ZnO thin films have been deposited on GaN and ZnO substrates at substrate temperatures up to 750 degrees C by radio-frequency sputtering using ZnO ceramic targets in pure argon or in a mixture of argon and oxygen. By optimizing the sputter parameters, such as sputtering power, Ar/O(2) sputtering gas ratio and temperature of the substrates high quality films were obtained as judged from the X-ray rocking curve half width and luminescence line width. The crystallinity of the ZnO films increases with increasing substrate temperature. Yet there are distinct differences between films grown on GaN templates and on O- and Zn-polar ZnO substrates. (C) 2009 Elsevier B.V. All rights reserved.