Strong ultraviolet and violet emissions from ZnO/TiO2 multilayer thin films

Abstract

ZnO/TiO2 multilayer thin films were prepared by electron beam evaporation and the influence of annealing temperature on their structural and optical properties was investigated. The analyses of X-ray diffraction (XRD) show that TiO2 in the as-deposited ZnO/TiO2 multilayer thin film as well as in that one annealed at 300°C is amorphous. When the annealing temperature is increased up to 400°C, the TiO2 is transformed into a brookite phase. Eventually, the TiO2 is transformed into an anatase phase with the annealing temperature rising up to 500°C. However, ZnO in these composite films all crystallizes in a wurtzite structure and has a preferred orientation along the (002) direction whether the ZnO/TiO2 multilayer thin film is annealed or not. The photoluminescence spectra show that all the samples have two strong emission peaks including an ultraviolet (UV) peak and a violet peak. The strong UV and violet co-emissions in ZnO materials is seldom reported previously. With the increase of annealing temperature, both the UV and violet emissions are enhanced. However, the intensity of UV emission is increased faster than that of the violet emission under the same annealing temperature. The mechanism of the violet emission and the reason for the variations of the UV and violet emissions with the different annealing temperatures are analyzed.