XPS study and electronic structure of non-doped and Cr⁺ ion implanted CuO thin films

CuO is a p-type semiconductor that can be found useful in various applications, sensing, photocatalysis or photovoltaics. Better material performance can be achieved by doping. In our study the doping was done using Cr ions and implantation method. Thin film samples were characterised with X-ray photoelectron spectroscopy (XPS) technique to study chemical properties of the films' surface and to determine the in-depth compositional profile of the films before and after annealing of an implanted sample. Spectroscopic ellipsometry was used to extract the dielectric function of CuO thin films. Depolarization measurements are shown as a useful method to quickly study differences between similar samples. XPS measurements proved that before annealing there is a peak of Cr concentration in depth of the sample, which is no longer present after annealing. Measurement of film resistance as function of temperature in range of 150-300 °C resulted with 0.82 eV bandgap. Electronic structure obtained with density functional theory calculations (DFT) showed that with Cr doping the energy band gap narrows and the material should become metallic.