Investigation of optical properties of nanoporous GaN films

In this study, we have investigated the optical properties of nanoporous GaN films on sapphire (0 0 0 1) prepared by UV-assisted electrochemical and electroless etching. Using various etching conditions, we can control the average pore size, pore depth and the density of pores on the GaN surface. Scanning electron microscopy (SEM) measurements reveal the nature of the pore morphology and microstructures. Optical properties of these nanoporous films have been studied using micro-photoluminescence and micro-Raman scattering. As compared to the as-grown GaN films, nanoporous layers exhibit a substantial photoluminescence (PL) intensity enhancement with red-shifted band-edge PL peaks associated with the relaxation of compressive stress. The red shifted E2 phonon peak in the Raman spectra of the porous GaN films further confirms such a stress relaxation. Microscopic optical measurements also suggest that light extraction from porous GaN surface is enhanced by such nanopatterning.