Optical and structural properties of copper oxide thin films grown by oxidation of metal layers

Abstract Copper oxide films were grown by oxidation of vacuum evaporated copper layers on silicon substrates. Oxidations were performed at atmospheric pressure, in a nitrogen–oxygen mixture 10% in oxygen and at temperatures varying between 185 and 450 °C. X-ray diffraction (XRD) patterns showed that, dependent on oxidation temperature, films were composed either of Cu2O at 225 °C, or of CuO above 350 °C. Films obtained at 185 °C were composed of copper silicide, formed near the interface with the substrate and Cu2O. At 280 °C films were composed of copper silicide, Cu2O and CuO. Fourier transform infrared transmittance spectra confirmed the results obtained from the XRD. The optical properties of films were studied with spectroscopic ellipsometry measurements within the energy range 1 to 3.5 eV. These measurements were analysed using the Tauc–Lorentz (TL) model to simulate the dispersion of the complex refractive index of disordered films. It was shown that the TL model describes satisfactorily the refractive index dispersion of these copper oxide films. The band gap, as defined by the TL model, was found equal to 2.3 eV for Cu2O and between 1.05 and 1.2 eV for CuO. It was shown that the gap of the Cu2O films was free of localized states, which was not the case for the gap of CuO.