Tri-Buffer Process: A New Approach to Obtain High-Quality ZnO Epitaxial Films on Sapphire Substrates
A tri-buffer method was applied to achieve layer-by-layer growth of high-quality ZnO films on sapphire (0001) substrates by rf plasma-assisted molecular beam epitaxy (MBE). After sufficient nitridation of the substrate, MgO and ZnO buffer layers were subsequently deposited on the resulting AlN layer. An atomically smooth ZnO surface with a roughness less than 1 nm in a 10 μm × 10 μm scanned area was obtained with this method. The crystal quality was also improved, as characterized by reflection high-energy electron diffraction (RHEED), x-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy (TEM). The results indicate that the tri-buffer process could reduce the large lattice mismatch between ZnO and nitrided sapphire and facilitate the two-dimensional (2-D) growth of the ZnO epilayer. A model is proposed to understand the observations.