Correlation Between the Structural, Morphological and Optical Characteristics of ZnO Thin Films Prepared by Thermal Evaporation

In this study, we used thermal evaporation to deposit thin films of zinc oxide (ZnO). The films were then subjected to annealing at various temperatures ranging from 350 °C to 500 °C, with a fixed annealing time of 2.5 hours. The film thickness was kept constant at 300 nm. The morphological, optical properties and structural changes of the ZnO films were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) and visible-ultraviolet spectroscopy (VIS-UV) techniques. The XRD pattern also confirmed that the ZnO films exhibited a hexagonal wurtzite crystal structure. The full width at half maximum (FWHM) values of the diffraction peaks decreased as the annealing temperature increased, indicating better crystallinity of the thin films at higher temperatures. SEM images show that the grain size of thin films tends to increase as the annealing temperature increases. The contact angles of the samples were significantly increased and the surface wettability of the layers changed from hydrophilic to hydrophobic after annealing temperature. The VIS-UV data showed that the ZnO films were transparent in the visible region. The optical transmittance slightly increased with increasing annealing temperature. The optical gap (Eg) of the films decreased as the annealing temperature increased. The calculated Urbach energy values indicated that the defects in the ZnO films decreased with annealing temperatures. Finally, the correlation between the structural, morphological, wettability and optical features of the samples was determined. The optical band gap was observed to correlate proportionally with crystallite size and inversely with Urbach energy as a function of annealing temperature.