The growth of non-polar ZnO and ZnO/Mg$_{0.25}$Zn$_{0.75}$O epi-films by radio-frequency magnetron sputtering

High quality non-polar ZnO and ZnO/Mg$_{x}$Zn$_{1-x}$O (x = 0.15, 0.25) epi-films have been successfully grown on m-plane and r-plane sapphire by using RF magnetron sputtering. The structural properties, including crystalline quality, strain state, and defect structures, of the ZnO and ZnO/Mg$_{x}$Zn$_{1-x}$O layers are thoroughly examined by synchrotron x-ray scattering, transmission electron microscopy and atomic force microscopy. We found the surface morphology of the a-plane oriented ZnO epi-films is smoother than that of m-plane oriented one. Moreover, the surface of Mg$_{0.25}$Zn$_{0.75}$O epi-layer is significantly better than the ZnO epi-film and exhibits the same orientation dependence. The optical properties of these samples are also investigated by temperature, polarization and power dependent photoluminescence, and polarization dependent Raman spectroscopy. The results reveal the a-plane ZnO epi-film grown on r-sapphire with a Mg$_{0.25}$Zn$_{0.75}$O buffer layer is promising for bright UV emission application.