Photoassisted chemical smoothing of AlGaN surface after laser lift-off

To fabricate AlGaN-based ultraviolet (UV) vertical cavity surface-emitting laser diodes, a pair of distributed Bragg reflectors (DBRs) having a smooth surface is desired to have a high quality factor (Q). In this work, photoassisted chemical etching (PCE) was attempted to smoothen the $-c(000-2)$ AlN surface after removing the sapphire substrate by means of the laser lift-off process. First, the conditions for PCE were optimized using an Xe lamp and KOH solution. The root-mean-square roughness of the $-c(000-2)$ AlN surface was reduced from 30.5 to 5.6 nm, which enables us to fabricate an improved HfO₂/SiO₂ DBR on the AlN surface. Then, using the optimized PCE technique, improved UV (Al,Ga)N planar microcavities were fabricated and distinct cavity-mode-related emissions were observed using the photoluminescence (PL) technique. By comparing the PL spectra between the samples with and without PCE treatment, the Q value at 303 nm for the case of the improved DBR was determined to increase from 174 to 270. The increase in the Q value is mainly attributed to the reduction of scattering losses in optical cavities. Furthermore, the discussion on the mechanism of improved surface during the PCE treatment is given. Consequently, PCE is demonstrated to be a feasible approach to refine the quality of ultraviolet nitride microcavities.