Nitridation process of sapphire substrate surface and its effect on the growth of GaN

The nitridated layer formed on a (0001) sapphire (alpha-Al2O3) substate surface by heating at 1050 degrees C in ammonia (NH3) gas was analyzed by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), transmission electron microscope (TEM) and energy dispersive x-ray spectrometry (EDX). Their influence on the growth of GaN in the combined usage of initial nitridation and successive deposition of a buffer layer was examined by AFM observations. The intensity of the N-1s nitrogen peak in the XPS rapidly increased with nitridation time, reaching saturation in a few minutes, and then continued to increase gradually. This change was found to correspond to morphological change revealed by AFM observations, that is, from a flat nitridated layer to high-density (10(9)-10(10) cm(-2)) nitridated protrusions. TEM observations and EDX measurements showed that the nitridation forms an amorphous layer consisting of AlNxO1-x. The flat nitridated layer, when combined with a buffer layer, favors two-dimensional growth of a thick GaN layer on it, while the layer with protrusions results in three-dimensional growth. Thus, thick GaN layers with smooth surfaces can be grown by controlling the surface of the nitridated layer, where a crystal-amorphous-crystal growth mechanism is successfully operating. (C) 1996 American Institute of Physics.