UBC Theses and Dissertations
Monolithically integrated 940 nm AlₓGa1-ₓAs distributed Bragg reflectors on bulk Si and Ge substrates Guo, Jia
To address the dramatically increased market demand for vertical-cavity surface-emitting lasers (VCSELs) as light sources in distance and three-dimensional (3D) sensing for consumer and industry products, monolithic integration of VCSELs on bulk Si and Ge substrates was investigated in this work. It has the benefits of much larger wafer size and thus higher throughput, lower manufacturing cost, and the compatibility to integrate with silicon complementary metal-oxide-semiconductor (CMOS) field-effect transistors. Distributed Bragg reflectors (DBRs), the first and fundamental part of VCSELs, were grown on three engineered bulk Si and Ge substrates: GaInAs/Ge substrate, graded-GaAsP/Si substrate, and aspect ratio trapping (ART)-Ge/Si substrate. A conventional bulk GaAs wafer was used as the control sample. Material characterization and optical performance measurement were performed on the fabricated DBRs. The GaInAs/Ge DBRs have comparable surface conditions and optical performance to the conventional bulk GaAs DBRs in terms of surface topology, thickness uniformity, and reflectance spectrum, qualifying GaInAs/Ge substrates for full VCSEL fabrication. The GaAsP/Si DBRs have defected surfaces with a relatively high density of surface bumps and a cross-hatch pattern. Moreover, the GaAsP/Si DBRs have irregular double-peak-shaped reflectance stopbands. The ART-Ge/Si DBRs have the worst surface quality, making them unsuitable for further VCSEL development. A surface scratch test was proposed and conducted to study the impact of the surface cross-hatch pattern on the DBR reflectance spectrum. A similar double-peak-shaped stopband was reproduced by manually introducing cross scratches on the high-quality bulk GaAs DBR surface. It strongly suggests that the cross-hatch pattern on the GaAsP/Si DBR surface is the root cause of the irregular double-peak-shaped stopband. A surface chemical mechanical polishing (CMP) was introduced as a potential solution to flatten the cross-hatch pattern and improve the reflectance spectrum. Data suggests the improvement of the double-peak-shaped stopband by reducing the surface roughness. However, the CMP process was with high variation and introduced new random scratches that affect the overall reflectance. Further epitaxy development and surface improvement are suggested to improve the bulk Si-based DBR performance.
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