UBC Theses and Dissertations
Monolithic integration of 850 nm and 940 nm AlGaAs vertical cavity surface emitting lasers on bulk Si and Ge substrates Zhao, Yunlong
The production of vertical-cavity surface-emitting lasers (VCSELs), which are widely used in short-reach optical communication and three-dimensional (3D) sensing for consumer and industry products, currently heavily relies on 4-inch and 6-inch bulk GaAs wafers. This GaAs-based production limits production volume and increases manufacturing costs. To address these challenges, the monolithic integration of VCSELs on alternative substrates, which can be made beyond 6-inch size, to enable large-scale production on larger wafer sizes, thus higher throughput and lower cost, was investigated. Three technical pathways to replace conventional GaAs substrates were proposed. Two were to replace the GaAs substrates with engineered Si substrates using aspect ratio trapping (ART)-Ge transition layers and graded GaAsP buffer layers. The third one is to apply Ge wafers as the substrates for VCSEL epitaxy. In this study, these three approaches were thoroughly analyzed and evaluated based on material characterization and optical spectra measurement. The distributed Bragg reflectors (DBRs), the first and one major structural part of VCSELs, and full VCSELs were epitaxially grown on ART-Ge/Si substrates. DBRs were grown on GaAsP/Si substrates. For Ge substrates, DBRs and half-VCSELs growth were conducted. Bulk GaAs wafers were used as control samples for comparison. The material quality of growth on ART-Ge/Si substrates is low. The surface exhibits a high density of bumps and cracks, and the thickness of DBR layers is not uniform. Multiple techniques were tried for improvement, but the effects were limited. Due to supply chain difficulty, this approach was abandoned. In contrast, the GaAsP/Si-based DBRs demonstrate better material quality, but a crosshatch pattern leads to disappointing optical performance. After evaluation, the possibility of successful integration on Si substrates is low with limited resources available to the research. On the other hand, the Ge-based DBRs and half-VCSELs exhibit the best material and optical quality, comparable to those of the conventional GaAs-based counterparts. The monolithic integration on Ge substrates was successful, and it is the first study to show successful DBR and half VCSEL results on engineered Ge substrates with structure and fabrication details. Industry companies can use this study much more readily to adopt this technology.
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