Growth of buffer-free high-quality ZnO epilayer on sapphire (0001) using radio-frequency magnetron sputtering

High-quality ZnO thin films were grown epitaxially on sapphire (0001) substrates by radio-frequency magnetron sputtering without a buffer layer at a high growth temperature of 750°C. The full width at half maximum of X-ray diffraction ω rocking measurement of the (0002) plane was 97.2 arcsec and that of the (101̄2) plane was 705.5 arcsec. Scanning electron microscope and atomic force microscope measurements showed that the epilayers were grown in a 2-dimensional growth mode and had a root mean square roughness of 1.1 nm. These results showed that the ZnO films have a high degree of out-of-plane and in-plane crystallinity and a remarkably good morphology. Low-temperature photoluminescence spectra also revealed a very sharp excitonic emission comprised of a neutral donor bound exciton emission and a very strong free exciton A emission with first, second, and third LO phonon replicas, indicating that the ZnO epilayer was of a high optical quality.