Doping-induced transition from polycrystals to single crystals in ZnO thin films

Abstract

The growth of high quality ZnO thin films is a hot research topic in the field of optoelectronic materials. In this study, high quality ZnO thin films with a double-layer structure were obtained by sol-gel method. Al-doping induced transition from polycrystals to single crystals in the ZnO thin films was found. The results show that Al-doping improves the c-axis orientation of ZnO thin films. When the Al-doping concentration is higher than 4at%, the frustum of hexagonal pyramid (FHP) shaped ZnO nanostructures are formed on the polycrystalline ZnO thin films surface. As the Al-doping concentration further rises, the areal density of the FHP-shaped nanostructures also gradually increases and the size becomes more uniform. Both SAED patterns and high-resolution TEM images indicate that the FHP-shaped ZnO nanostructures are single crystals. It is of great scientific significance to transform polycrystalline ZnO into single crystal ZnO by Al-doping combined with a simple heat-treatment process. Compared with pure ZnO thin films, the FHP-shaped nanostructures on the surface show better UV emission performance, suggesting these materials are ideal ones for the preparation of ZnO-based UV emitters. The authors propose that the growth mechanism of FHP-shaped ZnO nanostructure is a multi-step solid-solid reaction dominated by surface diffusion of zinc atoms.