UC Santa Barbara

GaN vertical-cavity surface-emitting lasers (VCSELs) show promise for numerous lighting, display, communications, and sensor applications due to their visible wavelength emission, low threshold current, high beam quality, and arraying capabilities. Primarily, research has been focused on short to medium cavity (L<5λ) VCSEL designs, prioritizing single longitudinal mode operation. However, GaN VCSELs struggle with thermal management due to self-heating from higher input power requirements, high optical losses from p-type GaN and current spreaders, and poor heatsinking from the typically low thermal conductivities of the bottomside distributed Bragg reflectors (DBRs). These issues result in a high thermal impedance, generally >1000 K/W, quick thermal rollover, and low device lifetimes. Recently, long cavity (L>>5λ) GaN VCSEL designs have shown significant promise towards addressing the issues of thermal stability and cavity length control but require substrate polishing and complex fabrication, limiting scalability for mass production. To address these issues, a topside lens fabrication method is developed. Then, a 65λ GaN VCSEL with a topside lens, a buried tunnel junction current aperture, and bottomside epitaxial nanoporous GaN DBR was fabricated using standard microfabrication techniques. First, a topside GaN lens was demonstrated, with CW lasing achieved at lower current densities than comparable planar cavity VCSELs. However, the output power was limited by the high temperature regrowth required to fabricate the GaN lens as well as the high turn-on voltage. Next, a topside dielectric lens was developed which enabled CW lasing performance above 2mW for a GaN VCSEL with a partially etched porous DBR, and single transverse mode operation for other VCSELs with fully etched porous DBRs. The devices show high thermal stability due to the long cavity, with an estimated thermal impedance of 600K/W measured on-chip.