PLASMA PRECONDITIONING OF SAPPHIRE SUBSTRATE FOR GAN EPITAXY

The crystalline quality of molecular beam epitaxy (MBE)-grown layers o f GaN on sapphire strongly depends on the initial stage of film nuclea tion and growth. Thus, pre-conditioning of the substrate is of vital i mportance. In this study we use X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) to examine in situ the cas e for surface cleaning and nitridation of c-plane sapphire substrates upon annealing in UHV and exposure to radio frequency (rf) plasmas of hydrogen and nitrogen, respectively. We find that low temperature (200 -300 degrees C) heat treatment in a hydrogen plasma offers effective r emoval of adventitious surface contaminants, contrary to annealing in vacuum. Moreover, our XPS data provide unambiguous evidence for format ion of surface nitride upon heat treatment in nitrogen plasma at 300-7 00 degrees C, in agreement with conclusions inferred from reflection h igh energy electron diffraction (RHEED). The surface nitride is found to remain stable on subsequent exposure to atmosphere. (C) 1997 Elsevi er Science S.A.