Abstract
Epitaxial ZnO films are grown on sapphire (α-Al2O3) (0001) substrates by metal-organic chemical vapor deposition under 6 and 0.05 Torr using diethyl zinc (DEZn, (C2H5)2Zn) and O2 as precursors and nitrogen as carrier gas. Measurements by X-ray diffraction and photoluminescence reveal that films grown under 6 Torr have better crystal quality. In ZnO films grown under both pressures, free excitonic transitions are observed by reflection. However, the emission behaviors are different. Low-temperature photoluminescence of the ZnO film grown under 6 Torr is dominated by donor-bound exciton emission, whereas that grown under 0.05 Torr exhibits strong acceptor-bound exciton emission, indicating pronounced acceptor incorporation in the latter. In-plane orientations of the ZnO films are well controlled by adjusting the growth process. Measurements by coaxial impact collision ion scattering spectroscopy (CAICISS) demonstrate that the ZnO films have +c polarity, regardless of their in-plane orientation. This finding differs from the results obtained by laser-molecular beam epitaxy where the polarity was found to depend on the in-plane orientation.
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