Off-axis sputter deposition of ZnO films on c-sapphire substrates with buffer layers prepared via nitrogen-mediated crystallization

High-quality epitaxial ZnO films on c-plane sapphire substrates have been obtained by utilizing off-axis sputtering configuration together with buffer layers prepared via nitrogen mediated crystallization (NMC). The role of NMC buffer layers is to provide high density of nucleation site and thus to reduce the strain energy caused by the large lattice mismatch (18%) between ZnO and sapphire. The NMC buffer layers allow two dimensional (2D) growth of subsequently grown ZnO films, being particularly enhanced by employing off-axis sputtering configuration, in which the substrate is positioned out of the high-energy particles such as negative oxygen ions originating from the targets. As a result, ZnO films with smooth surfaces (root-mean-square roughness: 0.76 nm) and high electron mobility of 88 cm²/Vsec are fabricated. Photoluminescence spectra of the ZnO films show strong near-band-edge emission, and the intensity of the orange-red defect emission significantly decreases with increasing the horizontal distance between the target and the substrate. From these results, we conclude that off-axis sputtering together with NMC buffer layers is a promising method for obtaining high quality epitaxial ZnO films.