Influence of annealing on microstructure and properties of Cr-doped ZnO thin films deposited on glass surface

Abstract

Cr-doped ZnO thin films were deposited onto glass substrates by radio frequency magnetron sputtering technique. The effects of annealing temperature and annealing time on the structural and optical properties of the Cr-doped ZnO films were comprehensively investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible (UV) and photoluminescence (PL) measurements. XRD studies reveal that the Cr-doped ZnO films are highly oriented along (002) direction. The intensity of ZnO (002) peak firstly increases and then decreases with increasing annealing temperature and annealing time. SEM analysis show that the grain size increases and the surface of films becomes rougher as the annealing temperature increases to 700 °C. The optical transmittance spectra indicate that the average transmittance in the range of the visible region is approximately 80%, and the band gaps of the films decrease with increasing annealing temperature. Two peaks of UV emission and green emission are observed in the PL spectra. The UV emission increases first and then decreases with increasing annealing temperature, which show that the increasing defect concentration in the films leads to the poor crystallinity. The green emission greatly increases with increasing annealing temperature, which could be attributed to the increase of oxygen vacancies and increased nonradiative process.