Luminescence Properties of Cobalt-Doped ZnO Films Prepared by Sol-Gel Method

Pure ZnO and Co-doped ZnO films have been deposited on coverslip substrates by sol-gel spin coating. The morphological, structural, and optical properties of the films were investigated. The microstructure of the ZnO films became increasingly fine and the crystalline size decreased with Co doping. Analysis of x-ray diffraction (XRD) and Raman spectra reveals that Co2+ ions are substituted for Zn2+ ions in the ZnO lattice without changing its wurtzite structure. Co doping induces a decrease of the band-gap energy and fluorescence quenching of the emission bands. The spectra related to transitions within the tetrahedral Co2+ ions in the ZnO host crystal were observed in absorption and luminescence spectra. Photoluminescence (PL) spectra under different excitation energies and PL excitation spectra for the visible emissions suggest that the orange-red emission and green emission could be related to interstitial zinc (Zn-i) shallow donors and oxygen vacancy (V (O)) deep donors, respectively. The red emission of Co-doped ZnO film could be assigned to the radiative transitions within the tetrahedral Co2+ ions in the ZnO host crystal after band-to-band excitation. A consistent explanation for the pure and Co-doped ZnO films is that the red emission under the excitation energy below the band gap is probably associated with extended Zn-i states.