Investigation of growth mode in ZnO thin films prepared at different temperature by plasma-molecular beam epitaxy

High-quality ZnO thin films on c-plane sapphire (Al2O3) substrates were prepared by plasma-molecular beam epitaxy (P-MBE). The influence of growth temperature on growth mode of ZnO was investigated. Real-time monitored by reflection high-energy electron diffraction (RHEED) images show that, below 500 °C, ZnO thin film was grown by three-dimension (3D) growth mode. While the two-dimension (2D) growth mode was obtained above growth temperature of 650 °C. Atomic force microscopy (AFM) images present that the surface morphology of ZnO thin film with 2D growth is improved and X-ray rocking curves (XRC) indicate that the full width at half maximum (FWHM) of the ZnO (0 0 2) peak becomes narrow. From the photoluminescence (PL) spectra, ultraviolet (UV) emission peak exhibits obvious blue-shift for the samples grown at lower temperature, which is attributed to the effect of the quantum confinement arisen from small crystal grain sizes. The minimum carrier concentration of N=7.66×1016 cm−3 was obtained in the ZnO thin films with the 2D grown, which is closed to that of bulk ZnO.