniques, e.g., passivation, contacting, or annealing, have been
conceived allowing for a fast implementation in optical devices.
However, besides the tremendous changes in the carrier mobility, there are additional effects coming into play with electrical
injection of charge carriers from indirect to direct band gap Ge.
Exemplarily in Figure 14, such an injection scheme in form of a
pin diode is discussed for the case of tensile strained Ge bridges
where the strain profile is shown in Figure 5B.
Far from the strained constriction, electrons can be injected
into the L-valleys of the conduction band as in standard Ge diodes.
However, close to the center the strain profile alters the band
structure with L- and γ-valley starting to cross, which allows for
intervalley scattering (Boucaud et al., 2013) from a high- into a
low-effective mass valley with a higher mobility, a process inverse
to the Gunn effect (Gunn, 1963). This may support current extraction and injection in optical devices. However, an actual impact
still needs to be proven.

experimentally by Carroll et  al. (2012) for Ge, this absorption
depends linearly on the excitation and inclines with decreasing
energy. This can be understood from the Drude dependence of
the free carrier absorption modified by dipole allowed intervalence
band transitions. Because the emission wavelength increases when
approaching the direct band gap configuration, and the initially
degenerate heavy and light hole bands split due to strain, the parasitic absorption will strongly increase in direct band gap systems
and may, thus, obstruct the efficiency of lasing (Wen and Bellotti,
2015). Applying the above introduced optical characterization
methods should allow to investigate these effects in detail, which,
together with evolving theoretical results, will enable to complete
our understanding.

Auger Recombination

On the material level, the performance of optical devices depends
strongly on the charge carrier recombination lifetime similarly
as described by rate Eq.  1. Here, both the radiative and the
Auger recombination lifetime depend on the carrier density
n: 1/τrad = B × n, 1/τAuger = C × n2. The quadratic carrier density
dependence implicates that Auger recombination becomes a
dominant loss mechanism at high-charge carrier densities, which
can be the order of >1019 cm−3 for typical laser devices.
Extensive theoretical work based on perturbation theory has
shown that, despite its indirect band gap, the band structure of
both, Si and Ge, is favorable for direct Auger recombination (Huldt,
1974; Lochmann, 1978) with Auger recombination coefficients of
the order of 10−30 cm6s−1. This is comparable to direct band gap
materials like GaAs or GaN used for optical devices in the visible
part of the spectrum but still much smaller than for low band gap
materials like InAs (Metzger et al., 2001).
Significantly, less is known about direct band gap group IV
Auger recombination. For direct band gap Ge0.9Sn0.1/Ge0.75Si0.1Sn0.15
multi-quantum-well structures, Sun et  al. recently showed
theoretically that the RT Auger recombination lifetime is of the
order of 50 ns compared to a radiative lifetime of 10 ns (Sun et al.,
2010). Other work on GeSn (Dutt et al., 2013) and n-doped or
tensile strained Ge (Liu et al., 2007; Jain et al., 2012) only refers
to unstrained bulk Ge recombination coefficients to include in
their gain models.
We believe, however, that this is unjustified considering that
there is an exponential dependence of the Auger lifetime on the
band gap and effective masses (Beattie and Landsberg, 1959; Huldt,
1971; Adams et al., 1980), both being strongly altered in direct
band gap Ge. Using the simple exponential dependence derived
by Beattie and Landsberg (1959) to scale the experimentally
determined Auger recombination coefficient of CGe = 10−30 cm6s−1
(Carroll et al., 2012) via the effective masses and band gaps of direct
band gap Ge, an Auger coefficient of the order of 10−26–10−27 cm6s−1
is obtained. Despite the overly strong simplicity of this comparison,
it shows that Auger coefficients will most probably increase and
need to be addressed and investigated in the future.

Cavity Design

Carrier Injection

For the usage of direct band gap materials in lasing structures,
high-quality factor optical resonators are necessary confining the

From an electronic point of view, Ge is one of the most interesting materials as it offers both, high electron and hole mobilities

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Group IV direct band gap photonics

Finding the intrinsic stress limits of Ge is another item of
interest in this context.
By saying this, we conclude our listing of fundamental and
materials-related challenges. This list may be incomplete. However,
it confirms that the research and development of a laser source
from a group IV material will involve many disciplines from
fundamental to device physics and from wave optics to material
and transport properties. To progress fast, a collaborative effort
is demanded.

FIGURE 14 | Schematic of the band structure of a forward biased
pin strained Ge bridge diode where the intrinsic layer and strained
region overlap.

Opportunities
Photonics

light in the gain region and, thus, allowing for stimulated emission.
Helpful in this regard is the high-refractive index contrast of the
Ge–air and GeSn–air interfaces (Kasper et al., 2013), which should
allow for good confinement properties. The design of suitable
cavities seems to be straightforward for GeSn lasers where the
wafer-scale direct band gap on Si or Ge already facilitated the
implementation of well-known laser cavities, such as FP cavities
(Wirths et al., 2015) or microdisks (Cho et al., 2011). At the same
time, optical microdisk cavities with quality factors of ~1400
have been demonstrated in tensile strained Ge using external
SiN stressors (Ghrib et  al., 2013) as well as waveguide cavities
(Capellini et al., 2014).
The situation is much more complex for uniaxially strained
Ge bridges where patterning of the bridge inevitably relaxes the
strain and, hence, prohibits a fundamental direct band gap. This
excludes many popular cavity designs, in particular, microdisks,
photonic crystals, and FP cavities. Hence, distributed feedback
structures, which do not rely on patterning of the strained region,
are currently under investigation (Marin et al., 2015).

Direct band gap group IV laser systems may permit a qualitative as well as a quantitative expansion of Si-photonics (group
IV photonics) into traditional but also new areas of applications.
However, it is requested that such lasers can be operated energy
efficiently, under ambient conditions and can be fully integrated
with current Si technology. An answer to whether this is possible
cannot be given yet as the research is at an early stage. We can
only speculate about the specifications of such a laser and, thus,
have to guess which of the applications would profit most from
a successful implementation of group IV lasers. Hence, for the
following discussion, let us assume that this all-group-IV laser does
indeed exist and it operates (i) under electrical injection, (ii) at RT
or above, and (iii) with reasonable power conversion efficiency.
What could we do with such a device, where is the highest impact,
and what is the platform of choice?
A high return is merely achievable when this laser device will be
combined with the current Si photonics by using the same platform.
Most advanced are photonic elements fabricated on SOI, except
for applications in the visible part of the spectrum – not covered
here – where SiN-based structures are often used. SOI for photonics typically consists of Si layers with a thickness of ~200–250 nm
and a several micrometer thick buried oxide to avoid leakage of
the propagating modes into the Si substrate. For strain engineering, the compatibility with SOI has already been shown (Süess
et al., 2013), c.f. Section “Microbridges,” and, as mentioned above,
bridges with even higher mechanical strength are fabricated from
GOI using wafer transfer (Sukhdeo et al., 2014). In fact, wafer-scale
fabrication of GOI using the SmartCut® process has already been
established several years ago for electronics (Augendre et al., 2009).
GOI for photonic applications, where thicker layers and a thicker
BOX are required, has been presented recently by Reboud et al.
(2015). A photonic platform based on GOI, in comparison to SOI,
has the advantage that all photonic elements, such as waveguides,
bends, and the resonant structures, can be reduced in size because
of the larger refractive index contrast. This allows for the potential
fabrication of more dense optical circuits and, hence, for an easier
integration with electronics. Furthermore, Ge provides coverage
of the longer wavelengths toward 10  μm and more. Moreover,
by using processes that are selective for either Ge or Si, the GOI
platform may provide additional fabrication opportunities. The
high quality (Si)GeSn presented by Wirths et al. (2013b) has been
deposited on a Ge VS on Si(001) indicating that the growth on SOI
and certainly GOI is possible, as well.

Band Gap Renormalization and Material Stability

Relevant for both here discussed direct band gap systems is a quantitative analysis of the band gap renormalization of the involved Γ
and L valleys in dependence of their respective carrier population.
So far, experiments suggest that the renormalization corrections
are comparable for the two valleys. Hence, the offset between the
Γ and L states would not depend on the injection density, which
is essential for a stable injection.
Moreover, material specific investigation concerns, for example,
the thermal stability of GeSn and SiGeSn metastable alloys with
regards to Sn diffusion and segregation where extensive segregation can result in changes of the emission wavelength and/or
emission efficiency. Recently, investigations have been pursued
to examine the temperature budget a GeSn or SiGeSn device would
be able to withstand, e.g., by in situ studies (Fournier-Lupien et al.,
2014) or annealing experiments (Wirths et al., 2014). First, in situ
results indicate phase separation of a 12% Sn containing ternary
SiGeSn and binary GeSn alloys at ~420 and 460°C, respectively,
which is surprising considering that the higher mixing entropy
usually results in a higher thermal stability of ternary alloys
(Fournier-Lupien et al., 2014). Annealing experiments revealed
distinct Sn diffusion at 300°C for GeSn with approximately the
same composition (Wirths et al., 2014).

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With a laser device implemented on the currently used SOI or
similarly on GOI, various new applications will emerge. Before
speculating, we may picture the many already existing photonic
elements. To select a few: Low-loss (<1 dB cm−1) single mode
waveguides in various designs, tapers to adiabatically match the
waveguide modes to fibers, and low-loss grating couplers (<1 dB)
in 1D or 2D providing polarization splitting. Other standard
elements include directional couplers, Mach Zehnder interferometers, ring resonators, and light modulators based on free
carrier injections or quantum confined Stark effect. Detectors
based on slightly tensile strained (0.2%) Ge provide more than
10  GHz speed for high-data rate transmission. Other device
elements include add-drop filters, buffers, and switches, which
can be integrated with fluidic channels for (bio-) sensing. The
short wavelength infrared at which the here discussed group IV
direct band gap lasers emit (see Figure 3) is certainly a clear asset
for sensing applications (Soref, 2010; Nedeljkovic et al., 2013).
Silicon-on-insulator is (and GOI could become) a very convenient platform to realize high-performing photonic crystal structures
enabling unique photonic circuits, such as compact high-Q cavities, which can operate stably at single and dual-wavelength, and
as wavelength division multiplexer as desired for optical signal
processing. This listing can be extended almost indefinitely, naming, e.g., switching and steering of optical signals, slow light, pulse
compression, customized reflectors, and filters. Together with the
expectation that such photonic circuits will be very cost effective,
compact, reliable, and efficient, a monolithically integrated laser
source will certainly bring new functionality, in particular when
optics can be merged with electronics.

physics, and Si electronics and photonics will cooperate and
define the routes to opto-electronics for fast and energy-efficient
data processing.

Conclusion and Outlook
We reviewed the methods for achieving a direct band gap in
group IV semiconductors in the most promising material system
for the prospect of a Si compatible laser, namely, Ge modified
either via tensile strain or by alloying with Sn. We expanded on
the methods to characterize these systems and gave examples
on their optical properties. The recent advances in numerous
approaches to achieve a direct band gap have finally concluded
in the first demonstration of lasing in a direct band gap GeSn
alloy (Wirths et al., 2015).
With this demonstration, we are at the beginning of an exciting journey in the field of silicon photonics. As shown in great
detail, the many optical characterization tools at hand allow us
to address a large amount of fundamental questions, including
band gap renormalization, various recombination processes, and
doping level-dependent lasing performance, but also material- and
technology-related issues, such as high Q-factor cavity design,
diffusive carrier transport, stress, and thermal diffusion limits.
We hope that with outlining these challenges, we can motivate
a vast amount of new researchers from various backgrounds in
optics, material science, and device physics to join this interesting
research field. We believe that combined efforts will converge in
a reasonable time to a demonstration of a practical laser source
being electrically pumped, highly efficient, and fully integrated on
an electro-optical CMOS platform. This building block will finally
pave the way for true monolithic on-chip integration of photonics
and CMOS electronics for new sensors in the long wavelength
infrared, and will eventually enable to build an on-chip or off-chip
electro-optical data distribution network for high-performance
computing.

