Gallium nitride (GaN) epitaxial layers have been grown on 0 face (000 (1) over bar) zinc oxide (ZnO) substrates by ammonia source molecular beam epitaxy. By adjusting the growth temperature and the III/V ratio during the nucleation stage, GaN layers with Ga (0001) or N (000 (1) over bar) polarities have been obtained. We show that low growth temperatures ( < 550 degrees C) and Ga-rich conditions lead to Ga-polar layers, whereas higher growth temperatures (> 600 degrees C) and N-rich conditions lead to N-polar layers. Furthermore, the formation of a zinc gallate (ZnGa2O4) interfacial layer between GaN and ZnO has been evidenced, which is responsible for the growth of Ga-polar GaN layers. The structural and optical properties of Ga- and N-polar GaN layers have been characterized and Ga-polar GaN layers exhibit higher crystal quality. (C) 2013 Elsevier B.V. All rights reserved.