Highly reflective distributed Bragg reflectors using a deeply etched semiconductor/air grating for InGaN/GaN laser diodes

Abstract

High reflectivity is achieved by deeply etched InGaN/GaN distributed Bragg reflector (DBR) mirrors with tilted sidewalls, which are appropriately designed by using the finite-difference time-domain method. The predicted optimal structure is different from the simple design consisting of a λ/(4n) semiconductor and λ/4 air. If the sidewall of the grating is tilted by 4°, the reflectivity of the DBR mirrors decreases to less than 40%. However, any degradation in the reflectivity of a perfectly vertical sidewall can be suppressed to just a few percent even with a sidewall tilt of 4°, if the DBR structure is properly designed. We fabricated InGaN/GaN multiple-quantum well lasers based on the optimal design. The devices operate as lasers with optical pumping at a lower threshold than devices without DBR mirrors. The DBR mirror reflectivity is characterized by the relation between the threshold pump intensity and the inverse of the cavity length, resulting in a high reflectivity of 62%.