CMOS Integration

The combination of optics with CMOS electronics to realize an
on-chip data distribution network (Heck and Bowers, 2014)
is – without any doubt – one of the most advanced and challenging applications for direct band gap group IV lasers. The
requirements are so complex (Miller, 2009) that before the start
of such a development, many fundamental questions have to be
answered, such as the efficiency issues among other challenges,
which have been addresses in the previous section. However,
once these hurdles are taken, we expect to arise a highly competitive and attractive platform solution for future data processing
applications. In fact, the extension of CMOS by integration of Ge
and (Si)GeSn may not just resolve the demands for a monolithic
laser gain medium, but, as discussed widely elsewhere (Kao et al.,
2014), (Si)GeSn would already advance the performance of the
electronic circuits. This appealing double benefit, together with
the potential compatibility to CMOS of such an all-group-IV
solution, bears an essential advantage in comparison to other
emerging technologies, such as spin- and/or valley-based electronics, which rely in part on non-conform chemical elements
and non-CMOS fabrication processes.
Hence, we expect that as soon as the fundamental lessons
of direct band gap lasing are learnt and a gain medium wellqualified for injection pumping at RT is defined, research and
development of a new opto-electronic platform will quickly
advance. Experts in CMOS technology, group IV epitaxy, laser

Frontiers in Materials | www.frontiersin.org

Acknowledgments
We would like to acknowledge the many scientific collaborators
we were fortunate to work with over the last few years. They
supported us in building up a strong portfolio in the investigation and understanding of lasing in group IV systems, and the
fabrication of direct band gap group IV materials. In particular,
we thank our previous group members Gustav Schiefler, Martin
J. Süess, and Renato Minamisawa for their contributions,
which led to this appealing strain concept, and the group of
Dan Buca (FZ Jülich), who contacted us for investigating their
high quality material and thus gave us the opportunity to learn
also about GeSn alloys. The tremendous progress achieved in a
short time is a shining example of our good collaboration. We
also thank Jérôme Faist and Ralph Spolenak (ETHZ) for their
whole-hearted support to this subject and their many essential
contributions. Finally, we acknowledge the Swiss Science
Foundation (SNF) for supporting part of the here reviewed
studies over several years.

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References

Chen, Y. T., Lan, H. S., Hsu, W., Fu, Y. C., Lin, J. Y., and Liu, C. W. (2011b). Strain
response of high mobility germanium n-channel metal-oxide-semiconductor
field-effect transistors on (001) substrates. Appl. Phys. Lett. 99, 022106.
doi:10.1063/1.3604417
Childs, G. N., Brand, S., and Abram, R. A. (1986). Intervalence band absorption in semiconductor laser materials. Semicond. Sci. Technol. 1, 116–120.
doi:10.1088/0268-1242/1/2/004
Cho, S., Chen, R., Koo, S., Shambat, G., Lin, H., Park, N., et al. (2011). Fabrication
and analysis of epitaxially grown Ge1-xSnx microdisk resonator with 20-nm
free-spectral range. IEEE Photon. Technol. Lett. 23, 1535–1537. doi:10.1109/
LPT.2011.2163929
Chow, W. W. (2012). Model for direct-transition gain in a Ge-on-Si laser. Appl. Phys.
Lett. 100, 191113. doi:10.1063/1.4714540
Chu, M., Sun, Y., Aghoram, U., and Thompson, S. E. (2009). Strain: a solution for
higher carrier mobility in nanoscale MOSFETs. Annu. Rev. Mater. Res. 39, 203–229.
doi:10.1146/annurev-matsci-082908-145312
Du, W., Ghetmiri, S. A., Conley, B. R., Mosleh, A., Nazzal, A., Soref, R. A., et al.
(2014). Competition of optical transitions between direct and indirect bandgaps
in Ge1−xSnx. Appl. Phys. Lett. 105, 051104. doi:10.1063/1.4892302
Dutt, B., Lin, H., Sukhdeo, D. S., Vulovic, B. M., Gupta, S., Nam, D., et al. (2013).
Theoretical analysis of GeSn alloys as a gain medium for a Si-compatible laser. IEEE J.
Sel. Top. Quantum Electron. 19, 1502706–1502706. doi:10.1109/JSTQE.2013.2241397
Dutt, B., Sukhdeo, D. S., Nam, D., Vulovic, B. M., Ze, Y., and Saraswat, K. C. (2012).
Roadmap to an efficient germanium-on-silicon laser: strain vs. n-type doping.
IEEE Photon. J. 4, 2002–2009. doi:10.1109/JPHOT.2012.2221692
El Kurdi, M., Fishman, G., Sauvage, S., and Boucaud, P. (2010). Band structure and
optical gain of tensile-strained germanium based on a 30 band k·p formalism.
J. Appl. Phys. 107, 013710. doi:10.1063/1.3279307
Fang, Z., Chen, Q. Y., and Zhao, C. Z. (2013). A review of recent progress in lasers
on silicon. Opt. Laser Technol. 46, 103–110. doi:10.1088/0034-4885/76/3/034501
Fischetti, M. V., and Laux, S. E. (1996). Band structure, deformation potentials, and
carrier mobility in strained Si, Ge, and SiGe alloys. J. Appl. Phys. 80, 2234–2252.
doi:10.1063/1.363052
Fitzgerald, E. A., Xie, Y. H., Green, M. L., Brasen, D., Kortan, A. R., Michel, J., et al.
(1991). Totally relaxed GexSi1−x layers with low threading dislocation densities
grown on Si substrates. Appl. Phys. Lett. 59, 811. doi:10.1063/1.105351
Fournier-Lupien, J. H., Chagnon, D., Levesque, P., AlMutairi, A. A., Wirths, S., Pippel,
E., et al. (2014). In situ studies of germanium-tin and silicon-germanium-tin
thermal stability. ECS Trans. 64, 903–911. doi:10.1149/06406.0903ecst
Geiger, R., Frigerio, J., Süess, M. J., Chrastina, D., Isella, G., Spolenak, R., et al. (2014a).
Excess carrier lifetimes in Ge layers on Si. Appl. Phys. Lett. 104, 062106–062106.
doi:10.1063/1.4865237
Geiger, R., Süess, M. J., Bonzon, C., Frigerio, J., Chrastina, D., Isella, G., et al. (2014b).
Carrier Lifetimes in Uniaxially Strained Ge Micro Bridges. IEEE, 227–228.
Geiger, R., Süess, M. J., Bonzon, C., Spolenak, R., Faist, J., and Sigg, H. (2014c). Strained
Ge Microbridges to Obtain a Direct Bandgap Laser. IEEE, 7–8.
Geiger, R., Frigerio, J., Süess, M. J., Minamisawa, R. A., Chrastina, D., Isella, G., et al.
(2013). Excess Carrier Lifetimes in Ge Layers on Si. IEEE, 103–104.
Ghrib, A., Kurdi El, M., de Kersauson, M., Prost, M., Sauvage, S., Checoury, X., et al.
(2013). Tensile-strained germanium microdisks. Appl. Phys. Lett. 102, 221112.
doi:10.1364/OE.23.006722
Ghrib, A., Kurdi El, M., Prost, M., Sauvage, S., Checoury, X., Beaudoin, G., et al. (2015).
All-around SiN stressor for high and homogeneous tensile strain in germanium
microdisk cavities. Adv. Opt. Mater. 3, 353–358. doi:10.1002/adom.201400369
Golikova, O. A., Moizhes, B. Y., and Stilbans, L. S. (1962). Hole mobility of germanium as a function of concentration and temperature. Sov. Phys. Solid State 3,
2259–2265.
Gray, A. X., Papp, C., Ueda, S., Balke, B., Yamashita, Y., Plucinski, L., et al. (2011).
Probing bulk electronic structure with hard X-ray angle-resolved photoemission.
Nat. Mater. 10, 759–764. doi:10.1038/nmat3089
Greil, J., Lugstein, A., Zeiner, C., Strasser, G., and Bertagnolli, E. (2012). Tuning the
electro-optical properties of germanium nanowires by tensile strain. Nano Lett.
12, 6230–6234. doi:10.1021/nl303288g
Gunn, J. B. (1963). Microwave oscillations of current in III–V semiconductors. Solid
State Commun. 1, 88–91. doi:10.1016/0038-1098(63)90041-3
Gupta, S., Chen, R., Vincent, B., Lin, D., Magyari-Kope, B., Caymax, M., et al. (2013a).
(Invited) GeSn channel n and p MOSFETs. ECS Trans. 50, 937–941. doi:10.114
9/05009.0937ecst

Adams, A. R., Asada, M., Suematsu, Y., and Arai, S. (1980). The temperature dependence
of the efficiency and threshold current of In 1- xGa xAs yP 1- y lasers related
to intervalence band absorption. Jpn. J. Appl. Phys. 19, L621–L624. doi:10.1143/
JJAP.19.L621
Akatsu, T., Deguet, C., Sanchez, L., Allibert, F., Rouchon, D., Signamarcheix, T., et al.
(2006). Germanium-on-insulator (GeOI) substrates – a novel engineered substrate
for future high performance devices. Mater. Sci. Semicond. Process. 9, 444–448.
doi:10.1016/j.mssp.2006.08.077
Aldaghri, O., Ikonić, Z., and Kelsall, R. W. (2012). Optimum strain configurations
for carrier injection in near infrared Ge lasers. J. Appl. Phys. 111, 053106.
doi:10.1063/1.3691790
Augendre, E., Sanchez, L., Benaissa, L., Signamarcheix, T., Hartmann, J.-M.,
Le Royer, C., et al. (2009). Challenges and progress in germanium-on-insulator
materials and device development towards ULSI integration. ECS Trans. 25,
351–362. doi:10.1149/1.3203972
Bauer, M. R., Tolle, J., Bungay, C., Chizmeshya, A. V. G., Smith, D. J., Menéndez, J.,
et al. (2003). Tunable band structure in diamond-cubic tin-germanium alloys
grown on silicon substrates. Solid State Commun. 127, 355–359. doi:10.1016/
S0038-1098(03)00446-0
Beattie, A. R., and Landsberg, P. T. (1959). Auger effect in semiconductors. Proc. R.
Soc. A Math. Phys. Eng. Sci. 249, 16–29. doi:10.1098/rspa.1959.0003
Bhargava, N., Coppinger, M., Gupta, J. P., Wielunski, L., and Kolodzey, J. (2013). Lattice
constant and substitutional composition of GeSn alloys grown by molecular beam
epitaxy. Appl. Phys. Lett. 103, 041908. doi:10.1063/1.4816660
Birner, S., Zibold, T., Andlauer, T., Kubis, T., Sabathil, M., Trellakis, A., et al. (2007).
Nextnano: general purpose 3-D simulations. IEEE Trans. Electron Devices 54,
2137–2142. doi:10.1109/TED.2007.902871
Boucaud, P., Kurdi El, M., Ghrib, A., Prost, M., de Kersauson, M., Sauvage, S., et al.
(2013). Recent advances in germanium emission. Photon. Res. 1, 102. doi:10.1364/
PRJ.1.000102
Bratland, K., Foo, Y., Soares, J., Spila, T., Desjardins, P., and Greene, J. (2003). Mechanism
for epitaxial breakdown during low-temperature Ge(001) molecular beam epitaxy.
Phys. Rev. B 67, 125322. doi:10.1103/PhysRevB.67.125322
Camacho-Aguilera, R. E., Cai, Y., Patel, N., Bessette, J. T., Romagnoli, M., Kimerling, L.
C., et al. (2012). An electrically pumped germanium laser. Opt. Express 20, 11316.
doi:10.1364/OE.20.011316
Capellini, G., Kozlowski, G., Yamamoto, Y., Lisker, M., Wenger, C., Niu, G., et al.
(2013). Strain analysis in SiN/Ge microstructures obtained via Si-complementary
metal oxide semiconductor compatible approach. J. Appl. Phys. 113, 013513.
doi:10.1063/1.4772781
Capellini, G., Reich, C., Guha, S., Yamamoto, Y., Lisker, M., Virgilio, M., et al. (2014).
Tensile Ge microstructures for lasing fabricated by means of a silicon complementary metal-oxide-semiconductor process. Opt. Express 22, 399–410. doi:10.1364/
OE.22.000399
Carroll, L., Friedli, P., Lerch, P., Schneider, J., Treyer, D., Hunziker, S., et al. (2011). Ultrabroadband infrared pump-probe spectroscopy using synchrotron radiation and
a tuneable pump. Rev. Sci. Instrum. 82, 063101–063101–9. doi:10.1063/1.3592332
Carroll, L., Friedli, P., Neuenschwander, S., Sigg, H., Cecchi, S., Isa, F., et al. (2012).
Direct-gap gain and optical absorption in germanium correlated to the density of
photoexcited carriers, doping, and strain. Phys. Rev. Lett. 109, 057402. doi:10.1103/
PhysRevLett.109.057402
Chang, G.-E., and Cheng, H. H. (2013). Optical gain of germanium infrared
lasers on different crystal orientations. J. Phys. D Appl. Phys. 46, 065103.
doi:10.1088/0022-3727/46/6/065103
Chen, R., Gupta, S., Huang, Y.-C., Huo, Y., Rudy, C. W., Sanchez, E., et al. (2013a).
Demonstration of a Ge/GeSn/Ge quantum-well microdisk resonator on silicon:
enabling high-quality Ge(Sn) materials for micro- and nanophotonics. Nano Lett.
14, 37–43. doi:10.1021/nl402815v
Chen, R., Huang, Y.-C., Gupta, S., Lin, A. C., Sanchez, E., Kim, Y., et al. (2013b).
Material characterization of high Sn-content, compressively-strained GeSn epitaxial
films after rapid thermal processing. J. Cryst. Growth 365, 29–34. doi:10.1016/j.
jcrysgro.2012.12.014
Chen, R., Lin, H., Huo, Y., Hitzman, C., Kamins, T. I., and Harris, J. S. (2011a).
Increased photoluminescence of strain-reduced, high-Sn composition
Ge1−xSnx alloys grown by molecular beam epitaxy. Appl. Phys. Lett. 99, 181125.
doi:10.1063/1.3658632

Frontiers in Materials | www.frontiersin.org

96

July 2015 | Volume 2 | Article 52

Geiger et al.

