Influence of surface roughness on the lasing characteristics of optically pumped thin-film GaN microdisks

Optically-pumped whispering-gallery mode (WGM) lasing is observed from thin-film GaN microdisk processed from GaN-on-Si InGaN/GaN multi-quantum well wafers by selective wet etch removal of the substrate. Compared to thinfilm microdisks processed from GaN-onsapphire wafers through laser lift-off of the sapphire substrate, the exposed surface is significantly smoother as laser-induced damage is avoided, with root-mean-square roughness of 1.3 nm compared to 5.8 nm of the latter wafer. The 8 µm diameter microdisks, fabricated by pattern transfer from a silica microsphere and dry etching, benefit from the surface smoothness to offer superior optical confinement within the cavity. WGM lasing thresholds of 2.9 mJ/cm 2 and 3.5 mJ/cm 2 with quality factors (Q) of 3100 and 1700 are observed at the peak lasing wavelengths of 453 nm and 532 nm respectively, which are significantly better than thin-film microdisks processed from GaN-on-sapphire wafers despite lower internal quantum efficiency, highlighting the importance of surface smoothness in such optical cavities.