Influence of Cr-doping on the structural and the optical properties of ZnO thin films prepared by sol-gel spin coating

Thin films of undoped ZnO and Cr-doped ZnO were deposited by sol-gel spin coating. The structural and the optical properties of Cr-doped ZnO thin films with different concentrations of Cr (0-2.5 at.% relative to Zn) were studied by using X-ray diffraction (XRD) and photoluminescence (PL). The XRD showed that the thin films grew with a preferred c-axis orientation. No traces of the Cr metal, oxides, or a binary Zn-Cr phase were detected. This means that the addition of Cr into the ZnO matrix preserved the formation of the wurtzite structure of ZnO. However, increasing the amount of Cr dopant in the ZnO thin films caused distortion due to the intercalation of the dopant atoms into the wurtzite structure. The PL spectra showed a blue shift in the near-band-edge emission and a decrease in the deep-level emission upon increasing the amount of Cr doping. The narrowest full width at half maximum (FWHM) was observed in ZnO thin films with 2.5 at.% Cr doping. The FWHM gradually decreased upon further increasing the Cr concentration.