Group IV direct band gap photonics

Gupta, S., Magyari-Köpe, B., Nishi, Y., and Saraswat, K. C. (2013b). Achieving direct
band gap in germanium through integration of Sn alloying and external strain.
J. Appl. Phys. 113, 073707. doi:10.1063/1.4792649
Hartmann, J. M., Sanchez, L., Van Den Daele, W., Abbadie, A., Baud, L., Truche, R.,
et al. (2010). Fabrication, structural and electrical properties of compressively
strained Ge-on-insulator substrates. Semicond. Sci. Technol. 25, 075010.
doi:10.1088/0268-1242/25/7/075010
Harwit, A., Pukite, P. R., Angilello, J., and Iyer, S. S. (1990). Properties of diamond
structure SnGe films grown by molecular beam epitaxy. Thin Solid Films 184,
395–401. doi:10.1016/0040-6090(90)90437-I
He, G., and Atwater, H. A. (1997). Interband transitions in SnxGe1−xAlloys. Phys.
Rev. Lett. 79, 1937–1940. doi:10.1103/PhysRevLett.79.1937
Heck, M. J. R., and Bowers, J. E. (2014). Energy efficient and energy proportional optical
interconnects for multi-core processors: driving the need for on-chip sources.
IEEE J. Sel. Top. Quantum Electron. 20, 332–343. doi:10.1109/JSTQE.2013.2293271
Huldt, L. (1971). Band-to-band auger recombination in indirect gap semiconductors.
Phys. Status Solidi A 8, 173–187. doi:10.1002/pssa.2210080118
Huldt, L. (1974). Auger recombination in germanium. Phys. Status Solidi A 24, 221–229.
doi:10.1088/0957-4484/22/43/435401
Huo, Y., Lin, H., Chen, R., Makarova, M., Rong, Y., Li, M., et al. (2011). Strong enhancement of direct transition photoluminescence with highly tensile-strained Ge grown
by molecular beam epitaxy. Appl. Phys. Lett. 98, 011111. doi:10.1063/1.3534785
Jacoboni, C., Nava, F., Canali, C., and Ottaviani, G. (1981). Electron drift velocity and
diffusivity in germanium. Phys. Rev. B 24, 1014–1026. doi:10.1103/PhysRevB.24.1014
Jain, J. R., Hryciw, A., Baer, T. M., Miller, D. A. B., Brongersma, M. L., and Howe, R. T.
(2012). A micromachining-based technology for enhancing germanium light emission via tensile strain. Nat. Photonics 6, 398–405. doi:10.1038/nphoton.2012.111
Kao, K.-H., Verhulst, A. S., Rooyackers, R., Douhard, B., Delmotte, J., Bender, H., et al.
(2014). Compressively strained SiGe band-to-band tunneling model calibration
based on p-i-n diodes and prospect of strained SiGe tunneling field-effect transistors. J. Appl. Phys. 116, 214506. doi:10.1063/1.4903288
Kasper, E., Kittler, M., Oehme, M., and Arguirov, T. (2013). Germanium tin: silicon
photonics toward the mid-infrared [Invited]. Photon. Res. 1, 69. doi:10.1364/
PRJ.1.000069
Koerner, R., Oehme, M., Gollhofer, M., Schmid, M., Kostecki, K., Bechler, S., et al.
(2015). Electrically pumped lasing from Ge Fabry-Perot resonators on Si. Opt.
Express 23, 14815–14822. doi:10.1364/OE.23.014815
Liu, J., Sun, X., Camacho-Aguilera, R., Kimerling, L. C., and Michel, J. (2010). Ge-on-Si
laser operating at room temperature. Opt. Lett. 35, 679. doi:10.1364/OL.35.000679
Liu, J., Sun, X., Pan, D., Wang, X., Kimerling, L. C., Koch, T. L., et al. (2007). Tensilestrained, n-type Ge as a gain medium for monolithic laser integration on Si. Opt.
Express 15, 11272. doi:10.1364/OE.15.011272
Lochmann, W. (1978). Phonon-assisted auger recombination in indirect gap semiconductors. Phys. Status Solidi A 45, 423–432. doi:10.1002/pssa.2210450208
Low, K. L., Yang, Y., Han, G., Fan, W., and Yeo, Y.-C. (2012). Electronic band structure
and effective mass parameters of Ge1-xSnx alloys. J. Appl. Phys. 112, 103715.
doi:10.1063/1.4767381
Marin, E., Bonzon, C., Geiger, R., Zabel, T., Sigg, H., and Faist, J. (2015). Proceedings
CLEO-Europe 2015. (in print)
Metzger, W. K., Wanlass, M. W., Ellingson, R. J., Ahrenkiel, R. K., and Carapella, J. J.
(2001). Auger recombination in low-band-gap n-type InGaAs. Appl. Phys. Lett.
79, 3272–3274. doi:10.1063/1.1418032
Michel, J., Liu, J., and Kimerling, L. C. (2010). High-performance Ge-on-Si photodetectors. Nat. Photonics 4, 527–534. doi:10.1038/nphoton.2010.157
Miller, D. A. B. (2009). Device requirements for optical interconnects to silicon chips.
Proc. IEEE 97, 1166–1185. doi:10.1109/JPROC.2009.2014298
Miller, D. A. B. (2010). Optical interconnects to electronic chips. Appl. Opt. 49, F59.
doi:10.1364/AO.49.000F59
Minamisawa, R. A., Süess, M. J., Spolenak, R., Faist, J., David, C., Gobrecht, J., et al.
(2012). Top-down fabricated silicon nanowires under tensile elastic strain up to
4.5%. Nat Commun 3, 1096. doi:10.1038/ncomms2102
Nakatsuka, O., Tsutsui, N., Shimura, Y., Takeuchi, S., Sakai, A., and Zaima, S. (2010).
Mobility behavior of Ge1-xSnx layers grown on silicon-on-insulator substrates.
Jpn. J. Appl. Phys. 49, 04DA10. doi:10.1143/JJAP.49.04DA10
Nam, D., Kang, J.-H., Brongersma, M. L., and Saraswat, K. C. (2014). Observation
of improved minority carrier lifetimes in high-quality Ge-on-insulator
using time-resolved photoluminescence. Opt. Lett. 39, 6205. doi:10.1364/
OL.39.006205

Frontiers in Materials | www.frontiersin.org

Nam, D., Sukhdeo, D., Cheng, S.-L., Roy, A., Chih-Yao Huang, K., Brongersma, M.,
et al. (2012). Electroluminescence from strained germanium membranes and
implications for an efficient Si-compatible laser. Appl. Phys. Lett. 100, 131112.
doi:10.1063/1.3699224
Nam, D., Sukhdeo, D., Roy, A., Balram, K., Cheng, S.-L., Huang, K. C.-Y., et al. (2011).
Strained germanium thin film membrane on silicon substrate for optoelectronics.
Opt. Express 19, 25866–25872. doi:10.1364/OE.19.025866
Nam, D., Sukhdeo, D. S., Kang, J.-H., Petykiewicz, J., Lee, J. H., Jung, W. S., et al.
(2013). Strain-induced pseudoheterostructure nanowires confining carriers at
room temperature with nanoscale-tunable band profiles. Nano Lett. 13, 3118–3123.
doi:10.1021/nl401042n
Nedeljkovic, M., Khokhar, A. Z., Hu, Y., Chen, X., Penades, J. S., Stankovic, S., et al.
(2013). Silicon photonic devices and platforms for the mid-infrared. Opt. Mater.
Express 3, 1205. doi:10.1364/OME.3.001205
Newman, R., and Tyler, W. (1957). Effect of impurities on free-hole infrared
absorption in p-type germanium. Phys. Rev. 105, 885–886. doi:10.1103/
PhysRev.105.885
Oda, K., Okumura, T., Tani, K., Saito, S.-I., and Ido, T. (2013). Improvement of photoluminescence from Ge layer with patterned Si3N4 stressors. Thin Solid Films 557,
355–362. doi:10.1016/j.tsf.2013.08.117
Oehme, M., Buca, D., Kostecki, K., Wirths, S., Holländer, B., Kasper, E., et al. (2013).
Epitaxial growth of highly compressively strained GeSn alloys up to 12.5% Sn. J.
Cryst. Growth 384, 71–76. doi:10.1016/j.jcrysgro.2013.09.018
Passaro, V., Tullio, C., Troia, B., Notte, M., Giannoccaro, G., and Leonardis, F. (2012).
Recent advances in integrated photonic sensors. Sensors 12, 15558–15598.
doi:10.3390/s121115558
Peschka, D., Thomas, M., Glitzky, A., Nurnberg, R., Gartner, K., Virgilio, M., et al.
(2015). Modeling of edge-emitting lasers based on tensile strained germanium
microstrips. IEEE Photon. J. 7, 1–15. doi:10.1109/JPHOT.2015.2427093
Pukite, P. R., Harwit, A., and Iyer, S. S. (1989). Molecular beam epitaxy of metastable, diamond structure SnxGe1-x alloys. Appl. Phys. Lett. 54, 2142–2144.
doi:10.1063/1.101152
Reboud, V., Widiez, J., Hartmann, J.-M., Dias, G. O., Fowler, D., Chelnokov, A., et al.
(2015). Structural and optical properties of 200 mm germanium-on- insulator
(GeOI) substrates for silicon photonics applications. Proc. SPIE 9367, Silicon
Photonics X 2015. doi:10.1117/12.2079393
Saito, S., Gardes, F. Y., Al-Attili, A. Z., Tani, K., Oda, K., Suwa, Y., et al. (2014). Group
IV light sources to enable the convergence of photonics and electronics. Front.
Mater. 1:15. doi:10.3389/fmats.2014.00015
Sánchez-Pérez, J. R., Boztug, C., Chen, F., Sudradjat, F. F., Paskiewicz, D. M., Jacobson,
R. B., et al. (2011). Direct-bandgap light-emitting germanium in tensilely strained
nanomembranes. Proc. Natl. Acad. Sci. U.S.A. 108, 18893–18898. doi:10.1073/
pnas.1107968108
Satta, A., Nicholas, G., Simoen, E., Houssa, M., Dimoulas, A., De Jaeger, B., et al.
(2006). Impact of germanium surface passivation on the leakage current of shallow
planar p-n junctions. Mater. Sci. Semicond. Process. 9, 716–720. doi:10.1016/j.
mssp.2006.08.037
Sau, J., and Cohen, M. (2007). Possibility of increased mobility in Ge-Sn alloy system.
Phys. Rev. B 75, 045208. doi:10.1103/PhysRevB.75.045208
Schetzina, J. F., and McKelvey, J. P. (1969). Strain dependence of the minority
carrier mobility in p-type germanium. Phys. Rev. 181, 1191. doi:10.1103/
PhysRev.181.1191
Schubert, E. F. (2006). Light-Emitting Diodes, 2nd Edn. Cambridge University
Press.
Senaratne, C. L., Gallagher, J. D., Jiang, L., Aoki, T., Smith, D. J., Menendez, J., et al.
(2014). Ge1-ySny (y = 0.01-0.10) alloys on Ge-buffered Si: synthesis, microstructure,
and optical properties. J. Appl. Phys. 116, 133509. doi:10.1063/1.4896788
Shockley, W., and Read, W. (1952). Statistics of the recombinations of holes and
electrons. Phys. Rev. 87, 835–842. doi:10.1103/PhysRev.87.835
Soref, R. (2010). Mid-infrared photonics in silicon and germanium. Nat. Photonics 4,
495–497. doi:10.1038/nphoton.2010.171
Strocov, V. N., Wang, X., Shi, M., Kobayashi, M., Krempasky, J., Hess, C., et al. (2014).
Soft-X-ray ARPES facility at the ADRESS beamline of the SLS: concepts, technical
realisation and scientific applications. J. Synchrotron. Radiat. 21, 32–44. doi:10.1107/
S1600577513019085
Süess, M. J., Geiger, R., Minamisawa, R. A., Schiefler, G. L., Frigerio, J., Chrastina, D.,
et al. (2013). Analysis of enhanced light emission from highly strained germanium
microbridges. Nat. Photonics 7, 466–472. doi:10.1038/nphoton.2013.67

97

July 2015 | Volume 2 | Article 52

Geiger et al.

