Effects of Co and/or Sn doping on crystal structures and optical properties of ZnO thin films

The Co and/or Sn doped ZnO thin films are deposited on the glass substrates by the sol-gel method. The effects of Co and/or Sn doping on surface morphologies and mircrostructures of ZnO films are investigated by metallurgical microscope and X-ray diffraction (XRD). The XRD results indicate that all the ZnO samples show preferential orientation along the (002) direction, and that the Sn-doped ZnO thin film exhibits the best c-axis orientation and largest grain size. XPS results reveal that Co and Sn elements exist as Co2+ and Sn4+, indicating that Co and Sn ions have entered into the ZnO crystal lattices successfully. Strong blue double emission and weak green emission are observed in the PL spectra of all the samples. In addition, the ultraviolet peaks appear in the undoped and the Co-doped ZnO thin films. Our results reveal that the Co and/or Sn doping can tune the band gap, meanwhile, such a doping can also affect oxygen dislocation, zinc oxygen and zinc interstial defect concentrations. Finally, the possible luminescence mechanisms of Co and/or Sn doped ZnO films are discussed.