Characterization of HVPE GaN layers by Atomic Force Microscopy and Raman spectroscopy

GaN layers were deposited on (0001) sapphire by hydride vapour phase epitaxy (HVPE) and then extensively characterized by Raman spectroscopy and atomic force microscopy. It was observed that the GaN epilayers directly deposited on sapphire generally have rough surfaces, mostly made of pyramids. The Raman scattering technique was found to be a powerful tool for quick and non-destructive assessment of the crystallographic quality of HVPE GaN, although care must be taken in the spectra analysis as the surface morphology can affect the spectra structure. The absence of forbidden modes indicate that the surface is planar and smooth and the material is of good crystalline quality. A discussion on the observed red-shift of the A1(LO) mode frequency is presented.