Surface roughness of nitrided (0001) Al2O3 and AlN epilayers grown on (0001) Al2O3 by reactive molecular beam epitaxy

Abstract

Nitridation of c-plane sapphire followed by deposition of a low temperature AlN or GaN buffer is commonly employed in the growth of GaN-based structures which have received a great deal of attention recently. In order to gain some needed insight, we undertook an investigation of nitridation of sapphire followed by the subsequent growth of AlN buffer layers in a reactive molecular beam epitaxy environment. Atomic force microscopy was used to characterize the surface roughness of samples after exposure to various nitridation conditions. Nitridation at higher temperatures was found to yield smoother substrate surfaces possibly due to smoothing of scratches introduced during substrate preparation. Incorporation of nitrogen into sapphire surfaces during the nitridation process was verified using x-ray photoelectron spectroscopy by observing the development of the N 1s peak with nitridation time. The surface roughness of AlN layers deposited on these nitrided surfaces was found to increase dramatically with thickness due to a significant coarsening of the surface topography. Surface roughness was found to decrease with increasing growth rate, the smoothest films being obtained with a growth rate of 140–200 nm/h at a substrate temperature of 800 °C.