Band gap widening and narrowing in Cu-doped ZnO thin films

• Band gap widening and narrowing upon copper doping in ZnO.
• Dissociation of CuO nanophase in ZnO host by high temperature annealing.
• Mechanisms of band gap tuning in framework of band gap renormalization.
• Effects of stress due to creation of intrinsic defects and secondary phase formation.

Cu-doped Zinc Oxide (CuZnO) thin films have been prepared on silicon and quartz substrates by sol-gel spin coating method. These films were further annealed at 700 °C in controlled oxygen environment for improving crystalline quality and stoichiometry. In order to understand the material properties of thin films, X-ray Diffraction, Scanning Electron Microscopy, Atomic Force Microscopy, Raman Spectroscopy and UV–Visible Spectroscopy methods have been used in the present study. Polycrystalline nature of the CuZnO thin films is confirmed by structural analysis with increased crystallite size upon increasing the doping concentrations of the thin films. The phonon evolution at different doping concentrations is understood by Raman spectroscopy, thus confirming the wurtzite phase stability of the thin films with increase in crystallite size and presence of mixed phases have also been reported. In addition, widening and narrowing of band gap for the CuZnO thin films have been discussed in optical studies. This anomalous behavior in band gap tuning such as widening (+0.5 eV) and narrowing (−0.6 eV) under different doping concentrations and annealing temperature is understood by the influence of induced stress effects due to creation of intrinsic defects with band gap renormalization and secondary phase formation.

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