Group IV direct band gap photonics

Sukhdeo, D. S., Nam, D., Kang, J.-H., Brongersma, M. L., and Saraswat, K. C. (2014).
Direct bandgap germanium-on-silicon inferred from 5.7% 〈100〉 uniaxial tensile
strain [Invited]. Photon. Res. 2, A8. doi:10.1364/PRJ.2.0000A8
Sun, G., Soref, R. A., and Cheng, H. H. (2010). Design of a Si-based lattice-matched
room-temperature GeSn/GeSiSn multi-quantum-well mid-infrared laser diode.
Opt. Express 18, 19957–19965. doi:10.1364/OE.18.019957
Tahini, H., Chroneos, A., Grimes, R. W., Schwingenschlögl, U., and Dimoulas, A.
(2012). Strain-induced changes to the electronic structure of germanium. J. Phys.
Condens. Matter 24, 195802. doi:10.1088/0953-8984/24/19/195802
Takeuchi, S., Sakai, A., Yamamoto, K., Nakatsuka, O., Ogawa, M., and Zaima, S.
(2006). Growth and structure evaluation of strain-relaxed Ge 1− xSn xbuffer
layers grown on various types of substrates. Semicond. Sci. Technol. 22, S231–S235.
doi:10.1088/0268-1242/22/1/S54
Velha, P., Dumas, D. C., Gallacher, K., Millar, R., Myronov, M., Leadley, D. R., et al.
(2013). Strained Germanium Nanostructures on Silicon Emitting at >2.2 µm
Wavelength. IEEE, 142–143.
Vincent, B., Gencarelli, F., Bender, H., Merckling, C., Douhard, B., Petersen, D. H.,
et al. (2011). Undoped and in-situ B doped GeSn epitaxial growth on Ge by
atmospheric pressure-chemical vapor deposition. Appl. Phys. Lett. 99, 152103.
doi:10.1063/1.3645620
Virgilio, M., Manganelli, C. L., Grosso, G., Schroeder, T., and Capellini, G. (2013).
Photoluminescence, recombination rate, and gain spectra in optically excited n-type
and tensile strained germanium layers. J. Appl. Phys. 114, 243102. doi:10.1063/1.4849855
Wang, L., Su, S., Wang, W., Gong, X., Yang, Y., Guo, P., et al. (2013). Strained germanium-tin (GeSn) p-channel metal-oxide-semiconductor field-effect-transistors
(p-MOSFETs) with ammonium sulfide passivation. Solid State Electron. 83, 66–70.
doi:10.1016/j.sse.2013.01.031
Wegscheider, W., Eberl, K., Menczigar, U., and Abstreiter, G. (1990). Single-crystal Sn/
Ge superlattices on Ge substrates: growth and structural properties. Appl. Phys.
Lett. 57, 875. doi:10.1063/1.104264
Wen, H., and Bellotti, E. (2015). Rigorous theory of the radiative and gain characteristics
of silicon and germanium lasing media. Phys. Rev. B 91, 035307. doi:10.1103/
PhysRevB.91.035307

Frontiers in Materials | www.frontiersin.org

Wirths, S., Buca, D., Mussler, G., Tiedemann, A. T., Holländer, B., Bernardy, P., et al.
(2013a). Reduced pressure CVD growth of Ge and Ge1-xSnx alloys. ECS J. Solid
State Sci. Technol. 2, N99–N102. doi:10.1149/2.006305jss
Wirths, S., Ikonić, Z., Tiedemann, A. T., Holländer, B., Stoica, T., Mussler, G., et al.
(2013b). Tensely strained GeSn alloys as optical gain media. Appl. Phys. Lett. 103,
192110. doi:10.1063/1.4829360
Wirths, S., Geiger, R., den Driesch von, N., Mussler, G., Stoica, T., Mantl, S., et al.
(2015). Lasing in direct-bandgap GeSn alloy grown on Si. Nat. Photonics 9, 88–92.
doi:10.1038/nphoton.2014.321
Wirths, S., Stange, D., Pampillón, M.-A., Tiedemann, A. T., Mussler, G., Fox, A., et al.
(2014). High-k gate stacks on low bandgap tensile strained Ge and GeSn alloys for
field-effect transistors. ACS Appl. Mater. Interfaces 7, 62–67. doi:10.1021/am5075248
Xiaochen, S., Jifeng, L., Kimerling, L. C., and Michel, J. (2010). Toward a germanium
laser for integrated silicon photonics. IEEE J. Sel. Top. Quantum Electron. 16,
124–131. doi:10.1109/JSTQE.2009.2027445
Xu, C., Beeler, R. T., Jiang, L., Grzybowski, G., Chizmeshya, A. V. G., Menéndez, J., et al.
(2013). New strategies for Ge-on-Si materials and devices using non-conventional
hydride chemistries: the tetragermane case. Semicond. Sci. Technol. 28, 105001.
doi:10.1088/0268-1242/28/10/105001
Zhu, Y.-H., Xu, Q., Fan, W.-J., and Wang, J.-W. (2010). Theoretical gain of strained
GeSn0.02/Ge1−x−y′SixSny′ quantum well laser. J. Appl. Phys. 107, 073108.
doi:10.1063/1.3329424
Conflict of Interest Statement: The authors declare that the research was conducted
in the absence of any commercial or financial relationships that could be construed
as a potential conflict of interest.
Copyright © 2015 Geiger, Zabel and Sigg. This is an open-access article distributed under
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or reproduction in other forums is permitted, provided the original author(s) or licensor
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98

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ORIGINAL RESEARCH ARTICLE

MATERIALS

published: 27 April 2015
doi: 10.3389/fmats.2015.00030

Direct growth of Ge1−x Snx films on Si using a cold-wall
ultra-high vacuum chemical-vapor-deposition system
Aboozar Mosleh 1,2 *, Murtadha A. Alher 2,3 , Larry C. Cousar 1,4 , Wei Du 2 , Seyed Amir Ghetmiri 1,2 ,Thach Pham 2 ,
Joshua M. Grant 5 , Greg Sun 6 , Richard A. Soref 6 , Baohua Li 4 , Hameed A. Naseem 2 and Shui-Qing Yu 2
1
2
3
4
5
6

Microelectronics-Photonics Graduate Program (µEP), University of Arkansas, Fayetteville, AR, USA
Department of Electrical Engineering, University of Arkansas, Fayetteville, AR, USA
Mechanical Engineering Department, University of Karbala, Karbala, Iraq
Arktonics, LLC, Fayetteville, AR, USA
Engineering-Physics Department, Southern Arkansas University, Magnolia, AR, USA
Department of Engineering, University of Massachusetts Boston, Boston, MA, USA

Edited by:
Jifeng Liu, Dartmouth College, USA
Reviewed by:
Fabio Iacona, National Research
Council, Italy
Christophe Labbé, Ecole Nationale
Supérieure d’Ingénieurs de Caen,
France
*Correspondence:
Aboozar Mosleh, Engineering
Research Center (ENRC), 700
Research Center Boulevard,
Fayetteville, AR 72701, USA
e-mail: amosleh@gmail.com

Germanium–tin alloys were grown directly on Si substrate at low temperatures using a coldwall ultra-high vacuum chemical-vapor-deposition system. Epitaxial growth was achieved
by adopting commercial gas precursors of germane and stannic chloride without any carrier
gases. The X-ray diffraction analysis showed the incorporation of Sn and that the Ge1−x Snx
films are fully epitaxial and strain relaxed. Tin incorporation in the Ge matrix was found to
vary from 1 to 7%. The scanning electron microscopy images and energy-dispersive X-ray
spectra maps show uniform Sn incorporation and continuous film growth. Investigation
of deposition parameters shows that at high flow rates of stannic chloride the films were
etched due to the production of HCl. The photoluminescence study shows the reduction
of band-gap from 0.8 to 0.55 eV as a result of Sn incorporation.
Keywords: chemical-vapor-deposition, Si photonics, Ge alloys, photoluminescence, Ge–Sn

INTRODUCTION
The discovery and development of Ge1−x Snx epitaxy technology
has enabled silicon photonics to be explored in a different scope of
a material platform. The ability of band-gap engineering by varying Sn mole fraction, along with its compatibility to the complementary metal–oxide–semiconductor (CMOS) process, has paved
the way for highly competitive Si-based near and mid-infrared
optoelectronic devices. Recent reports on the fabrication and
characterization of high performance Ge1−x Snx devices such as
modulators (Kouvetakis et al., 2005), photodetectors (Conley et al.,
2014a,b), and light emitting diodes (LEDs) (Du et al., 2014a) show
great potential for Ge1−x Snx being adopted by industry in the
near future. Cutting-edge reports on Ge1−x Snx , achieving a direct
band-gap group IV alloy (Du et al., 2014b; Ghetmiri et al., 2014a;
Li et al., 2014; Wirths et al., 2014), is a turning point for the technology to be pursued for the demonstration of an efficient group
IV laser. In addition, due to the tunable lattice constant and formation of Lomer dislocations, Ge1−x Snx has been shown to work as
a universal compliant buffer layer to grow high quality lattice mismatched materials, like III–Vs, on Si (Beeler et al., 2011a; Mosleh
et al., 2014).
A variety of challenges exist for the growth of Ge1−x Snx alloys
on Si such as large lattice mismatch between Ge1−x Snx and Si
(more than 4.2%), low solid solubility of Sn in Ge (less than 0.5%),
and instability of diamond lattice Sn (α-Sn) above 13°C. Therefore, growth can only possibly be done under non-equilibrium
conditions. Different growth methods have been demonstrated
for Ge1−x Snx growth in which molecular beam epitaxy (MBE)
and chemical-vapor-deposition (CVD) have obtained device quality material and high Sn incorporation. For the MBE method,

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both gas source and solid source MBE have been used by different
groups to grow Ge1−x Snx films (Gurdal et al., 1998; Takeuchi et al.,
2007; Chen et al., 2011; Werner et al., 2011; Stefanov et al., 2012;
Bhargava et al., 2013; Oehme et al., 2013; Wang et al., 2013).
The other parallel approach of Ge1−x Snx growth is CVD. The
early results of CVD growth by Kouvetakis and Chizmeshya (2007)
at Arizona State University (ASU) showed the ability to grow
Ge1−x Snx film directly on Si using a hot-wall ultra-high vacuum
CVD (UHV-CVD) system with deuterated Stannane (SnD4 ) as
the Sn precursor along with different chemistries of germanium.
Due to the high cost and instability of SnD4 , other precursors
such as tetramethyl tin [Sn(CH3 )4 ] and stannic chloride (SnCl4 )
have been explored to grow Ge1−x Snx alloys. Vincent et al. (2011)
(from IMEC using atmospheric pressure CVD) and Kim et al.
(Chen et al., 2013) [from Applied Materials/Stanford University
using reduced pressure-CVD (RP-CVD)] have reported successful
growth of Ge1−x Snx by using SnCl4 and a high cost Ge precursor digermane (Ge2 H6 ) and carrier gases on a Ge-buffered
Si substrate. Using the same SnCl4 and Ge2 H6 precursors and
carrier gases, Mantl et al. (Wirths et al., 2013) (from PGI9-IT)
demonstrated direct growth of Ge1−x Snx on Si using showerhead
technology in an RP-CVD chamber. In the recent report, Tolle
et al. (Margetis et al., 2014; Mosleh et al., 2014a) (ASM company)
have achieved Ge1−x Snx growth using an industry prevail RPCVD reactor in collaboration with University of Arkansas (UA).
Low-cost Germane (GeH4 ) and SnCl4 with carrier gasses of N2 /H2
were used to grow Ge1−x Snx . A Ge buffer was deposited between
the Si substrate and the Ge1−x Snx layer in order to compensate
the lattice mismatch between the layers. Table 1 lists the different
research groups that have grown Ge1−x Snx using CVD. Different

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Direct Ge–Sn growth on Si using UHV-CVD

Table 1 | A summary of reports on Ge1−x Snx growth using CVD methods by different research groups.
Growth team

Deposition system

Deposition gas precursors
Ge

ASU (Kouvetakis and Chizmeshya, 2007)

Cost

Carrier gas

Sn

Cost

Buffer layer

UHV-CVD

Different chemistries

High

SnD4

High

Yes

No

IMEC (Vincent et al., 2011)

AP-CVD

Ge2 H6

High

SnCl4

Low

Yes

Ge

Applied materials (Chen et al., 2013)

RP-CVD

Ge2 H6

High

SnCl4

Low

Yes

Ge

PGI9-IT (Wirths et al., 2013)

RP-CVD

Ge2 H6

High

SnCl4

Low

Yes

No

ASM/UA (Margetis et al., 2014; Mosleh et al., 2014a)

RP-CVD

GeH4

Low

SnCl4

Low

Yes

Ge

UHV-CVD

GeH4

Low

SnCl4

Low

No

No

UA (this work)

growth methods and the cost effectiveness of the gas precursors
are compared.
In this paper, we report direct growth of strain-relaxed
Ge1−x Snx films on Si substrates with Sn mole fractions up to 7%
using a cold-wall UHV-CVD system. Stannic chloride and germane were chosen as the precursors which are low-cost and commercially available. The growth of Ge1−x Snx films was achieved
without using any carrier gases and buffer layers. In order to
investigate the material quality, the X-ray diffraction (XRD),
high-resolution transmission electron microscopy (TEM), energydispersive X-ray spectroscopy (EDX), Raman spectroscopy, and
photoluminescence (PL) measurements have been conducted.

