Abstract
ABSTRACTZnO single crystal thin film is grown on Al2O3(0001) substrate by plasma-assisted molecular beam epitaxy employing a thin MgO buffer layer. Using reflection high-energy electron diffraction (RHEED), we investigated the surface morphology evolution during the buffer and ZnO growth. We found that a few nanometers thick MgO layer deposited on Al2O3(0001) substrate strongly influences the subsequent growth of ZnO by promoting lateral epitaxial growth, which eventually leads to an atomically flat surface. As a result, (3×3) surface reconstruction of ZnO is observed and RHEED intensity oscillations are recorded. Structural investigations indicate that MgO with rock-salt crystallographic structure forms on the Al2O3(0001) surface as a template between the substrate and ZnO epilayer. Above that, the ZnO epilayer grows with little strain. The mosaicity in the ZnO film is suppressed by more than two orders as indicated by both symmetric and asymmetric X-ray rocking curves. The twin defect with a 30° in-plane crystal orientation difference is completely eliminated. Free exciton emissions at 3.3774 eV (XA) and 3.383 eV (XB) are observed in photoluminescence at 4.2 K further indicating the high quality of the resulting ZnO epilayers.
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