Continuous-wave electrically pumped 1.55-mu m
edge-emitting 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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http://dx.doi.org/10.1109/LPT.2009.2019261
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 13, JULY 1, 2009 827
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 tech-nology to produce 6- m-thick platelet lasers with cleaved facets, microscale pick. and place assembly to position them on the sub-strate, 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
op-toelectronics, laser cavity resonators, ridge waveguides, semicon-ductor lasers, silicon.
I. INTRODUCTION
F
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 pas-sive optical devices such as waveguides, splitters, couplers, and wave-division multiplexers [5]. In fact, silicon, and sil-icon-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 devel-opment 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. Sil-icon 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 pub-lished April 03, 2009; current version pubpub-lished June 10, 2009. This work was supported in part by the MIT-Singapore Alliance, in part by the MARCO Inter-connect 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: fonstad@mit.edu).
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 semi-conductor lasers onto silicon substrates. Most often these ef-forts 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 ma-terial inefficiently and can only integrate devices fabricated from the same heterostructure layer sequence.
This letter reports the successful integration of small foot-print, low-threshold electrically pumped CW III–V semicon-ductor edge-emitting lasers on silicon by a new approach incor-porating a microcleaving technology to produce platelet lasers with cleaved facets, microscale pick and place assembly to po-sition 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 waveg-uides 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 va-rieties 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].
II. DEVICEFABRICATION ANDINTEGRATION
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.1The 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.
828 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 21, NO. 13, JULY 1, 2009
Fig. 1. Top view scanning electron microscope (SEM) photograph of a micro-cleaved 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 pho-tolithography 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 repla-narized 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 pho-tolithography 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 a solution of H SO : H O : H O.
Next, the bars of devices were released from the carrier sub-strate by dissolution of the protective polymer. The individual devices were then separated (and their end facets were simulta-neously 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 mi-cropipette (pulled to form a 20- m inner diameter tip that was subsequently beveled at 45 ) affixed to a three-axis mi-cropositioning 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. 2. SEM photograph of one facet end of a microcleaved platelet ridge laser on silicon.
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 withI = 30 mA.
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).
III. EXPERIMENTALRESULTS ANDDISCUSSION CW characterization of microcleaved ridge lasers on silicon was performed with the Si substrate mounted on a tempera-ture-controlled stage. Output powers as high as 26.8 mW (at C) were detected and the device lased at stage tem-peratures as high as 55 C (Fig. 3). At 20 C, the peak differen-tial external quantum efficiency was 73%; at 10.3 C it was 81%.
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 yields a value for the characteristic temperature of 42.9 K.
Derivative analysis of the electrical characteristics gives a value of 8.8 for the diode series resistance, , and a
RUMPLER AND FONSTAD: CW ELECTRICALLY PUMPED 1.55- m EDGE-EMITTING PLATELET RIDGE LASER DIODES 829
value of 1.3 for the diode ideality factor . With more atten-tion spent on the contact metallurgy, this resistance value could likely decrease by an order of magnitude [19]. The thin mi-crocleaved ridge platelet lasers integrated onto silicon outper-formed conventionally cleaved ridge lasers processed simulta-neously 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].
IV. CONCLUSION
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 perfor-mance 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 waveg-uides, 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.
ACKNOWLEDGMENT
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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