We have grown GaN films on atomically flat Zn-face ZnO at room temperature (RT) and 700 degrees C, and compared their structural properties. Although the film quality of GaN grown at 700 degrees C was quite poor due to the serious interface reaction between GaN and ZnO, GaN with an atomically flat stepped and terraced surface grows epitaxially at RT due to the suppression of the interface reaction. The growth of GaN at RT proceeds in the layer-by-layer mode, while at 700 degrees C it grows three-dimensionally. Atomic force microscope observations after alkali etching of GaN surfaces revealed that GaN grown at RT exhibits an N-polarity while that grown at 700 degrees C has a Ga-polarity. (c) 2007 Elsevier B.V. All rights reserved.