Role of defects in tailoring optical, electronic, and luminescent properties of Cu-doped ZnO films

We present a detailed characterization study on copper-doped ZnO films to correlate the films' electronic and optical properties with the existing native defects in the lattice. In addition, we describe the variation in the concentration of these defects with Cu dopant and temperature. The results of XRD confirmed the single-phase wurtzite-structure of the synthesized films. The SEM images showed a homogeneous and dense grain morphology with a granular form and a signature for a hexagonal-like shape. The EDX, XPS, and UV-Vis spectra showed the proper doping of Cu ions into the lattice. The XPS analysis indicated mixed electronic states of both Cu2+ and Cu1+ and showed a clear increase in the Cu2+ intensity relative to Cu1+, with Cu dopant. The transmittance spectra exhibited an average value above 80% in all doped films in the visible and infrared regions. The overall results indicated a clear link between the films' optical and electronic responses and the level of the intrinsic defects in the lattice. By increasing the Cu dopant, we find a slight reduction in the energy bandgap (Eg). This is correlated with a clear reduction in the blue emission luminescence band associated with the VZn and in the yellow emission band associated with the Oi. On the other hand, we observed a clear enhancement in the green emission band originating from the VO, and in the emission band related to possible transitions from Zni levels to Oi levels. The slight reduction in the Eg signals a weak sp-d hybridization between the ZnO conduction band electrons and the Cu2+ ions, which is mediated by the intrinsic defects. With reducing the temperature, the photoluminescence temperature profiles indicated a slight increase in the Eg values and a negligible effect on the distribution of the native defects.