Interface Engineering in ZnO Epitaxy

We report interface engineering in ZnO epitaxy to grow high-quality layers by plasma-assisted molecular beam epitaxy. Through interface engineering, we have succeeded in two-dimensional layer-by-layer growth of ZnO both on sapphire and GaN, and control of lattice polarity of ZnO films on a Ga-polar GaN template. MgO buffer has been used to convert the growth mode from a three-dimensional to a two-dimensional mode in ZnO epitaxy on sapphire. O-plasma pre-exposure on Ga-polar GaN templates has been employed to form a monoclinic Ga2O3 interface layer at the ZnO/GaN heterointerface, while Zn pre-exposure prevents oxidation of the GaN surface resulting in a ZnO/GaN heterointerface without an interface layer. The lattice polarity of ZnO films on Ga-polar GaN templates with and without a Ga2O3 interface layer with inversion symmetry have been revealed as Zn- and O-polar, respectively, by coaxial impact collision ion scattering spectroscopy. Structural properties of ZnO films grown with MgO buffer on sapphire or with Zn pre-exposure on GaN templates are better than those grown without MgO buffer or with an O-plasma pre-exposure, respectively. A buffer mechanism of MgO is discussed based on reflection high-energy electron diffraction and high-resolution X-ray diffraction analyses, while the mechanism for controlling the polarity in ZnO epitaxy is discussed using an interface layer with inversion symmetry. It is suggested that the present method offers a general approach to control the crystal polarity.