EXPERIMENT
GROWTH METHOD

A cold-wall UHV-CVD system was adopted to grow Ge1−x Snx
films (see Figure 1 for machine schematic). The system composes a load-lock chamber with a base pressure of 10−6 Pa and
a process chamber whose base pressure reaches 10−8 Pa using the
turbo-molecular and cryogenic pumps, respectively. Due to lowtemperature growth of the films, removal of oxygen and water
vapor is critical which was achieved by using a cryogenic pump.
The turbo-molecular pumps are backed by mechanical pumps.
The heating stage consisted of a pyrolytic graphite heater with a
thermocouple placed at the same distance away from the heater
as the wafer. The sample holder rotates up to 80 rpm for uniform
film growth. The gas flow is through a side entry port, controlled
by mass flow controllers (MFCs). Stannic chloride is a volatile liquid with vapor pressure of 2.4 kPa at one atmospheric pressure.
Therefore, the evaporation could produce enough pressure to be
passed through the MFC.
Germanium–tin films were grown on 400 (001) p-type Si substrates with 5–10 Ω cm resistivity. Prior to loading the samples,
they were cleaned in a two-step process: (1) Piranha etch solution
[H2 SO4 :H2 O2 (1:1)], (2) oxide strip HF dipping [H2 O:HF (10:1)
using 48% pure HF] followed by nitrogen blow drying. The final
oxide strip step was not followed by a water rinse as it reduces the
life-time of hydrogen passivation and exposes the surface to ambient oxygen (Mosleh et al., 2013, 2014b). The experiments were
carried out at reduced pressures of 13, 40, 65, 95, 130, 200, and
260 Pa and at temperatures as low as 300°C. Germane (GeH4 ) and
stannic chloride (SnCl4 ) were used as the precursors for Ge1−x Snx
growth. The gas flow ratio (GeH4 /SnCl4 ) was set to 5, 3.3, 2.5,
and 1.6. Depending on the growth parameters such as gas flow

Frontiers in Materials | Optics and Photonics

ratio and deposition pressure, a growth rate of 20–3.3 nm/min
was achieved.
CHARACTERIZATION METHOD

Analysis of Sn mole fraction, lattice constant, growth quality,
and strain in the Ge1−x Snx films were conducted using a highresolution X-ray diffractometer. High-resolution TEM (TITAN)
with an accelerating voltage of 300 kV was used to investigate
crystal orientation and defects in the grown epi-layers as well as
determining the thicknesses of the samples. Surface morphology
of the samples was investigated by a scanning electron microscope equipped with EDX. Room temperature PL measurements
were carried out using a 690-nm excitation laser. The PL signal
was collected by a grating-based spectrometer equipped with a
thermoelectric-cooled PbS detector (cut-off at 3 µm) for spectral
analysis.

RESULTS AND DISCUSSION
MATERIAL CHARACTERIZATION

The 2θ-ω XRD scan was performed from the symmetric (004)
plane to obtain the out-of-plane lattice constant of the Ge1−x Snx
films. Figure 2A shows the peak at 69° corresponding to a satisfaction of the Bragg condition by Si (001) substrate, and the peaks at
lower angles of 66–65° due to larger lattice size of the Ge1−x Snx layers. The difference in the position of Ge1−x Snx peaks is due to the
difference in the Sn mole fractions of Ge1−x Snx layers. Different
compositions were achieved from 1 to 7% with desirable crystal
quality. The Ge1−x Snx peaks are broadened for two reasons: (1)
thin film thickness of the layers and (2) presence of mosaicity in the
Ge–Sn crystal and formation of defects as a result of strain relaxation. The full width at half maximum (FWHM) of the Ge1−x Snx
peaks are between 0.28 for 1% Sn film and 0.36 for 7% Sn film. The
change in FWHM depends on various factors such as film thickness, relaxation, and quality and there is no trend showing that the
FWHM of the peaks change as the Sn composition increases.
In order to calculate the total lattice constant and the strain
in the film, an asymmetric reciprocal space mapping (RSM)
from (−2, −2, 4) plane was performed. The RSM scans provide
measurement of the in-plane (a k ) and out-of-plane (a ⊥ ) lattice
constant of Ge1−x Snx alloys. The total lattice constant a0GeSn was
calculated by taking into account the elastic constants of Ge1−x Snx
(Beeler et al., 2011b). Knowing the total lattice constant, the Sn
mole fractions is calculated through Vegard’s law with the bowing factor of b = 0.0166 Å (Moontragoon et al., 2012). Figure 2B

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Direct Ge–Sn growth on Si using UHV-CVD

FIGURE 1 | Cold-wall UHV-CVD system with a substrate rotation. Samples are transferred through a load-lock chamber equipped with a turbo-molecular
pump. The growth chamber is equipped with a turbo-molecular pump and a cryogenic pump. Side entry of the gases is controlled by mass flow controllers.

shows the RSM of 6% Sn sample. The x-axis shows Q z in reciprocal lattice unit (rlu) which is related to the out-of-plane lattice
constant (L) and the y-axis shows Q x which is related to the inplane lattice constant (H or K). Direction of the spread in the
Ge0.94 Sn0.06 peak does not show a compositional gradient in the
sample because it is related to the relaxation of the lattice on Si
substrate. Large lattice mismatch between Sn and Ge is the main
reason for a large spread in the omega direction. The relaxation
line in Figure 2B shows that the films which are grown above are
tensile strained and the films grown underneath are compressively
strained. The Ge0.94 Sn0.06 peak is observed to be on the relaxation
line and the relaxation is measured to be 97%.
Calculation of total strain in other samples shows that all the
films are more than 95% relaxed. Table 2 shows the lattice constants of the Ge1−x Snx alloys, their Sn mole fraction, and strain
relaxation percentage. Ge1−x Snx films were almost fully relaxed
mainly due to large lattice mismatch between Si (5.431 Å) and
Ge1−x Snx (above 5.658 Å) and small critical thickness (Mosleh
et al., 2014a). The other reason for relaxation of Ge (and similarly Ge1−x Snx ) films on Si is the thermal mismatch between
these two materials. High temperature growth (above 500°C) and
rapid cool down has been the main method for achieving tensile
strained Ge on Si (Conley et al., 2014a). The Ge1−x Snx samples
were grown at 300°C for 30 min and we have not achieved tensile strained films; however, the thermal mismatch between Si
and Ge1−x Snx has helped relaxing the compressive strain. The
strain has been mainly relieved through formation of misfit dislocations including Lomer misfit dislocation. The cross-sectional
TEM image in Figure 2C shows formation of such dislocations

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at the Ge1−x Snx /Si interface. In addition, Figure 2B shows that
strain relaxation occurred by formation of misfit dislocations at
the interface. The TEM image shows that the grown film was fully
epitaxial. Film thickness of the samples is listed in Table 2.
The SEM scan/EDX spectra of the samples show surface morphology of the sample as well as Sn incorporation in the Ge matrix.
The EDX spectra in Figure 2D show the presence of Ge, Si, and Sn
in the Ge0.94 Sn0.06 film. Due to the high count collection of secondary electrons from the substrate, the ratio of Sn and Ge cannot
exactly reveal the percentage of Sn in Ge. The presence of carbon
and oxygen in the EDX spectra is mainly due to the contamination and oxidation of the film after exposure to ambient air. The
EDX maps for Ge (Figure 2E) and Sn (Figure 2F) display uniform
incorporation of Sn. The SEM image shows continuous growth
of Ge1−x Snx without observation of locally crystalline patches.
No segregation and precipitation of Sn was observed on the films
which indicates robust and stable growth of the films.
GROWTH MECHANISM

Growth of Ge1−x Snx on a Si substrate requires considering the
reaction of byproducts and reduction of activation energy by
introducing carrier gases. Stannic chloride has a tendency to etch
Ge due to the presence of chlorine in the chemistry of the molecule. The byproduct of GeH4 + SnCl4 reaction is HCl which is an
etchant gas for germanium and silicon (Bogumilowicz et al., 2005).
Following reactions show different mechanisms of film deposition
as well as HCl production in the chamber:
GeH4 → Ge + 2H2

(1)

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Direct Ge–Sn growth on Si using UHV-CVD

FIGURE 2 | (A) Symmetric (004) 2θ-ω scan of Ge1− x Snx films which are
grown on a Si substrate. The peak at 69° shows the Si substrate peak and
the peaks between 66° and 65° belong to Ge1− x Snx films. (B) Reciprocal
space map from asymmetrical plane (−2, −2, 4) for Ge0.94 Sn0.06 grown on a
Si substrate. The x -coordinate shows out-of-plane lattice constant and the
y -coordinate shows in-plane lattice constant in units of reciprocal lattice
unit. The relaxation line shows that the films grown above are tensile
strained and below are compressively strained. Presence of the

Table 2 | Tin mole fraction calculation, lattice constant, and relaxation
percentage of the grown samples.
Sample Sn (%) a kΠ (nm) a ⊥ (nm) a (nm) Relaxation Thickness
no.
5.668

(%)

(nm)

98

615

1

1.2

5.666

5.671

2

2.1

5.673

5.679

5.676

98

423

3

2.9

5.678

5.687

5.682

97

295

4

4.2

5.689

5.695

5.692

98

207

5

5.8

5.699

5.712

5.706

97

108

6

7.0

5.715

5.719

5.717

99

532

2H2 + SnCl4 → Sn + 4HCl
GeH4 + SnCl4 → Ge + Sn + 4HCl

(2)
(3)

Higher temperature of the substrate results in higher density
of depositing ad-atoms (Ge and Sn); however, it will result in production of HCl at a higher rate. In addition, higher flow rate of
SnCl4 increases the production rate of HCl as well. Controlling the
temperature and flow rate of the gases could control the process

Frontiers in Materials | Optics and Photonics

Ge0.94 Sn0.06 on the relaxation line shows that the film is strain relaxed.
(C) Transmission electron microscopy images of Ge0.94 Sn0.06 film shows
epitaxial growth of Ge–Sn on a Si substrate. Arrows show misfit
dislocations formed at the Ge1− x Snx /Si interface. (D) The EDX spectrum of
Si/Ge0.94 Sn0.06 film shows the presence of Si (substrate), Ge and Sn (film),
O (native oxide), and C (carbon contamination from the ambient). (E) The
EDX surface maps of Ge and (F) Sn taken from scanning electron
micrographs for Ge0.94 Sn0.06 film shows uniform growth of Ge1− x Snx alloy.

so that growth is the dominant process in the chamber. The Ge/Sn
film will be etched by HCl through the following reactions:
4HCl + Ge → GeH4 + 2Cl2

(4)

4HCl + Sn → SnCl4 +2H2

(5)

Domination of etching over growth is the main mechanism
that prevents direct growth of Ge1−x Snx on Si.
By controlling the flow through MFCs, we have grown
Ge1−x Snx films on Si at different pressures with a fixed flow ratio
of GeH4 /SnCl4 = 1.6. Growth was observed at 13 Pa of deposition
pressure and continued until the deposition pressure increased to
130 Pa. Figure 3A shows the thickness of Ge1−x Snx films versus
deposition pressure of the chamber as well as Sn incorporation
percentage. Incorporation of Sn in the Ge lattice is increased by
raising the pressure due to the higher residence time of the precursors in the chamber. The residence time of the gases has increased
from 2 s at 13 Pa to 19 s at 130 Pa. Meanwhile, HCl etched more
of the Ge1−x Snx films after deposition at higher pressures. This
trend has continued to 130 Pa and no growth has been observed
at 200 and 265 Pa. The increase in Sn composition from 1 to 6%

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Direct Ge–Sn growth on Si using UHV-CVD

FIGURE 3 | (A) Variation of Sn incorporation percentage versus deposition
pressure. Films were etched away for deposition pressures higher than
130 Pa. The secondary axis on the right shows the reduction of film thickness

as a result of increase in the deposition pressure. (B) Tin incorporation and
film thickness of the samples grown at 65 Pa growth pressure versus
GeH4 /SnCl4 flow ratios.

