Tuning photonic crystal fabrication by nanosphere lithography and surface treatment of AlGaN-based ultraviolet light-emitting diodes

Abstract

Photonic crystal processing was performed using nanosphere lithography as a low-cost procedure to enhance the quantum efficiency of AlGaN-based ultraviolet light emitting diodes. Spectral transmissivity/reflectivity and photoluminescence measurements, in conjunction with finite-element modeling and electromagnetic simulations, provide an indication of radiance enhancement with a photonic crystal periodicity comparable to the emission wavelength. To recondition the plasma-damaged sidewalls, post-processing methods based on high temperature annealing and surface treatment were evaluated, which in general, established a significant increase in light extraction efficiency. X-ray photoelectron spectroscopy clarified the formation of surface oxides and hydroxides on the as-fabricated nanostructures, and their dissolution after wet-chemical processing is linked to enhanced optical output. Hydroxyl-termination was found to prevail after KOH etching, but significantly reduced after HCl or H3PO4 treatment. The two-step sequence of HCl followed by KOH treatment provided the best quality nanotextured surface for optical emission in this study, as indicated by the nearly 14.5-fold enhancement in photoluminescence intensity.