Structural, optical, and electrical properties of cadmium oxide thin films prepared by sol–gel spin-coating method

Abstract

Cadmium oxide thin films were prepared on glass substrates by the sol–gel spin-coating technique. The effects of the annealing temperature and Cd2+ concentration at the initial solution on the physical properties of the thin films are studied. It was found that 450 °C is the optimum annealing temperature for the preparation of cadmium oxide thin films with high transparency in visible and near infrared spectral regions, as well as high electrical conductivity. Then, by using this optimum annealing temperature, cadmium oxide thin films of various molar concentrations were deposited at the same experimental conditions. X-ray diffraction patterns show that the obtained films are polycrystalline. The structural analysis shows that all the samples have a cubic structure. The crystallite size and the preferred orientation were calculated from the X-ray diffraction data. Optical measurements have shown that an increase in the molar concentrations results in a reduction in the optical transmission of the layer. The films are highly absorbing in the visible region lower than 600 nm and their absorption is strongly influenced by the film thickness, with an average value above 60% for the current growth conditions. At the same time, the optical band gap decreases from 2.35 to 2.15 eV when the molar concentration of precursor solutions increases. The electrical measurements show that the resistivity of the films varies slightly from 2.7 × 10−3 to 7.5 × 10−3 Ω cm with the molar concentration. The cadmium oxide film, with the minimum resistivity of 2.7 × 10−3 Ω cm, was prepared by an initial concentration of Cd2+ of 0.1 M, with carrier concentration and mobility 1.16 × 1019 cm−3 and 199 cm2/Vs, respectively. The dependence of the band gap and the electrical properties of CdO thin films elaborated at different annealing temperature and molar concentrations.