Effect of in Situ UV Irradiation on the Optical and Structural Properties of ZnO Thin Films Prepared by Dip-Coating Method

Abstract

In this study, zinc oxide (ZnO) thin films were grown on glass substrate using a sol gel dip-coating method assisted with in situ ultraviolet (UV) irradiation. The films were irradiated for 20 to 80 s during dipping. Growing the layers under the influence of UV illumination provides an important opportunity for obtaining films with enhanced properties. The effect of UV irradiation on the microstructural, morphological, and optical properties of the as-deposited films was undertaken using X-ray diffraction (XRD), scanning electron microscopy (SEM), and UV-visible spectroscopy. XRD patterns revealed that all the synthesized films are polycrystalline phase ZnO with hexagonal structure and without any impurity phases, indicating their high purity. Furthermore, increasing the UV exposure time to 60 s improved the crystallinity of the films. UV irradiation had no effect on the lattice parameters, but increased exposure time resulted in an increase in crystallite

VI size. In addition, the UV-treated grown films for 20 and 40 s improved the (103) peak intensity, whereas the films irradiated for 60 and 80 s improved the (112) peak intensity. The SEM morphology observations revealed that the surface of all synthesized films was continuous, rough, and dense, with streamer-shaped features ranging in large from 0.8 to 1.6 𝜇𝑚 and length from 2.3 to 9 𝜇𝑚. The UV-vis absorption spectra of the untreated and UV-treated dip-coated ZnO films showed that all the films exhibited a high transmittance of visible light with an average optical transparency over 85% and 100% at 700nm. Besides, all of the UV-treated films exhibited narrow optical absorption that extended to the near-ultraviolet region. Estimated band-gap values rise from 3.12 eV for the untreated film to 3.20 eV for UV-treated film for 60s, which could be due to the Moss-Burstein effect observed in degenerate semiconductor materials.