Effects of Sapphire (0001) Surface Modification by Gallium Pre-Exposure on the Growth of High-Quality Epitaxial ZnO Film

ZnO films were grown on (0001) sapphire substrates using rf plasma-assisted molecular beam epitaxy. Distinct rotation domain structures were observed in ZnO films grown on Al-terminated or O-terminated sapphire surfaces. Therefore, the Ga pre-exposure process was adopted to modify the sapphire (0001) surface just before the buffer layer growth. It was revealed that the sapphire surface modification by Ga pre-exposure had significant effects on the elimination of the rotation domains in the ZnO epilayer. The full width at half maximum (FWHM) of the X-ray diffraction (XRD) rocking curve of the ZnO (002) reflection plane was as narrow as 67 arcsec. Consequently, a high-quality ZnO film with very good optical and electrical properties was obtained.