FIGURE 4 | (A) Raman spectra of the Ge1− x Snx film grown on a Si
substrate. The shift in the Ge–Ge peak is due to the incorporation of Sn in
Ge lattice. The shoulder on the left side of the Ge–Ge peak is due to the
Ge–Sn peak at 285 cm−1 . The Ge–Sn peak is shown at lower
wavenumber of 250–260 cm−1 . (B) Ge–Ge and Ge–Sn peak shifts versus

Sn mole fraction. The solid symbols are experimental data and the
curves are theoretical predictions for relaxed films. The Ge–Ge peak is
expected to shift 0.8310 cm−1 for every 1% Sn incorporation in relaxed
films. The expected shift (0.8311 cm−1 ) for Ge–Sn peak is very close to
that of Ge–Ge.

has been accompanied with reduction in the thickness from 615 to
108 nm. Films that were expected to have higher than 6% Sn content were totally etched off. Therefore, in order to grow higher Sn
content films, growth mechanism under fixed pressure and changing the SnCl4 flow was studied. Higher film thickness and higher
Sn incorporation was achieved as a result of domination of growth
over etching. Figure 3B shows Sn incorporation in Ge1−x Snx films
versus SnCl4 flow rate at 95 Pa deposition pressure. The secondary
axis of Figure 3B shows film thicknesses of the samples. Due to the
dominance of etching for higher SnCl4 flow rate, the films were
mostly etched and the film thickness was less than 100 nm.
Introduction of carrier gases has different effects on the growth
of Ge1−x Snx films. Hydrogen changes the balance in the reaction to
produce more HCl. Consequently, the GeH4 /SnCl4 ratio at which
the Ge1−x Snx films were depositing will not result in growth when
hydrogen is introduced in the chamber. In addition, introduction
of nitrogen and argon as carrier gases will reduce the activation

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energy of the growth (Wirths et al., 2013). Although reduction
of activation energy enables easier breakdown of the molecules
on the surface and enhances the growth quality and growth rate,
it would prepare the conditions for easier etch due to the presence of an etchant agent. Therefore, the presence of carrier gases
pushes the competition between growth and etching toward etching, resulting in film etching at even lower flow rates of carrier
gases when the flow rate of SnCl4 is of the same order of GeH4 .
OPTICAL CHARACTERIZATION

Raman spectroscopy

The Ge1−x Snx films were further investigated by Raman spectroscopy in order to analyze the crystal structure. Room temperature Raman spectra of the grown samples as well as a Ge reference
sample are plotted in Figure 4A. The Ge–Ge longitudinal optical
(LO) peak was observed at 300 cm−1 for the Ge reference sample
while the Ge–Ge peak in the Ge1−x Snx films was shifted to lower

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Mosleh et al.

FIGURE 5 | (A) Photoluminescence spectra of the Ge1− x Snx films
with 2, 4, 6, and 7% Sn mole fraction showing a red-shift in the
band-gap of the films. Incorporation of Sn has shifted both direct
band-gap and indirect band-gap toward lower energies. (B) The

wavenumbers due to the change in bonding energy of Ge–Ge by
incorporation of Sn atoms. The intensity of the Ge–Ge LO peak
at 300 cm−1 is normalized for all the samples for comparison of
the peak positions. In addition to the main Ge–Ge peak, Raman
spectra of Ge1−x Snx films show other peaks that are induced as
a result of Sn incorporation. The Ge–Sn LO peaks for different
Sn mole fractions were observed at 250–260 cm−1 in the films. A
second peak of Ge–Sn is observed at 285 cm−1 , which can be seen
as a shoulder of Ge–Ge main peak.
The peak positions are obtained by Lorentzian fitting to find
the exact position for further analysis. The shift in the Ge–Ge
LO peak depends on both strain and Sn composition of the
films. Theoretical calculations for ∆ω are different for strainrelaxed films and strained films for different Sn (x) content
[∆ωGe−Ge (x) = bx cm−1 ]. The Ge–Ge peak is expected to shift
by a factor of b = −30.30 for a strained alloy while this factor
varies to b = −83.10 for a strain-relaxed film (Cheng et al., 2013).
Figure 4B shows the experimental data obtained for Ge–Ge and
Ge–Sn Raman shift from the sample compared with the theoretical calculations. The peak shifts match well with the theoretical
calculations for strain-relaxed films.
Photoluminescence

Germanium has an indirect band-gap in the L valley with the
energy of 0.644 eV and a direct band-gap at the γ point with 0.8 eV
energy at room temperature. Incorporation of Sn in Ge lattice lowers the conduction band edge at the γ-point at a faster rate than
that at the L-point. PL measurements on Ge1−x Snx samples allow
determination of the band-gap edge for various Sn compositions.
Figure 5 depicts room temperature PL intensity spectra for
as-grown Ge1−x Snx films with 2, 4, 6, and 7% Sn mole fractions. As indicated in Figure 5A, increase of the Sn mole fraction
results in a band-gap reduction. Both direct and indirect PL peaks
exhibit red-shift with Sn compositions increase from 2 to 7%. A
Gaussian fitting function was employed to extract the PL peak
positions of both direct and indirect transitions as described in

Frontiers in Materials | Optics and Photonics

Direct Ge–Sn growth on Si using UHV-CVD

bowed Vegard’s law interpolation for the direct (solid line) and
indirect band-gap (dash line) of Ge1− x Snx alloy is plotted for different
Sn compositions and is overlaid with experimental data (solid
symbols).

Ghetmiri et al. (2014b). In Ge0.94 Sn0.06 and Ge0.93 Sn0.07 samples,
the energies difference between direct and indirect transitions are
very small, therefore the PL emissions from these indirect and
direct transitions cannot be identified. A temperature-dependent
study is needed to differentiate the direct and indirect peak positions which will be reported in the future. The PL peaks from
the samples with 2, 4, 6, and 7% Sn compositions are shown in
Figure 5B as solid symbols. The solid and the dashed lines show
the direct and indirect band-gap energies based on bowed Vegard’s law for the relaxed Ge1−x Snx alloy (Ghetmiri et al., 2014b),
respectively. Since the Ge1−x Snx films are almost strain-free, as
confirmed by XRD measurements, the experimental results closely
follow the predicted values from Vegard’s law.

CONCLUSION
Direct growth of Ge1−x Snx layers on Si substrates was achieved
using a cold-wall UHV-CVD system. The films were grown by
employing low-cost commercial available GeH4 and SnCl4 precursors without using any carrier gases and buffer layers. Characterizations of the samples with XRD showed successful incorporation
of Sn up to 7%. The TEM images show fully epitaxial growth of the
samples without any precipitation of Sn from the Ge lattice. The
Raman results verified the Sn incorporation and PL measurements
showed reduction of the band-gap to 0.55 eV for 7% Sn sample.
The low-cost and CMOS compatible growth method and the performance of the samples indicate a promising future for Ge1−x Snx
applications in Si photonics. Moreover, the samples were grown
strain-relaxed enabling this material to be a universal compliant
buffer layer which can be used in hybrid integration.

ACKNOWLEDGMENTS
The work at the UA was supported by NSF (EPS-1003970),
the Arkansas Bioscience Institute, the Arktonics, LLC (Air Force
SBIR, FA9550-14-C-0044, Dr. Gernot Pomrenke, Program Manager), and DARPA (W911NF-13-1-0196, Dr. Dev Palmer, Program
Manager). Drs. RS and GS acknowledge support from AFOSR

April 2015 | Volume 2 | Article 30 | 104

Mosleh et al.

(FA9550-14-1-0196, Dr. Gernot Pomrenke, Program Manager).
JG acknowledges the support of NSF REU Program under Grant
number EEC-1359306.

REFERENCES
Beeler, R. T., Grzybowski, G. J., Roucka, R., Jiang, L., Mathews, J., Smith, D.
J., et al. (2011a). Synthesis and materials properties of Sn/P-doped Ge on Si
(100): photoluminescence and prototype devices. Chem. Mater. 23, 4480–4486.
doi:10.1021/cm201648x
Beeler, R., Roucka, R., Chizmeshya, A., Kouvetakis, J., and Menéndez, J. (2011b).
Nonlinear structure-composition relationships in the Ge1-y Sny Si (100) (y<
0.15) system. Phys. Rev. B 84, 035204. doi:10.1103/PhysRevB.84.035204
Bhargava, N., Coppinger, M., Gupta, J. P., Wielunski, L., and Kolodzey, J. (2013). Lattice constant and substitutional composition of GeSn alloys grown by molecular
beam epitaxy. Appl. Phys. Lett. 103, 041908. doi:10.1063/1.4816660
Bogumilowicz, Y., Hartmann, J. M., Truche, R., Campidelli, Y., Rolland, G., and Billon, T. (2005). Chemical vapour etching of Si, SiGe and Ge with HCl; applications
to the formation of thin relaxed SiGe buffers and to the revelation of threading
dislocations. Semicond. Sci. Technol. 20, 127. doi:10.1088/0268-1242/20/2/004
Chen, R., Huang, Y., Gupta, S., Lin, A. C., Sanchez, E., Kim, Y., et al. (2013).
Material characterization of high Sn-content, compressively-strained GeSn
epitaxial films after rapid thermal processing. J. Cryst. Growth 365, 29–34.
doi:10.1016/j.jcrysgro.2012.12.014
Chen, R., Lin, H., Huo, Y., Hitzman, C., Kamins, T. I., and Harris, J. S. (2011).
Increased photoluminescence of strain-reduced, high-Sn composition Ge1-x Snx
alloys grown by molecular beam epitaxy. Appl. Phys. Lett. 99, 181125. doi:10.
1063/1.3658632
Cheng, R., Wang, W., Gong, X., Sun, L., Guo, P., Hu, H., et al. (2013). Relaxed and
strained patterned germanium-tin structures: a Raman scattering study. ECS J.
Solid State Sci. Technol. 2, 138–145. doi:10.1149/2.013304jss
Conley, B. R., Mosleh, A., Ghetmiri, S. A., Du, W., Soref, R. A., Sun, G., et al. (2014a).
Temperature dependent spectral response and detectivity of GeSn photoconductors on silicon for short wave infrared detection. Opt. Express 22, 15639–15652.
doi:10.1364/OE.22.015639
Conley, B. R., Margetis, J., Du, W., Tran, H., Mosleh, A., Ghetmiri, S. A., et al. (2014b).
Si based GeSn photoconductors with a 1.63 A/W peak responsivity and a 2.4 µm
long-wavelength cutoff. App. Phys. Lett. 105, 221117. doi:10.1063/1.4903540
Du, W., Zhou, Y., Ghetmiri, S. A., Mosleh, A., Conley, B. R., Nazzal, A., et al. (2014a).
Room-temperature electroluminescence from Ge/Ge1-x Snx /Ge diodes on Si substrates. Appl. Phys. Lett. 104, 241110. doi:10.1063/1.4884380
Du, W., Ghetmiri, S. A., Conley, B. R., Mosleh, A., Nazzal, A., Soref, R. A.,
et al. (2014b). Competition of optical transitions between direct and indirect
bandgaps in Ge1-x Snx . Appl. Phys. Lett. 105, 051104. doi:10.1063/1.4892302
Ghetmiri, S. A., Du, W., Margetis, J., Mosleh, A., Cousar, L., Conley, B. R., et al.
(2014a). Direct-bandgap GeSn grown on Silicon with 2230 nm photoluminescence. Appl. Phys. Lett. 105, 151109. doi:10.1063/1.4898597
Ghetmiri, S. A., Du, W., Conley, B. R., Mosleh, A., Naseem, H. A., Yu, S., et al.
(2014b). Shortwave-infrared photoluminescence from Ge1-x Snx thin films on
silicon. J. Vac. Sci. Technol. B 32, 060601. doi:10.1116/1.4897917
Gurdal, O., Desjardins, P., Carlsson, J., Taylor, N., Radamson, H., Sundgren, J.,
et al. (1998). Low-temperature growth and critical epitaxial thicknesses of fully
strained metastable Ge1-x Snx (x.0.26) alloys on Ge (001) 2 × 1. J. Appl. Phys.
83, 162–170. doi:10.1063/1.366690
Kouvetakis, J., and Chizmeshya, A. V. G. (2007). New classes of Si-based photonic
materials and device architectures via designer molecular routes. J. Mater. Chem.
17, 1649–1655. doi:10.1039/b618416b
Kouvetakis, J., Menendez, J., and Soref, R.A. Strain-Engineered Direct-Gap
Ge/SnxGe1-x Heterodiode and Multi-Quantum-Well Photodetectors, Laser, Emitters and Modulators Grown on SnySizGe1-y-z Buffered Silicon. United States
patent US 6897471 B1 (2005).
Li, H., Brouillet, J., Salas, A., Chaffin, I., Wang, X., and Liu, J. (2014). Low
temperature geometrically confined growth of pseudo single crystalline GeSn
on amorphous layers for advanced optoelectronics. ECS Trans. 64, 819–827.
doi:10.1149/06406.0819ecst
Margetis, J., Ghetmiri, S. A., Du, W., Conley, B. R., Mosleh, A., Soref, R., et al. (2014).
Growth and characterization of epitaxial Ge1-X Snx alloys and heterostructures

