Temperature dependence of microstructure and strain evolution in strained ZnO films on Al(2)O(3)(0001)

We have studied the temperature dependence of the growth mode and microstructure evolution in highly mismatched sputter-grown ZnO/Al(2)O(3)(0001) heteroepitaxial films. The growth mode was studied by real-time synchrotron x-ray scattering. We find that the growth mode changes from a two-dimensional (2D) layer to a 3D island in the early growth stage with temperature (300-600 °C), in sharp contrast to the reported transition from three dimensions to two dimensions in metal-organic vapor phase epitaxy. At around 400 °C intermediate 2D platelets nucleate in the early stage, which act as nucleation cores of 3D islands and transform to a misaligned state during further growth. Meanwhile, at high temperature (above 500 °C), the spinel structure of ZnAl(2)O(4) grows in the early stage, and it undergoes a transition to wurtzite-ZnO (w-ZnO) with thickness. The spinel formation is presumably driven by high temperature and large incident energy of impacting atoms during sputtering. The results of the strain evolution as functions of temperature and thickness during growth suggest that the surface diffusion is a major factor determining the microstructural properties in the strained ZnO/Al(2)O(3)(0001) heteroepitaxy.