The monolithic integration of epitaxially-grown InGaAs/GaAs self-organized quantum dot lasers with hydrogenated amorphous silicon (a:Si-H) waveguides on silicon substrates is demonstrated. Hydrogenated amorphous silicon waveguides, formed by plasma-enhanced-chemical-vapor deposition (PECVD), exhibit a propagation loss of ~10 dB/cm at a wavelength of 1.05 µm. The laser-waveguide coupling, with coupling coefficient of 22%, is achieved through a 3.2 µm-width groove etched by focused-ion-beam (FIB) milling which creates high-quality etched GaAs facets.
© 2008 Optical Society of America
The subject of silicon photonics, which has gathered considerable momentum recently, envisions the incorporation of optical interconnects on silicon-based complementary-metal-oxide-semiconductor (CMOS) chips [1, 2, 3]. Optical interconnects involve light emitters, preferably lasers, waveguides and guided-wave devices, and detectors. In spite of materials-related issues, some progress has been made in all fronts. An important component of silicon photonics is the light guide and associated guided-wave devices such as modulators. With the development of microfabrication and silicon-on-insulator (SOI) technologies, reasonably low loss (≤0.5 dB/cm) silicon waveguides operating in the wavelength range of 1.3–1.55 µm have been developed [3, 4, 5]. Similarly, silicon-based modulators, based on the electric-field induced delocalization of the electron wavefunctions in shallow well , quantum-confined Stark effect (QCSE) , and free-carrier plasma dispersion effect (FCPDE)  with various geometries such as a Mach-Zehnder interferometer  and microring resonator , have also been developed.
While the above developments have been achieved with crystalline silicon and SOI technology, hydrogenated amorphous silicon (a:Si-H) may offer benefits in terms of lower cost, low temperature processing, as well as other unique characteristics in photonics applications . These include refractive index and bandgap tunability dependent on H composition, and a desirable thermo-optic effect specifically for low-power and low-frequency switching. In contrast to crystalline silicon, amorphous silicon does not have a clearly defined band structure and an abrupt band edge. In addition, the dangling bonds in a:Si-H can be saturated by H. As a result, a:Si-H exhibits an acceptable absorption loss in the wavelength range of 0.95~1.15 µm (whilst crystalline silicon has much higher loss in this range), and certainly a lower absorption loss at longer wavelengths . Moreover, a silicon waveguide/modulator technology realized by plasma-enhanced-chemical-vapor deposition (PECVD), or similar techniques, will introduce flexibility in design and fabrication that is important for the development of silicon photonics. With this in mind, we have investigated the integration of light sources with a:Si-H waveguides, both formed on silicon substrates. We demonstrate here the on-chip integration of InGaAs/GaAs self-organized quantum dot (QD) lasers grown directly on silicon with a:Si-H waveguides formed by PECVD. The waveguides exhibit a propagation loss of ~10 dB/cm for λ=1.05 µm. The laser-waveguide coupling is achieved through a groove etched by focused-ion-beam (FIB) milling. A coupling coefficient of 22% is measured.
2. Device growth, fabrication, and characteristics
The groove-coupled edge-emitting QD laser/a:Si-H waveguide is schematically shown in Fig. 1. A 2-µm-thick GaAs buffer layer was first grown by metal-organic vapor phase epitaxy on (001)-oriented silicon substrates with 4° mis-orientation towards <111>. The GaAs-AlGaAs-In0.5Ga0.5As QD separate confinement laser heterostructure was then grown by molecular beam epitaxy with incorporation of ten layers of InAs QDs as a dislocation filter in the GaAs buffer layer . The In0.5Ga0.5As QDs, GaAs and Al0.7Ga0.3As layers were grown at 500 °C, 600 °C, and 620 °C, respectively. The wafer is patterned and then dry etched, using Cl2/Ar inductively coupled plasma (ICP), to delineate the regions where the waveguide is to be deposited. The SiOx/a:Si-H/SiOx multimode waveguide is deposited by PECVD at 380 °C. Before the deposition of the SiOx upper cladding layer, the transverse dimension of the a:Si-H waveguide is defined by ICP dry etching. It is important that the depth of the etched trench and the thickness of the waveguide core/cladding are carefully adjusted such that the laser active region and waveguide core are closely aligned. The groove-coupled laser/waveguide is fabricated by using standard photolithography, wet and dry etching, and contact metallization techniques. The wafer substrates are lapped down to ~80 µm for optimized cleaving and the integrated laser-waveguide segments are cleaved along the <110> direction. An antireflection (AR) coating consisting of λ/4-thick Al2O3 is deposited on the a:Si-H waveguide output facet by e-beam evaporation. The coating has the refractive index of 1.58, which is measured using ellipsometry and yields a calculated transmission of 97%.