www.frontiersin.org

Direct Ge–Sn growth on Si using UHV-CVD

using a commercial CVD system. ECS Trans. 64, 1830–1830. doi:10.1149/06406.
0711ecst
Moontragoon, P., Soref, R., and Ikonic, Z. (2012). The direct and indirect bandgaps
of unstrained Si x Ge1-xy Sny and their photonic device applications. J. Appl. Phys.
112, 073106–073106–8. doi:10.1063/1.4757414
Mosleh, A., Benamara, M., Ghetmiri, S. A., Conley, B. R., Alher, M. A., Du, W., et al.
(2014). Investigation on the formation and propagation of defects in GeSn thin
films. ECS Trans. 64, 1845–1845. doi:10.1149/06406.0895ecst
Mosleh, A., Ghetmiri, S. A., Conley, B. R., Abu-Safe, H., Waqar, Z., Benamara,
M., et al. (2013). “Nucleation-step study of silicon homoepitaxy for lowtemperature fabrication of Si solar cells,” in Photovoltaic Specialists Conference
(PVSC), IEEE 39th . Tampa, FL.
Mosleh, A., Ghetmiri, S. A., Conley, B. R., Hawkridge, M., Benamara, M., Nazzal, A.,
et al. (2014a). Material characterization of Ge1-x Snx alloys grown by a commercial CVD system for optoelectronic device applications. J. Electron. Mater. 43,
938–946. doi:10.1007/s11664-014-3089-2
Mosleh, A., Ghetmiri, S. A., Conley, B. R., Abu-Safe, H. H., Benamara, M., Waqar,
Z., et al. (2014b). Investigation of growth mechanism and role of H2 in very low
temperature Si epitaxy. ECS Trans. 64, 967–975. doi:10.1149/06406.0967ecst
Oehme, M., Buca, D., Kostecki, K., Wirths, S., Holländer, B., Kasper, E., et al. (2013).
Epitaxial growth of highly compressively strained GeSn alloys up to 12.5% Sn. J.
Cryst. Growth 384, 71–76. doi:10.1016/j.jcrysgro.2013.09.018
Stefanov, S., Conde, J., Benedetti, A., Serra, C., Werner, J., Oehme, M., et al. (2012).
Laser synthesis of germanium tin alloys on virtual germanium. Appl. Phys. Lett.
100, 104101. doi:10.1063/1.3692175
Takeuchi, S., Sakai, A., Yamamoto, K., Nakatsuka, O., Ogawa, M., and Zaima, S.
(2007). Growth and structure evaluation of strain-relaxed Ge1-x Sn buffer layers grown on various types of substrates. Semicond. Sci. Technol. 22, S231.
doi:10.1088/0268-1242/22/1/S54
Vincent, B., Gencarelli, F., Bender, H., Merckling, C., Douhard, B., Petersen, D.
H., et al. (2011). Undoped and in-situ B doped GeSn epitaxial growth on
Ge by atmospheric pressure-chemical vapor deposition. Appl. Phys. Lett. 99,
152103–152103–3. doi:10.1063/1.3645620
Wang, L., Su, S., Wang, W., Gong, X., Yang, Y., Guo, P., et al. (2013). Strained
germanium–tin (GeSn) p-channel metal-oxide-semiconductor field-effecttransistors (p-MOSFETs) with ammonium sulfide passivation. Solid State Electron. 83, 66–70. doi:10.1016/j.sse.2013.01.031
Werner, J., Oehme, M., Schmid, M., Kaschel, M., Schirmer,A., Kasper, E., et al. (2011).
Germanium-tin pin photodetectors integrated on silicon grown by molecular
beam epitaxy. Appl. Phys. Lett. 98, 061108–061108–3. doi:10.1063/1.3555439
Wirths, S., Buca, D., Mussler, G., Tiedemann, A., Holländer, B., Bernardy, P., et al.
(2013). Reduced pressure CVD growth of Ge and Ge1-x Snx alloys. ECS J. Solid
State Sci. Technol. 2, N99–N102. doi:10.1149/2.006305jss
Wirths, S., Geiger, R., Scherrer, P., Ikonic, Z., Tiedemann, A. T., Mussler, G., et al.
(2014). “Epitaxy and photoluminescence studies of high quality GeSn heterostructures with Sn concentrations up to 13 at.%,” in 11th International Conference on Group IV Photonics 27–29 August. Paris.

Conflict of Interest Statement: The authors declare that the research was conducted
in the absence of any commercial or financial relationships that could be construed
as a potential conflict of interest.
Received: 28 January 2015; accepted: 23 March 2015; published online: 27 April 2015.
Citation: Mosleh A, Alher MA, Cousar LC, Du W, Ghetmiri SA, Pham T, Grant JM,
Sun G, Soref RA, Li B, Naseem HA and Yu S-Q (2015) Direct growth of Ge1−x Snx films
on Si using a cold-wall ultra-high vacuum chemical-vapor-deposition system. Front.
Mater. 2:30. doi: 10.3389/fmats.2015.00030
This article was submitted to Optics and Photonics, a section of the journal Frontiers
in Materials.
Copyright © 2015 Mosleh, Alher, Cousar, Du, Ghetmiri, Pham, Grant , Sun, Soref,
Li, Naseem and Yu. This is an open-access article distributed under the terms of the
Creative Commons Attribution License (CC BY). The use, distribution or reproduction
in other forums is permitted, provided the original author(s) or licensor are credited
and that the original publication in this journal is cited, in accordance with accepted
academic practice. No use, distribution or reproduction is permitted which does not
comply with these terms.

April 2015 | Volume 2 | Article 30 | 105

ORIGINAL RESEARCH ARTICLE

MATERIALS

published: 23 February 2015
doi: 10.3389/fmats.2015.00008

Room-temperature near-infrared electroluminescence
from boron-diffused silicon pn-junction diodes
Si Li , Yuhan Gao, Ruixin Fan, Dongsheng Li and Deren Yang*
State Key Lab of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou, China

Edited by:
Dan-Xia Xu, National Research
Council Canada, Canada
Reviewed by:
Jifeng Liu, Dartmouth College, USA
Tatiana S. Perova, The University of
Dublin, Ireland
*Correspondence:
Deren Yang, State Key Lab of Silicon
Materials, Department of Materials
Science and Engineering, Zhejiang
University, Zheda Road 38, Hangzhou
310027, China
e-mail: mseyang@zju.edu.cn

Silicon pn-junction diodes with different doping concentrations were prepared by boron
diffusion into Czochralski n-type silicon substrate. Their room-temperature near-infrared
electroluminescence (EL) was measured. In the EL spectra of the heavily boron doped
diode, a luminescence peak at ~1.6 µm (0.78 eV) was observed besides the band-to-band
line (~1.1 eV) under the condition of high current injection, while in that of the lightly boron
doped diode only the band-to-band line was observed. The intensity of peak at 0.78 eV
increases exponentially with current injection with no observable saturation at room temperature. Furthermore, no dislocations were found in the cross-sectional transmission
electron microscopy image, and no dislocation-related luminescence was observed in
the low-temperature photoluminescence spectra. We deduce that the 0.78 eV emission
originates from the irradiative recombination in the strain region of diodes caused by the
diffusion of large number of the boron atoms into a silicon crystal lattice.
Keywords: boron diffusion, silicon pn-junction diode, near-infrared electroluminescence

INTRODUCTION
With the development of integrated circuits (ICs), the disadvantage of traditional metal interconnection structure, such as
interlayer interference, energy dissipation, and signal delay, has
become a bottleneck restricting the development of ultra-largescale integration circuits (USLIs). Optical interconnection, which
uses photons to transform information, will be an ultimate solution for future progress in USLIs. Because silicon is an indirectband-gap semiconductor and fundamentally unable to emit light
efficiently, achieving efficient silicon-based light sources compatible with current IC manufacturing technology has become the key
issue of silicon optoelectronics. Many routes to fabricate efficient
silicon light emitters have been proposed: porous silicon (Canham, 1990; Qin et al., 1996; Bisi et al., 2000; Zhao et al., 2005a,b),
Si nanoprecipitates in SiO2 (Pavesi et al., 2000; Wang et al., 2007),
erbium-doped Si (Ennen et al., 1983; Zheng et al., 1994; Polman
et al., 1995), Si/SiO2 superlattice structures (Lu et al., 1995), and
silicon pn-junction diodes (Sveinbjörnsson, 1996; Martin et al.,
2001; Ng et al., 2001; Sun et al., 2004; Lourenco et al., 2005). Among
these ways, silicon pn-junction diodes have attracted much attention. The most standout advantage of this kind of light-emitter
is that the fabrication process is totally compatible with USLI
technology. Both ion-implantation (Sveinbjörnsson, 1996; Martin et al., 2001; Ng et al., 2001; Sun et al., 2004; Lourenco et al.,
2005; Sobolev, 2010) and thermal diffusion (Kveder et al., 2004;
Hoang et al., 2006, 2007) have been used to manufacture silicon
pn diodes. The past few years has seen great advances in the development of silicon pn-junction diodes. Electroluminescence (EL)
efficiency of 0.1–1% has been achieved (Martin et al., 2001; Ng
et al., 2001). In addition to the band-to-band emission around
1.1 µm, other near-infrared emissions have been found in boronimplanted and boron-diffused silicon pn diodes (Sveinbjörnsson,
1996; Sun et al., 2004). Sveinbjörnsson (1996) reported strong

Frontiers in Materials | Optics and Photonics

~1.6 µm (0.78 eV) EL emission related to dislocation-related center D1 at room temperature from dislocation-rich silicon diodes.
Sun et al. (2004) reported two luminescence bands around 1.05
and 0.95 eV related to doping spikes in boron-implanted silicon pn
diodes. These emissions show great application potential in silicon
optoelectronics. But the mechanism is still in dispute.
In this paper, we fabricated silicon pn-junction diodes with
different boron doping concentrations. Their room-temperature
EL was measured and their cross-sectional transmission electron
microscopy (TEM) images were studied. The result shows that
the heavily boron doped silicon pn-junction diode without dislocation loops can emit strong 0.78 eV luminescence under the
condition of high current injection besides the band-to-band
emission. It is considered that the 0.78 eV emission originates
from the irradiative recombination in the strain regions caused by
the diffusion of large number of boron atoms into silicon crystal
lattice.

MATERIALS AND METHODS
Two kinds of boron diffusion sources were prepared by dissolving B2 O3 into SiO2 latex with B3+ concentration of 0.203 mol/L
(marked as A) and 0.569 mol/L (marked as B), respectively. Boron
sources were spin onto the surface of (100) oriented n-type
Czochralski-grown Si substrates (2 ~ 10 Ω cm, 500 µm in thickness) after the substrate wafer was cut into 15 mm × 15 mm slices
and carefully cleaned by standard RCA process. Rapid thermal
treating method was used to form shallow pn junction by boron
diffusion at 1100°C for 5 min in the flowing high-purity N2 atmosphere. After a pn junction was formed, an indium tin oxide (ITO)
electrode with a thickness of 100 nm was deposited on the p-layer
side by magnetron sputtering, and an Al electrode with a thickness
of 100 nm was evaporated on the n-layer side. Thus, a pn-junction
diode was prepared.

