Nonpolar, a-plane (11 (2) over bar0) ZnO thin films were epitaxially grown on r-plane (11 (1) over bar2) sapphire substrate by radio frequency magnetron sputtering. The influence of oxygen partial pressure was studied. With increasing O-2 partial pressure of the Ar/O-2 sputtering gas, the ridge-like facet structure of the ZnO film altered to a smooth film. Full width half maximum of X-ray rocking curves for the on-axis (11 (2) over bar0) reflections were 0.45 degrees, 0.36 degrees, 0.09 degrees and 0.25 degrees for samples grown at Ar/O-2 ratios of 2/1, 1/1, 1/2 and 1/3, respectively. A smooth surface could be obtained under oxygen rich conditions (Ar/O-2 ratio: 1/2), enhancing the lateral growth along the c-axis direction. (c) 2013 Elsevier B.V. All rights reserved.