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A continuous-wave hybrid AlGaInAs-silicon evanescent laser
Fang, A W
Paniccia, M J
Bowers, J E, University of California, Santa Barbara
Postprints, UC Santa Barbara
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complimentary metal-oxide-semiconductor, semiconductor lasers, silicon-on-insulator (SOI)
We report a novel laser architecture, the hybrid silicon evanescent laser (SEL), that utilizes
offset AlGaInAs quantum wells (QWs) bonded to a silicon waveguide. The silicon waveguide
is fabricated on a silicon-on-insulator wafer using a complimentary metal-oxide-semiconductor-
compatible process, and is subsequently bonded with the AlGaInAs QW structure using low
temperature 02 plasma-assisted wafer bonding. The optical mode in the SEL is predominantly
confined in the passive silicon waveguide and evanescently couples into the III-V active region
providing optical gain. The SEL lases continuous wave (CW) at 1568 nm with a threshold of 23
mW. The maximum temperature for CW operation is 60 degrees C. The maximum single-sided
fiber-coupled CW output power at room temperature is 4.5 mW.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 10, MAY 15, 2006 1143
A Continuous-Wave Hybrid AlGaInAs–Silicon
Alexander W. Fang, Student Member, IEEE, Hyundai Park, Student Member, IEEE, Richard Jones, Member, IEEE,
Oded Cohen, Mario J. Paniccia, Senior Member, IEEE, and John E. Bowers, Fellow, IEEE
Abstract—We report a novel laser architecture, the hybrid
silicon evanescent laser (SEL), that utilizes offset AlGaInAs
quantum wells (QWs) bonded to a silicon waveguide. The silicon
waveguide is fabricated on a silicon-on-insulator wafer using a
complimentary metal–oxide–semiconductor-compatible process,
and is subsequently bonded with the AlGaInAs QW structure
using low temperature O? plasma-assisted wafer bonding. The
optical mode in the SEL is predominantly confined in the passive
silicon waveguide and evanescently couples into the III–V active
region providing optical gain. The SEL lases continuous wave
(CW) at 1568 nm with a threshold of 23 mW. The maximum tem-
perature for CW operation is 60 C. The maximum single-sided
fiber-coupled CW output power at room temperature is 4.5 mW.
semiconductor lasers, silicon-on-insulator (SOI) technology.
tion wavelengths of 1.3 and 1.5
siveness in the integrated electronics industry. Silicon’s inef-
ficient light generation has been the major hindrance for the
realization of an electrically pumped laser on silicon, a key el-
ement for photonic integrated circuits. This has been addressed
in the form of a Raman laser ,  and material engineered
light-emitting diode structures ,  aimed at increasing light
emission. We report here an approach that utilizes AlGaInAs
cent tail overlapping into the offset quantum-well region. We
recently reported a pulsed silicon evanescent laser (SEL) oper-
ating at 20 C. We report here a continuous-wave (CW) SEL
operating at a maximum temperature of 60 C. At 20 C, the
devices lase with a threshold of 23 mW and maximum fiber-
coupled output of 4.5 mW. The confinement factor in the ac-
tive region and device length was increased in the CW device to
above 4.1% and 800 m, respectively, from 3.6% and 600 m
ILICON is an attractive material for a silicon photonics
platform because of its transparency at the communica-
m and because of its perva-
Manuscript received January 18, 2006; revised February 27, 2006. This work
(DARPA), and by Intel Corp.
A. W. Fang, H. Park, and J. E. Bowers are with the Department of Electrical
and Computer Engineering, University of California Santa Barbara, Santa Bar-
bara, CA 93106 USA (e-mail: firstname.lastname@example.org).
R. Jones and M. J. Paniccia are with the Photonics Technology Laboratory,
Intel Corporation, Santa Clara, CA 95054 USA.
O. Cohen is with the Photonics Technology Laboratory, Intel Corporation,
Jerusalem 91031, Israel.
