SURFACE-ROUGHNESS OF NITRIDED (0001)AL2O3 AND ALN EPILAYERS GROWN ON (0001)AL2O3 BY REACTIVE MOLECULAR-BEAM EPITAXY

Nitridation of c-plane sapphire followed by deposition of a low temper ature AlN or GaN buffer is commonly employed in the growth of GaN-base d structures which have received a great deal of attention recently. I n order to gain some needed insight, we undertook an investigation of nitridation of sapphire followed by the subsequent growth of AlN buffe r layers in a reactive molecular beam epitaxy environment. Atomic forc e microscopy was used to characterize the surface roughness of samples after exposure to various nitridation conditions. Nitridation at high er temperatures was found to yield smoother substrate surfaces possibl y due to smoothing of scratches introduced during substrate preparatio n. Incorporation of nitrogen into sapphire surfaces during the nitrida tion process was verified using x-ray photoelectron spectroscopy by ob serving the development of the N 1 s peak with nitridation time. The s urface roughness of AlN layers deposited on these nitrided surfaces wa s found to increase dramatically with thickness due to a significant c oarsening of the surface topography. Surface roughness was found to de crease with increasing growth rate, the smoothest films being obtained with a growth rate of 140-200 nm/h at a substrate temperature of 800 degrees C. (C) 1997 American Vacuum Society.