ZnO films have been grown on Si(0 0 1) substrates by plasma-assisted molecular beam epitaxy. In growing the ZnO films, effects of Zn pre-exposures at temperatures of 50-400 degrees C have been investigated by X-ray diffraction and cross-sectional transmission electron microscopy. All the ZnO films were grown with a preferred c-axis orientation, regardless of Zn pre-exposure temperatures and the Zn pre-exposure was not effective in protecting the surface oxidation of the substrate, which resulted in polycrystalline films growth. However, the Zn pre-exposures at 200-400 degrees C were effective in improving the crystal quality, where the columns of the ZnO films were ordered with only the two zone axes of ZnO < 2 <(1)over bar>(1) over bar 0 > and ZnO < 01<(1)over bar>0 >, while the ZnO films where Zn pre-exposures were performed at 50 and 100 degrees C showed completely polycrystalline features with columns of high value of tilt component and/or random orientations along the < 0 0 0 1 > direction. The ZnO films with the Zn pre-exposures at 200-400 degrees C showed full-width at half-maximums of 1.6-1.8 degrees for the X-ray rocking curves with the ZnO(0 0 0 2) reflections, while those for the ZnO films with the Zn pre-exposures at 50 and 100 degrees C were 5.4 degrees and 9.8 degrees, respectively. (C) 2007 Elsevier B.V. All rights reserved.