We report on an atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) investigation of hard nitridation of sapphire (alpha.-Al2O3) substrate, using an Epi UNI-Bulb RF plasma cell at substrate temperatures ranging from 250 to 600 degrees C. Our results show that an AlN1-xOx layer forms on sapphire after extended nitridation at all temperatures, following a Stranski-Krastanov growth mode, with less islands forming at higher temperatures. We also observe a layer-dependent charging shift in XPS, separating smooth AlN1-xOx layers from rough AlN1-xOx islands due to their different electronic coupling to the substrate. Although the island growth is suppressed at higher temperatures, the surface roughness increases at higher temperatures as seen by AFM. We also observe sputtering effects with protrusions and pits. (C) 2011 Elsevier B.V. All rights reserved.