Preconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridation

The crystalline quality of molecular beam epitaxy grown layers of GaN on sapphire is generally improved by nitridation of the substrate. In this study, we use x-ray photoelectron spectroscopy, low energy electron diffraction, and atomic force microscopy to examine the case for nitridation of c-plane sapphire upon exposure to rf plasma generated nitrogen radicals. We find that a monolayer of surface nitride is formed after similar to 300 min exposure with the substrate at 400 degrees C. Extended exposure causes growth of protrusions from the c-plane sapphire and thus leads to a rough surface morphology. Moreover, we report removal of adventitious surface carbon upon heat treatment at 300 degrees C in nitrogen plasma, albeit with reduced efficiency compared with hydrogen plasma cleaning. (C) 1997 American Institute of Physics.