Determination of optical constants of vacuum annealed ZnO thin films using Wemple Di Domenico model, Sellmier's model and Miller's generalized rules

Abstract

Thin films of zinc oxide (ZnO) were coated by a spin coater on the glass substrate and are annealed at 300 °C and 500 °C under the reduced atmosphere of 10−3 mbar. The structural properties were examined by X-ray diffractogram (XRD), atomic force microscopy (AFM), and optical study of ZnO thin films by UV–Vis spectrophotometer. The XRD study showed that the films had a preferred orientation towards the c-axis, and the AFM study revealed a columnar growth. Optical parameters such as band gap, Urbach energy, electron-phonon interaction strength, refractive index, carrier concentration to the effective mass (N/m*) ratio, carrier concentration, and plasma frequency were extracted from transmittance and absorption curves by using Wemple Di Domenico and Sellmier's models. The non-linear optical parameters such as susceptibility and refractive index are obtained from Miller's generalized rules. The optical bandgap was found to decrease with an increase in dopant concentrations and also with annealing temperature but for 0.5 M bandgap increase at 300°C. The Urbach energy follows the increasing trend for both increase in concentration and annealing temperature. This indicates vacuum annealing induces defect sites in the material under investigation.