Effect of ultrathin buffer on the microstructure of highly mismatched epitaxial ZnO films on Al2O3 (0001)

The effect of the ultra thin (4 nm) ZnO buffer layer grown at a low temperature of 300 deg. C on the microstructural evolution of highly mismatched ZnO/Al2O3(0001) films was investigated. Based on the real time synchrotron x-ray scattering, atomic force microscopy, and high resolution electron microscopy, it was shown that the ultrathin two-dimensional (2D) layers play a critical role for improving the ZnO layer quality by inducing 2D growth mode instead of three-dimensional mode at 500 deg. C in early stage. The ZnO films grown on the ultrathin buffer exhibited structural coherence between the surface and the interface in the substrate normal direction in early stage. The great enhancement of the microstructural quality was attributed to the strain accommodation by the 2D ultrathin buffer