Digital Object Identifier 10.1109/LPT.2006.874690
Fig. 1. Device structure cross section and cross section SEM image (inset).
in previous pulsed devices in order to increase gain and reduce
effective mirror losses.
II. DEVICE STRUCTURE AND FABRICATION
The device structure is shown in Fig. 1. The device is divided
into two regions: the silicon-on-insulator (SOI) passive-wave-
guide structure and the III–V active region that provides the
optical gain. The SOI structure consists of a Si substrate, a
1- m-thick SiO
lower cladding layer, and a Si rib wave-
guide with a height (
) and rib-etch depth ( ) of 0.7 and
m, respectively. The waveguide width (
from 1 to 5
m. The III–V region consists of a two-period
InP/1.1- m InGaAsP superlattice (SL), a 110-nm-thick InP
spacer, a 50-nm-thick unstrained 1.3- m AlGaInAs separated
confinement heterostructure (SCH) layer, strain-compensated
AlGaInAs QWs, a 500-nm-thick unstrained 1.3- m AlGaInAs
SCH layer, and an InP upper cladding layer. The SL region
employs 7.5-nm-thick alternating layers of InP–InGaAsP to
inhibit the propagation of defects from the bonded interface to
the QW region .
Five 7-nm-thick AlGaInAs QWs with compressive strain
(0.85%) and 10-nm-thick AlGaInAs barriers with tensile strain
( 0.55%) are used. The barrier layers have a bandgap corre-
sponding to a wavelength of 1.3 m.
a lightly p-doped (doping concentration
substrate by standard photolithography and reactive ion etching
(RIE) plasma of Cl –HBr–Ar. A thin layer of SiO was used
as a hard mask. The SOI wafer and III–V epitaxial wafer are
) is varied
1041-1135/$20.00 © 2006 IEEE
1144IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 10, MAY 15, 2006
treated by buffered HF and NH OH, respectively, after a thor-
ized water. The two samples are bonded together via oxygen
plasma assisted bonding . After a low temperature anneal
(300 C), the InP substrate is removed with HCl. The devices
are diced, the facets are polished, and the devices are character-
ized. Finally, the facets are coated with a broadband dielectric
HR coating ( 80%) consisting of three periods of SiO –Ta O
and characterizedagain. The final device lengthafter dicing and
polishing is 800 m. The bonded layer is continuous across the
ishing of the facets. The inset in Fig. 1 shows a scanning elec-
tion. The particles on the facet surface are due to the polishing
The thermal expansion coefficient mismatch between Si
K and InP
cracks in the III–V layers when bonding temperatures above
300 C are used. Low temperature oxide mediated bonding
was utilized to avoid these surface nonuniformities typically
seen in direct wafer bonding conducted at 600 C. The oxygen
plasma treatment generates a thin oxide layer ( 5 nm) whose
surface is very smooth and highly chemically reactive . As
a result, this bonding process creates a thin oxide layer at the
bonded interface; this does not significantly alter the optical
mode because it is so thin and transparent at 1.55 m.
K can introduce
III. EXPERIMENT AND RESULTS
The device is optically pumped perpendicular to the laser by
a 1250-nm wavelength fiber. The light from the pump laser is
focused by a cylindrical lens illuminating a 12 by 916 m rect-
angular spot incident on the device through the top InP cladding
layer. For the purpose of comparison, the incident pump power
reaching the device was determined by measuring the reflected
pump power and calculating the overlap of the pump beam with
the laser mode. The power reflectivity from the bonded wafer at
1250 nm was measured to be 40%, and the laser mode was cal-
culated by the length of the cavity multiplied by the computed
mode widths of 9.36, 4.98, 4.38, 4.48, 5.18, and 5.18
waveguide widths of 1, 1.5, 2.5, 3, 4, and 5
The laser output is collected with a multimode fiber from the
alyzer or photodetector. The fiber coupling efficiency is experi-
mentally measured to be approximately
1- and 3- m-wide devices, respectively, and can be assumed to
be at least
5 dB. The TE/TM near-field images of the output
mode are recorded on an IR camera through a polarizing beam
splitter and an 80x lens at the opposite waveguide facet.