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Li et al.

The carrier concentration and the depth of pn junctions were
studied by an SSM350 instrument of spreading resistance profile (SRP). The microstructure of pn junctions was measured by a
transmission electron microscope (TEM, JEOL 2010). Photoluminescence (PL) and EL signals were recorded using an Edinburgh
FLS920P Spectrometer with a nitrogen-cooled near-infrared photomultiplier tube. The low-temperature PL measurements were
performed over the range of 20 ~ 300 K by using a helium flow
cryostat.

Electroluminescence from silicon pn junction

EL spectra of Sample A, only band-to-band emission is observed
as the current increases. In contrast, Sample B emits 0.78 eV EL
besides the band-to-band emission under the condition of high
current injection (>705 mA) and its intensity increases greatly
with the current. The band-to-band emission of both pn diodes
demonstrates a small red shift with the increasing current; this is
related to the device heating in response to the current injection.
Sveinbjörnsson (1996) and Xiang et al. (2012a,b) have reported
strong emission of 0.78 eV EL at room temperature from silicon pn diodes containing dislocations. They have also found

RESULTS AND DISCUSSION
Figure 1 is the SRP results of pn junctions made from the two
boron sources (A and B). It is clear from the figure that shallow pn
junctions were formed. It can be seen that Sample B made from the
B boron source has the higher carrier concentration than Sample A
made from the A boron source. The surface carrier concentration
of Sample B reaches 4.6 × 1017 cm−3 , while that of Sample A is
5 × 1016 cm−3 . It is necessary to notify that the SPR measures the
activated dopant density only, which does not take into account
the possible dopant clustering at the surface, so there can be a large
amount of boron doping, which is inactive. The depth of Sample
B pn junction is about 250 nm, a little deeper than that of Sample
A, which is about 200 nm.
Figure 2 shows the I –V curves of the two pn diodes. As shown
in the figure, both the pn diodes perform good rectifying properties. The forward current increases quickly with the voltage while
the reverse current stays low. Under the same forward bias, the current of Sample B is greater than that of Sample A. This is because
Sample B has the higher carrier concentration and less resistance.
In addition, the turn-on voltage of Sample B is about 0.7 V, bigger
than that of Sample A, which is about 0.6 V. The reason is that the
pn-junction barrier of Sample B with higher doping concentration is larger, so that the forward voltage needed to overcome the
barrier is larger.
The room-temperature EL spectra of Sample B and Sample
A are different, as shown in Figure 3. In the room-temperature

FIGURE 2 | I –V curves of the two pn diodes at room temperature.

A

B

FIGURE 1 | Spreading resistance profile spectra of pn junctions made
from Sample A and Sample B fabricated with two boron sources.

www.frontiersin.org

FIGURE 3 | Room-temperature EL spectra at different electrical
currents of two pn diodes: (A) for sample B and (B) for sample A.

February 2015 | Volume 2 | Article 8 | 107

Li et al.

Electroluminescence from silicon pn junction

FIGURE 4 | (A) Photoluminescence spectra registered at different temperatures under laser excitation of 808 nm (500 mW) and (B) cross-sectional TEM image
of sample B.

lattice distortion is caused by large number of boron atoms diffusing into silicon lattice. Under the condition of high current
injection, these regions can trap carriers and form effective irradiative recombination centers, which are related to the 0.78 eV
luminescence.
Figure 5 shows the dependence of peak intensity of 0.78 eV
on the input power. When the input power is low, there is no
0.78 eV EL emission. When the input power reaches ~15 W, Sample B starts to emit 0.78 eV luminescence and its intensity increases
almost exponentially with the input power with no observable
saturation. This means that if the turn-on power of the o.78 eV
emission could be sufficiently decreased, a highly efficient light
source would be achieved.

CONCLUSION

FIGURE 5 | The dependence of peak intensity of EL band at 0.78 eV on
the input power of pn-junction diodes.

dislocation-related bands in the low-temperature PL spectra.
They tended to regard the peak at 0.78 eV in PL spectrum at
room temperature as a red-shifted luminescence band D1. However, for Sample B, no other luminescence band is found in
the low-temperature PL spectrum (the pump power intensity is
0.22 W/cm2 ) except the band-to-band emission and no dislocations are observed in the cross-sectional TEM image as shown
in Figure 4. It can be seen from Figure 4B that Sample B is
free from dislocations. Our work suggests that the 0.78 eV EL
at room temperature has no direct connection with dislocations
or dislocation-related luminescence bands. In fact, lots of lattice
damage regions can be seen near the surface of Sample B in the
TEM image. As mentioned before, although the measured dopant
concentration by SPR is relatively low, a significant amount of
inactive boron doping may exist, so we think that the observed

Frontiers in Materials | Optics and Photonics

In this paper, two silicon pn diodes with different boron doping concentrations were fabricated by boron diffusion. We studied
their room-temperature near-infrared EL. The results show that
in the EL spectra of the heavily boron doped diode, a luminescence peak at ~1.6 µm (0.78 eV) was observed besides the
band-to-band line (~1.1 eV) under the condition of high current
injection, while in that of the lightly boron doped diode, only
the band-to-band line was observed. In addition, no dislocations
were found in the cross-sectional TEM image and no dislocationrelated luminescence was observed in the low-temperature PL
spectra. The 0.78 eV emission is proved to have no direct connection with dislocations or dislocation-related luminescence bands.
In fact, lots of lattice damage regions can be seen near the surface of the highly doped diode in the TEM image. We deduce that
the 0.78 eV emission may originate from the irradiative recombination in these regions. What is more, the intensity of 0.78 eV
emission increases exponentially with the input power without
observable saturation, which may be used as an efficient light
source in future.

ACKNOWLEDGMENTS
This work is supported by the National Basic Research Program
of China (973 Program) (No. 2013CB632102).

February 2015 | Volume 2 | Article 8 | 108

Li et al.

REFERENCES
Bisi, O., Ossicini, S., and Pavesi, L. (2000). Surf. Sci. Rep. 38, 1–126. doi:10.1016/
S0167-5729(99)00012-6
Canham, L. T. (1990). Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers. Appl. Phys. Lett. 57, 1046–1048.
doi:10.1063/1.103561
Ennen, H., Schneider, J., Pomerenke, G., and Axmann, A. (1983). 1.54-µm Luminescence of erbium implanted III–V? semiconductors and silicon. Appl. Phys.
Lett. 43, 943–945. doi:10.1063/1.94190
Hoang, T., Holleman, J., LeMinh, P., Schmitz, J., Mchedlidze, T., Arguirov, T., et al.
(2007). Influence of dislocation loops on the near-infrared light emission from
silicon diodes. IEEE Trans. Electron Devices 54, 1860–1866. doi:10.1109/TED.
2007.901072
Hoang, T., LeMinh, P., Holleman, J., and Schmitz, J. (2006). Strong efficiency
improvement of SOI-LEDs through carrier confinement. IEEE Electron Device
Lett. 27, 383–385. doi:10.1109/LED.2007.895415
Kveder, V., Badylevich, M., Steinman, E., Lzotov, A., Seibt, M., and Schroter, W.
(2004). Room-temperature silicon light-emitting diodes based on dislocation
luminescence. Appl. Phys. Lett. 84, 2106–2108. doi:10.1063/1.1689402
Lourenco, M. A., Milosavljevic, M., Galata, S., Siddiqui, M. S. A., Shao, G., Gwilliam,
R. M., et al. (2005). Silicon-based light-emitting devices. Vacuum 78, 551–556.
doi:10.1016/j.vacuum.2005.01.085
Lu, Z. H., Lockwood, D. J., and Baribeau, J. M. (1995). Quantum confinement
and light emission in SiO2 /Si superlattices. Nature 378, 258–260. doi:10.1038/
378258a0
Martin, A. G., Zhao, J., Wang, A., Peter, J. R., and Michal, G. (2001). Efficient silicon
light-emitting diodes. Nature 412, 805–808. doi:10.1038/35090539
Ng, W. L., Lourenco, M. A., Gwilliam, R. M., Ledain, S., Shao, G., and Hoomwood,
K. P. (2001). An efficient room-temperature silicon-based light-emitting diode.
Nature 410, 192–194. doi:10.1038/35069092
Pavesi, L., Dal, N. L., Mazzoleni, C., Franzo, G., and Priolo, F. (2000). Optical gain
in silicon nanocrystals. Nature 408, 440–444. doi:10.1038/35044012
Polman, A., Van, D., Hoven, G. N., Custer, J. S., Shin, J. H., and Sema, R. (1995).
Erbium in crystal silicon: optical activation, excitation, and concentration limits.
J. Appl. Phys. 77, 1256–1262. doi:10.1063/1.358927
Qin, G. G., Song, H. Z., Zhang, B. R., Lin, J., Duan, J. Q., and Yao, G. Q. (1996).
Experimental evidence for luminescence from silicon oxide layers in oxidized
porous silicon. Phys. Rev. B 54, 2548–2555. doi:10.1103/PhysRevB.54.2548
Sobolev, N. A. (2010). Defect engineering in implantation technology of silicon
light-emitting structures with dislocation-related luminescence. Semiconductors
44, 1–23. doi:10.1134/S106378261001001X
Sun, J. M., Dekorsy, T., Skorupa, W., Schmdt, B., Mucklich, A., and Helm,
M. (2004). Below-band-gap electroluminescence related to doping spikes in

www.frontiersin.org

Electroluminescence from silicon pn junction

boron-implanted silicon pn diodes. Phys. Rev. B 70, 1–11. doi:10.1103/PhysRevB.
70.155316
Sveinbjörnsson, E. Ö, and Weber, J. (1996). Room temperature electroluminescence
from dislocation-rich silicon. Appl. Phys. Lett. 69, 2686–2688. doi:10.1063/1.
117678
Wang, M., Li, D., Yuan, Z., Yang, D., and Que, D. (2007). Photoluminescence of
Si-rich silicon nitride: defect-related states and silicon nanoclusters. Appl. Phys.
Lett. 90, 1–3. doi:10.1063/1.2717014
Xiang, L., Li, D., Jin, L., and Yang, D. (2012a). Dislocation-related electroluminescence of silicon after electron irradiation. Solid State Commun. 152, 1956–1959.
doi:10.1016/j.ssc.2012.08.011
Xiang, L., Li, D., Jin, L., Pivac, B., and Yang, D. (2012b). The origin of 0.78eV
line of the dislocation related luminescence in silicon. J. Appl. Phys. 112, 1–4.
doi:10.1063/1.4754825
Zhao, Y., Yang, D., Li, D., and Jiang, M. (2005a). Annealing and amorphous silicon
passivation of porous silicon with the blue light emission. Appl. Surf. Sci. 252,
1065–1069. doi:10.1016/j.apsusc.2005.01.176
Zhao, Y., Yang, D., Li, D., and Jiang, M. (2005b). Photoluminescence of oxidized
porous silicon under UV-light illumination. Mater. Sci. Eng. B 116, 95–98.
doi:10.1016/j.mseb.2004.08.014
Zheng, B., Michel, J., Ren, F. G., Kimerling, L. C., Jacobson, D. C., and Poate, J.
M. (1994). Room-temperature sharp line electroluminescence at 1.54 µm from
an erbium-doped silicon light-emitting diode. Appl. Phys. Lett. 64, 2842–2844.
doi:10.1063/1.111977

Conflict of Interest Statement: The authors declare that the research was conducted
in the absence of any commercial or financial relationships that could be construed
as a potential conflict of interest.
Received: 17 August 2014; accepted: 20 January 2015; published online: 23 February
2015.
Citation: Li S, Gao Y, Fan R, Li D and Yang D (2015) Room-temperature near-infrared
electroluminescence from boron-diffused silicon pn-junction diodes. Front. Mater. 2:8.
doi: 10.3389/fmats.2015.00008
This article was submitted to Optics and Photonics, a section of the journal Frontiers
in Materials.
Copyright © 2015 Li, Gao, Fan, Li and Yang . This is an open-access article distributed
under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s)
or licensor are credited and that the original publication in this journal is cited, in
accordance with accepted academic practice. No use, distribution or reproduction is
permitted which does not comply with these terms.

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