Thickness dependent study of RF sputtered ZnO thin films for optoelectronic device applications

Abstract

This work reports thickness dependent structural and optical studies of ZnO thin films grown over p-type silicon (Si) and glass substrates using RF sputtering technique. The effect of variation of thickness on different microstructural and surface morphological parameters such as lattice constant, grain size, dislocation density, residual stress, lattice strain and surface roughness have been studied. All samples under study exhibited a hexagonal wurtzite structure with preferential growth along the c-axis perpendicular to the surface. It has been observed that structural disorder, stress and strain in the film reduces and as a result the crystalline quality of deposited ZnO thin films improves with increasing thickness. Optical characterization of deposited films has been done using UV-visible spectrophotometer, ellipsometer and photoluminescence spectroscope. Transmittance spectra of all ZnO thin film samples (190 - 342 nm) have exhibit a high transmittance over 85% in the visible region, and refractive index was found to vary between 0.85 - 3.1 in UV-Visible wavelength region (300 - 800 nm). The optical bandgap recorded a marginal increase from 3.22 eV to 3.26 eV by varying the thickness from 190 - 342 nm. Effect of film thickness on other optical parameters such as absorbance, reflectance, urbach energy, dielectric constant and photoluminescence has also been analyzed and reported.