Investigation of the structural, electrical and optical properties of Zr-doped CdO thin films for optoelectronic applications

Cadmium oxide (CdO) thin films were successfully deposited on glass substrates by the sol–gel spin coating method. The effects of the annealing temperature and Zirconium (Zr) doping on the physical properties of the CdO thin films were investigated. It was found that 550 °C is the optimal annealing temperature with a good crystallinity for the CdO thin films. Then, by using this temperature, different Zr doping concentrations (0, 1, 3, 5 and 7%) were prepared at the same experimental conditions. The X-ray diffraction (XRD) patterns of all the as-prepared thin films showed a polycrystalline nature with a face centered cubic (FCC) structure and that Zr incorporated the sites of Cadmium (Cd) up to 3% concentration, while the excess Zr doping ( x > 3%) aggregated at the grain boundaries of the CdO lattice. Scanning electron microscopy (SEM) analysis revealed homogeneous and dense grains distribution with a film thickness of 30 μm. The optical study showed an increase in optical transmittance of CdO with Zr doping up to 3% and then a decrease onwards within the same wavelength region. The optical band gap values increased from 2.41 to 2.67 eV with increasing Zr doping which was explained by the Burstein–Moss effect. The minimum electrical resistivity (5.32 × 10 −1 Ω .cm) was obtained for the film coated with 3% Zr using the four-probe method.