Tailoring the optical properties of ZnO thin films with a double-layer structure: The role of annealing temperature

ZnO thin film is an important optical material. Therefore, using simple methods or techniques to improve its optical quality is an important research topic. In this study, a sol-gel spin-coating method was used to deposit ZnO thin films co-doped with Fe and Cl on glass substrates and the effect of annealing temperature on their microstructure and optical properties was explored. Field emission scanning electron microscope observes that all samples have a double-layer structure: the upper layer is composed of single-crystal nanocolumns and the lower layer is made up of polycrystalline nanoparticles. The increase of annealing temperature leads to some important changes: (1) the UV emission is gradually enhanced; (2) the absorption edge is gradually red-shifted; (3) cavities are formed in the lower layer of ZnO thin films. As far as we know, there are few reports on cavities in the lower layer of ZnO thin films. The appearance of these cavities, we speculate, is due to the mass transport from the lower layer to the upper layer, that is, the formation of single crystal nanocolumns is the result of sacrificing the polycrystalline nanoparticles in the lower layer of ZnO thin films.