Continuous-wave electrically pumped 1.55-mu m edgeemitting platelet ridge laser diodes on silicon
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Rumpler, J.J., and C.G. Fonstad. “Continuous-Wave Electrically
Pumped 1.55- \mu m Edge-Emitting Platelet Ridge Laser Diodes
on Silicon.” Photonics Technology Letters, IEEE 21.13 (2009):
827-829. © Copyright 2010 IEEE
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Continuous-Wave Electrically Pumped 1.55-m
Edge-Emitting Platelet Ridge Laser Diodes on Silicon
Joseph J. Rumpler and Clifton G. Fonstad, Jr., Fellow, IEEE
Abstract—We report the successful integration on silicon of
small footprint, low-threshold electrically pumped edge-emitting
lasers by a new approach incorporating microcleaving technology to produce 6- m-thick platelet lasers with cleaved facets,
microscale pick. and place assembly to position them on the substrate, and diaphragm pressure solder bonding to attach/connect
them permanently in place. InP-based ridge-waveguide platelet
lasers integrated on silicon lase at 1550-nm continuous-wave to
55 C (pulsed to 80 C) with output powers as high as 26.8 mW,
external differential quantum efficiencies as high as 81%, and
threshold currents as low as 18 mA.
Index Terms—Fabrication, indium compounds, integrated optoelectronics, laser cavity resonators, ridge waveguides, semiconductor lasers, silicon.
OR OVER 40 years, silicon has been the material of
choice for high-density microelectronics in large part
because of the performance advantages of high speed, low
static power, complementary metal–oxide–semiconductor
technology. With the maturity of silicon fabrication processes
gained over this time, and the ever-increasing prominence of
silicon devices in the marketplace, researchers are now looking
to extend the technology by manufacturing optoelectronic
devices directly on silicon substrates for applications such
as optical interconnects [1], [2] and biomedical sensors and
instrumentation [3], [4].
Silicon-based compounds are regularly used to make passive optical devices such as waveguides, splitters, couplers,
and wave-division multiplexers [5]. In fact, silicon, and silicon-based materials such as silicon–dioxide, silicon–nitride,
and silicon–oxy–nitride, are the predominant materials used in
commercial optical planar waveguides today [6]. The development of silicon active optical devices like lasers and optical
amplifiers, however, has proved to be much more challenging.
The difficulty lies in the fact that silicon is an inefficient
light-emitting material due to its indirect energy bandgap. Silicon laser research efforts have investigated ways to circumvent
this limitation by using materials such as nano-porous silicon
[7], rare-earth-doped silica glasses [8], [9], silicon nano-crystals
[10], and strained germanium-on-silicon [11], or have exploited
phenomena such as the Raman effect [12]. Although these
Manuscript received January 07, 2009; revised March 02, 2009. First published April 03, 2009; current version published June 10, 2009. This work was
supported in part by the MIT-Singapore Alliance, in part by the MARCO Interconnect Focus Center, and in part by the Army Research Office.
The authors are with the Massachusetts Institute of Technology, Cambridge,
MA 02139 USA (e-mail: [email protected]).
Digital Object Identifier 10.1109/LPT.2009.2019261
demonstrations represent tremendous breakthroughs, each
requires an additional laser to pump the devices and achieve
light emission. To make silicon the material of choice for
monolithic optoelectronic integration, the development of an
efficient electrically pumped laser is necessary.
Approaches to achieve electrically pumped lasers on silicon
have typically involved the hybrid integration of III–V semiconductor lasers onto silicon substrates. Most often these efforts have involved bonding epitaxial III–V active structures
directly on silicon [13], [14]. Pollentier et al. and Seo et al.
reported pulsed lasing of GaAs-AlGaAs single quantum-well
(QW) lasers on silicon and InP-InGaAsP multiple QW ridge
lasers on silicon, respectively; however, continuous-wave (CW)
lasing was not achieved [13], [14]. Rojo Romeo et al. reported
ultralow threshold pulsed lasing of microdisk lasers consisting
of InP membranes molecularly bonded on Si [15]. Researchers
at the University of California at Santa Barbara and Intel have
recently demonstrated an electrically pumped CW diode laser
[16] made by oxide-bonding a III–V semiconductor QW active
gain region and upper cladding layers to silicon waveguides on
SiO on Si. This novel evanescently coupled, hybrid cavity laser
structure has several attractive features, but it uses epitaxial material inefficiently and can only integrate devices fabricated from
the same heterostructure layer sequence.
This letter reports the successful integration of small footprint, low-threshold electrically pumped CW III–V semiconductor edge-emitting lasers on silicon by a new approach incorporating a microcleaving technology to produce platelet lasers
with cleaved facets, microscale pick and place assembly to position them on the substrate, and diaphragm pressure bonding
to solder and connect them permanently in place. The lasers
reported here are designed for coaxial integration with waveguides on silicon, but they have features that make them excellent
candidates for many other applications as well. The technology
described is highly modular so that it can be applied to wide varieties of devices, materials, and substrates; it is also well suited
for integrating different wavelength lasers on the same substrate.
More details of the integration technology can be found in [17]
and [21].
The starting semiconductor material used in this work was
a standard 1550 nm ridge laser epitaxial structure grown on a
(100) InP substrate by a commercial foundry.1 The only change
made in the foundry’s standard heterostructure product was the
incorporation of a 500-nm-thick n InGaAs etch stop layer next
1Land-Mark Optoelectronics Corporation, No. 1, Lane 217, Chung-Cheng
Rd., Yung-Kang 710, Tainan, Taiwan.
