Epitaxial GaN films were grown on sputtered ZnO buffer layers of thickness 25-200 nmover c-plane sapphire by reactive sputtering of GaAs in nitrogen at 700 degrees C. The epitaxial quality and microstructure have been studied by high resolution X-ray diffraction in phi (phi) and omega (omega) scan geometries. The surface morphology of epilayers was studied by atomic force microscopy and scanning electron microscopy and their crystalline quality was assessed by Raman spectroscopy. These studies have shown that ZnO buffer layers of 50-100 nm facilitate growth of GaN epilayers of high crystalline quality, compared to those grown on thinner and thicker ZnO buffer layers. (C) 2013 Elsevier B. V. All rights reserved.