Monolithic integration of 940 nm vertical-cavity surface-emitting lasers on bulk Germanium substrate : a comparative study and systematic diagnosis

Germanium (Ge) substrates offer a viable path to scale Vertical-Cavity Surface-Emitting Lasers (VCSELs) to 8’’ and 12’’ wafers. Compared with GaAs wafers, Ge reduces bow/warp and is compatible with silicon complementary metal–oxide–semiconductor manufacturing. Although Ge-based VCSELs with high performance were reported in 2021, process details were undisclosed, leaving the open literature without a reproducible framework or understanding of yield and uniformity mechanisms on Ge. This thesis develops an independent Ge-based VCSEL technology route, benchmarks it against GaAs-based controls, and systematically studies performance variation. First, a half-VCSEL structure comprising n-type distributed Bragg reflectors (n-DBRs) and multiple quantum wells (MQWs) validated the engineered Ge substrate consisting of GaAs/InGaAs/InGaP layers grown on a 6° offcut Ge substrate. The half-VCSEL epitaxy validated suppression of anti-phase domains and achieved a surface roughness of 0.84 nm. Photoluminescence (PL) from the MQWs showed peak intensities comparable to GaAs-based controls at 200 mW excitation. Second, a full VCSEL structure on engineered Ge substrates exhibited comparable epitaxial quality and better surfaces than GaAs-based counterparts, with a 72% reduction in wafer bow/warp and a 40% reduction in roughness. Fabricated Ge-based VCSELs demonstrated successful lasing with static performance comparable to GaAs-based devices; the best Ge-based devices exhibited a 19% higher differential efficiency. Despite improved flatness and roughness, Ge-based VCSELs exhibited lower yield and larger performance variation than GaAs-based counterparts. A power-dependent PL study on half-VCSELs without a p-DBR found a 1.26 nm thermal redshift, implicating defect-related non-radiative recombination heating and revealing a p-DBR masking effect: thick p-DBRs conceal strain in the buried MQW. An isolated MQWs+ structure grown on engineered Ge substrates revealed 70–85 MPa residual stress and a dislocation-induced crosshatch pattern, causing a five- to eleven-fold reduction in PL intensity and loss of X-ray diffraction Pendellösung fringes. This confirms the p-DBR masking effect and shows the importance of MQW epitaxial quality. Overall, this thesis establishes an independently developed Ge-based VCSEL process with details disclosed and provides one of the first systematic studies of variation and yield in Ge-based VCSELs, identifying accumulated stress and defect control in the buried MQWs as essential for improving yield, uniformity, and reliability.