1041-1135/$25.00 © 2009 IEEE
Fig. 2. SEM photograph of one facet end of a microcleaved platelet ridge laser
on silicon.
Fig. 1. Top view scanning electron microscope (SEM) photograph of a microcleaved platelet ridge laser. The scratches and similar marks on the device were
caused by the probes used during electrical measurements.
to the InP substrate. The front-side device processing began
with the deposition and lift-off of Ti–Pt–Au Ohmic metal stripe
contacts to the p InGaAs contact layer. The semiconductor
waveguide ridges were then formed using standard contact photolithography and concentrated hydrochloric acid etching. The
wet etch stopped on a 7-nm InGaAsP etch stop layer, thereby
precisely controlling the ridge height. The wafer was then replanarized using benzocyclobutane (BCB), the BCB surface was
etched back to reveal the stripe contacts on top of the ridges,
and large area Cr–Au electrical contacts were deposited and
patterned on top of the devices. Grooves were etched through
the BCB and semiconductor layers down to the lower n etch
stop layer to delineate long notched bars containing four lasers.
At this stage, the wafer front-side was coated with a protective
polymer and mounted face-down on a rigid carrier substrate.
Backside processing began with the removal of the InP
substrate with hydrochloric acid. This etchant stopped on the
500-nm n InGaAs layer. A combination of contact photolithography and e-beam deposition of Ni–Au–Ge–Au–Ni–Au
was then used to form the backside ohmic contacts to the n
InGaAs layer on each device. Then, the remaining n InGaAs
not covered by the ohmic metal was removed by wet etching in
H SO : H O : H O.
a solution of
Next, the bars of devices were released from the carrier substrate by dissolution of the protective polymer. The individual
devices were then separated (and their end facets were simultaneously formed) using a novel microcleaving process [18] that
results in cleaved end-facets and a nominal laser cavity length
of 300 m (Fig. 1).
Fully processed microcleaved ridge laser platelets were
assembled on a silicon substrate by a microscale pick and
place technique. The assembly tool is a small quartz micropipette (pulled to form a 20- m inner diameter tip that
was subsequently beveled at 45 ) affixed to a three-axis micropositioning stage, which allows precise positioning of the
platelets under a microscope. The platelets were positioned
on a silicon substrate covered with 250-nm Au and 400-nm
Fig. 3. CW light output as a function of drive current measured at a number
of different stage temperatures of a microcleaved platelet ridge laser on silicon.
The inset shows the output spectrum at room temperature with I
= 30
In, and thermo-compressively solder bonded in place under
a thermoplastic membrane at a pressure of 40 to 45 PSI and
temperature of 210 C for approximately 6 min (Fig. 2).
CW characterization of microcleaved ridge lasers on silicon
was performed with the Si substrate mounted on a temperature-controlled stage. Output powers as high as 26.8 mW (at
C) were detected and the device lased at stage temperatures as high as 55 C (Fig. 3). At 20 C, the peak differenwas 73%; at 10.3 C it was
tial external quantum efficiency
The output emission spectrum measured just above threshold
at a drive current of 30 mA (Fig. 3, insert) shows an output
emission peak just below 1542 nm, and a Fabry–Pérot mode
spacing of 1.1 nm, consistent with a group index of 3.6.
Pulsed lasing (1-kHz repetition rate, 0.45% duty cycle) was
measured to a temperature of at least 80 C, the temperature
limit of the thermoelectric cooler used. A log-linear plot of the
threshold current (pulsed) as a function of stage temperature
of 42.9 K.
yields a value for the characteristic temperature
Derivative analysis of the electrical characteristics gives a
for the diode series resistance,
, and a
value of 8.8
value of 1.3 for the diode ideality factor . With more attention spent on the contact metallurgy, this resistance value could
likely decrease by an order of magnitude [19]. The thin microcleaved ridge platelet lasers integrated onto silicon outperformed conventionally cleaved ridge lasers processed simultaneously with the microcleaved lasers, but left on their native
InP substrates and cleaved by conventional techniques, in terms
of maximum operating temperature, characteristic temperature,
output power, and differential efficiency [18]. The improved
thermal properties are believed to be due in part to the fact that
silicon is a better thermal conductor than InP, and in part to the
fact that the gold-based bonding layers act to spread heat from
the active region [20].
The platelet lasers integrated on silicon in this study operate
CW with performance superior to that of comparable lasers
on their native substrates. While this work used a relatively
simple ridge laser design and rudimentary process tool set, the
platelet fabrication and microcleaving processes developed can
just as easily be used with more sophisticated, higher performance laser diode geometries and processes incorporating, for
example, better lateral current confinement, narrower stripe
widths, and lower resistance contacts. The integration process
itself is modular, places few restrictions on the laser processing,
and can be used with different types of devices, as well as
with a wide variety of substrates. It is particularly significant
that the platelet devices are integrated as single units, and
that they can be tested and screened prior to being integrated.
Integrating individual units also means that lasers emitting at
widely different wavelengths can be integrated together on a
single substrate. Integrating laser diodes as individual platelets
in dielectric recesses coaxially aligned with dielectric waveguides, with little or no gap between the laser and recess facets,
also makes efficient use of heteroepitaxial materials and results
in a unit with a relatively small footprint.
The authors gratefully acknowledge the importance of work
done by E. Barkley, J. Perkins, Y.-S. V. Lei, and M. S. Teo
in contributing to the success of this research. They also
acknowledge the interest and support of Prof. J. Meindl,
Prof. A. Chandrakasan, Dr. J. Shah, and Dr. W. H. Chang.
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