The polarity of molecular-beam epitaxy grown ZnO films was controlled on nitrided c-sapphire substrate by modifying the interface between the ZnO buffer layer and the nitrided sapphire. The ZnO film grown on nitrided sapphire was proven to be Zn-polar while the O-polar one was obtained by using gallium predeposition on nitrided sapphire, which was confirmed by coaxial impact collision ion scattering spectroscopy and chemical etching effect. The Zn-polar ZnO film showed higher growth rate, slightly better quality, and different surface morphology in comparison to the O-polar one.

Rotation domains are easily and often introduced into ZnO epilayers grown on c-Al 2 O 3 substrate, resulting in degraded epilayer quality. In this paper, effects of the sapphire surface properties on the epitaxy of ZnO were studied; the surface was carefully treated by high temperature thermal cleaning (TC), atomic hydrogen (H*), oxygen radical (O*) treatments, decoration by a few monolayers gallium and nitridation, and then the ZnO epilayers grown on these surfaces were characterized. Three kinds of rotation domains were observed in total and the sub-domains were completely suppressed by Ga decoration and sapphire nitridation. The effect of the above treatments on polarities of ZnO epilayer was also studied.

ZnO epilayers were grown on nitridated c-Al 2 O 3 substrates by plasma-assisted molecular beam epitaxy and the effect of the deposition temperature for GaN buffer and/or intermediate layers on the polarity of ZnO epilayer was investigated. First, it was found that the polarity of the ZnO epilayers grown on low temperature-grown GaN buffer and/or intermediate layers was uncertain and it became often +c polarity, i.e., Zn-polarity. This seems strange because the polarity of the GaN-underlying layer was -c polarity, i.e., N-polarity, and then the polarity of the under layer was not kept unchanged by the following epilayer. On the other hand, when the growth temperature of the GaN buffer/intermediate layer was increased above 850 °C, it was found that the polarity of the GaN layer was followed by the ZnO epilayers; the polarity became -c polarity, i.e., O-polarity. This reason was estimated by the effect of surface oxides on GaN which were formed just before the deposition of ZnO buffer layer on it.

A low-temperature thin GaN layer was used to wet the grown ZnO buffer layer effectively in the epitaxy of a ZnO film on a nitridated c-sapphire substrate by plasma-assisted molecular beam epitaxy. An atomically smooth Zn-polar ZnO epilayer was achieved with an rms roughness of 0.13 nm in a 3 µm ×3 µm scanned area. Triangular terraces with monolayer steps (0.26 nm) were observed by atomic force microscope. The crystalline quality of the ZnO epilayer was also improved with the full width at half maximum (FWHM) values for (002) and (102) ω-scans of 41 arcsec and 378 arcsec, respectively.

. Ps, 78.55.Et, 78.40.Fy, 81.15.Hi ZnO epilayers were grown on nitrided c-Al 2 O 3 substrates by MBE and the effect of the deposition temperature for GaN buffer layers on the polarity of ZnO epilayer and their properties such as structural and optical properties were investigated. Zn-polar ZnO was normally obtained by using the low temperature GaN buffer layer at 650 ˚C, while O-polar ZnO were obtained by the high-temperature GaN buffer layer at 850 ˚C. PL spectra for Zn-polar and O-polar ZnO epilayers, PL spectra were dominated by donorbound exciton at 3.361 eV and acceptor-bound exciton at 3.356 eV, respectively. It was found that the position of emission peak was affected by the strain in the ZnO layer. PL spectra observed in ZnO epilayers with different polarities are discussed together with their reflectance spectra and strains of ZnO epilayers with different polarities.