Figs. 2 and 3 show the laser output power as a function
of pump power and temperature for two different waveguide
widthsof4and1 m.InFig.2,a4- m-widedeviceisoperating
with a threshold pump power of 23 mW with a fiber-coupled
maximum output powerof 4.5 mW and a slope efficiency of 3%
at 20 C. The total maximum output power taking into account
the light from both facets and the coupling losses of 5 dB is
approximately 28 mW and the corresponding slope efficiency
is 16%. The threshold increases from 23 to 105 mW between
5.3 and6.3 dB for
(inset) threshold versus temperature.
LL curves and mode profiles for 800-?m-long 4-?m-wide device
Fig. 3.LL curves and mode profile for 800-?m-long 1-?m-wide device.
20 C and 60 C and the structure exhibits a temperature coef-
of 27 K. The kinks in the LL curves are due to the
multimode lasing with wide waveguide dimension. It is clearly
shown from two different mode profiles in Fig. 2 that higher
modes are superimposed with a fundamental mode at the region
II of the LL curve while only a fundamental mode is lasing at
a threshold of 120 mW and a slope efficiency of 0.5% at 20 C.
Since this waveguide width is narrower, the fundamental mode
is lasing without other higher order modes up to 0.6 mW. This
device demonstrates a maximum fiber-coupled output power of
0.9 mW. The total maximum output power including the output
from both facets and coupling losses is approximately of 5 mW
with a slope efficiency of 2.8%.
In Fig. 4, the threshold pump power dependence on wave-
guide width is shown for different temperatures. The wider
stripe lasers have lower threshold pump power than narrower
devices, since the wider devices have a lower modal overlap
with device sidewalls and greater overlap in the silicon region
over the III–V region. This results in lower scattering losses
and lower overall propagation losses.
Fig. 5 shows the lasing spectrum of a 4- m-wide device at
sists of the expected Fabry–Pérot response for the 800- m-long
FANG et al.: CW HYBRID AlGaInAs-SEL1145 Download full-text
Threshold pump power with different waveguide widths for 800-?m
Fig. 5. Lasing spectra of a 4-?m-wide and 800-?m-long device.
cavity, with a group index of 3.68. The calculated group index
from simulations is 3.77.
Sixty devices (ten devices at each of six widths) were charac-
terized. Forty-seven of the sixty devices are lasing with a vari-
ation of threshold power for each waveguide width of less than
9%. The yield of the four wider widths is 98%, but the yield
is lower for the narrower stripe widths due to damage during
The modal loss for the 4- m-wide devices was measured
experimentally to be
of devices with lengths of 700
ment was confirmed by taking Hakki–Paoli measurements in
the long wavelength limit. The 700- m HR-coated devices had
a maximum output power of 2.7 mW at 20 C and operated
up to 60 C for wider devices. They showed similar high yield,
low device-to-device variation, and threshold versus waveguide
width behavior to that of the 800- m-long devices.
by fabricating a second set
m. The modal loss measure-
We present here an optically pumped SEL operating CW at
1568 nm up to 60 C. It has a maximum fiber-coupled output
power of 4.5 mW with a threshold pump power of 23 mW. The
laser utilizes low temperature oxide mediated bonding of offset
AlGaInAs QWs to a silicon rib waveguide to achieve optical
gain. The process provides high yield and low device-to-device
performance variation. This structure can be extended to elec-
trically pumped devices, such as lasers, amplifiers, and modu-
lators, through the doping of III–V layers and minor backside
The authors would like thank K. Callegari and G. Zeng for
sample prep and C. S. Suh for taking SEM images.
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