High-quality [0 0 0 1]-oriented ZnO films were grown in a single growth experiment on GaN/AlO3 applying a two-step metal organic vapor phase epitaxy growth technique. The essence of this method is the heteroepitaxy of a low-temperature ZnO buffer layer using dimethyl-zinc and tertiary-butanol precursors on GaN/Al2O3 and the subsequent homoepitaxial growth of a high-temperature layer using NO as O-precursor. The layers show smooth surface morphology and high crystalline quality as demonstrated by X-ray diffraction (FWHM of (0 0 0 2) omega-scans for a 2.28 mum thick layer is 160"). The bright luminescence is dominated by narrow excitonic emission lines (e.g., FWHM < 1.3 meV for bound exciton I-8). Our method opens broad prospect for the growth of ZnO-based device structures. (C) 2004 Elsevier B.V. All rights reserved.