Effects of Zn pre-exposure temperature on the microstructures of ZnO films grown on Si(0 0 1) substrates by plasma-assisted molecular beam epitaxy

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 °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 °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¯ 1¯ 0〉 and ZnO〈0 1 1¯ 0〉, while the ZnO films where Zn pre-exposures were performed at 50 and 100 °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 °C showed full-width at half-maximums of 1.6–1.8° 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 °C were 5.4° and 9.8°, respectively.