Abstract
We report the control of the in-plane orientation of ZnO epitaxial films grown on as-polished sapphire (α-Al2O3) (0001) substrates by metal organic chemical vapor deposition (MOCVD). When passing certain flows of Zn-precursor over the substrate before introducing oxygen-precursor at certain substrate temperatures, unit cells of the ZnO film show the same in-plane orientation as the substrate. Otherwise, 30° rotation is observed between unit cells of ZnO and the substrate. In case of using dietylzinc (DEZn) and oxygen gas as precursors, the substrate temperature and the flow rate of DEZn, under which no-twist ZnO films can be obtained, are determined experimentally. Measurements by coaxial impact collision ion scattering spectroscopy (CAICISS), however, demonstrate that the ZnO films all have +c polarity, regardless of their in-plane orientation. This is different from the results obtained by laser-molecular beam epitaxy by which +c polarity on as-polished sapphire (0001) substrates are hardly obtained. Since the +c polarity film is preferable in applications of wurtzite semiconductors, our results reveal that MOCVD technique is particularly important in future applications of ZnO-related oxides.
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