Effect of ultra-thin buffer on the structure of highly mismatched epitaxial ZnO during sputter growth

We investigated the microstructural evolution of ZnO/Al2O3(0001) films with and without an ultra-thin (4nm) ZnO buffer that was grown at a low temperature (LT) of 300°C using real time synchrotron X-ray scattering, atomic force microscopy, and high resolution electron microscopy. It is shown that the ultra-thin two-dimensional (2D) layers play a critical role for improving the ZnO layer quality, by inducing 2D growth mode instead of 3D mode at 500°C in early stage. The ZnO films grown on the ultra-thin buffer exhibited structural coherence between the surface and the interface in the substrate normal direction in early stage. The great enhancement of the structural quality was attributed to the strain accommodation by the 2D ultra-thin buffer.