A critical issue in the monolithic integration of an edge-emitting laser and planar waveguide is optimization of the optical coupling between the two devices, which is dependent on the etched facet quality and groove dimension in a groove-coupled scheme . Dry etching techniques such as reactive ion etching (RIE) and ICP usually generate microscopic surface roughness and non-vertical sidewalls. In comparison, FIB has the advantage of maskless etching which can produce smoother surfaces and vertical sidewalls. We therefore used FIB to etch GaAs facets with a reflectivity of R~0.28 , which is comparable to that of a cleaved GaAs facet. The cross-sectional scanning electron microscope (SEM) image of an edge-emitting InGaAs/GaAs QD laser with an etched GaAs facet is shown in Fig. 2. In addition to high-quality etched facets, the dimension of the etched groove is critical for optimum coupling. We have calculated this coupling using a generalized transmission matrix model . The model, which is not limited to a Gaussian beam, more accurately describes the coupling behavior of an edge- emitting laser groove-coupled with another guided-wave section. In our experiment, the length of the laser and a:Si-H waveguide segment are 400 and 250 µm, respectively, separated by a FIB-etched groove with a width of 3.20 µm. SEM images of the groove-coupled laser/waveguide are shown in Fig. 3.
The InGaAs/GaAs QD lasers on silicon emit at a wavelength of 1.02 µm [inset of Fig. 4(a)]. To determine the propagation loss of the PECVD a:Si-H waveguides at this wavelength range, light from a 1.05 µm Nd:glass laser was coupled into and out of the waveguide segments using single mode fibers. The output power was measured for waveguide segments of varying length, with λ/4-thick Al2O3 AR coating deposited on the facets. From these measurements, the waveguide propagation loss is estimated to be 10 dB/cm. The light-current (L-I) characteristics from the QD laser and coupled a:Si-H waveguide ends have been measured under pulsed bias (500 µs pulses with 1% duty cycle) and the results are shown in Fig. 4(a) and 4(b), respectively. With the measured waveguide loss and L-I characteristics, we have estimated the laser-waveguide coupling using the generalized matrix transmission model through the relation 
Here, P1 and P2 are the output power from the laser and waveguide ends, respectively. t1(t21~0.69) is the transmittance of the cleaved GaAs facet and t3 (t23~0.97) is the transmittance of the waveguide output facet with AR coating. t2=exp[(ik-α/2)Lw], where α~10 dB/cm is the a:Si-H waveguide loss and Lw is the waveguide length. The coupling coefficient |S12|2 measured for the groove width of 3.20 µm is 22% at an injection current of J=1.5Jth, which is in good agreement with calculated values.
Laser-waveguide integration is an important aspect of integrated photonics and has been extensively investigated. The main point of this study is to demonstrate laser-waveguide integration on silicon, for potential application in optical interconnects, with a lower cost/CMOS-compatible a:Si-H waveguide technology. In addition, it should be noted that the loss of a:Si-H at longer wavelengths is smaller [11, 14], which will generate interest in the integration of these waveguides with 1.3-1.55 µm QD lasers. Such lasers have been demonstrated on GaAs substrates , and we are currently in the process of realizing them on silicon.
We demonstrate the monolithic integration of an epitaxially-grown quantum dot laser with a PECVD-formed a:Si-H waveguide on silicon substrates for the first time. The two devices are coupled by a FIB-etched groove and the coupling coefficient is estimated to be 22%. Waveguide loss could be decreased by optimizing PECVD processing and waveguide fabrication. This technology can be extended to integrate the laser with other passive or quasi-active a:Si-H guided-wave devices on silicon.
The work is being supported by the Defense Advance Research Projects Agency (EPIC program) under grant W911NF-04-1-0429. Part of the fabrication was done in Michigan Nanofabrication Facility and EMAL at University of Michigan, Ann Arbor.
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