Polarity of heavily doped ZnO films grown on sapphire and SiO2 glass substrates by pulsed laser deposition

The crystalline polarity of undoped and impurity-doped ZnO films grown on SiO2 glass substrates was investigated with the goal of achieving polarity-selective growth of ZnO films on non-crystalline substrates. We first demonstrated that hard X-ray photoelectron spectroscopy (HX-PES) is an appropriate method for determining the crystalline polarity of ZnO. We then characterized the ZnO films grown by pulsed laser deposition using HX-PES. The resulting films deposited with a 1 mol% Al-doped ZnO target had the (0001) surface, whereas films grown with nominally undoped, 0.1 mol% Al-doped, 1 mol% Ga-doped, and 1 mol% In-doped ZnO targets had the (0001¯) surface. Since a clear polarity change due to Al-doping was seen at the ZnO/glass structure, we conclude that the essential parameter governing the polarity of the ZnO films is unlikely lattice matching (alignment of the lattice on the atomic scale) at the heterointerface between the ZnO films and substrates.