Extended-analysis for ZnO monolayers, (TiO2 & WO3)/ZnO bilayers, and WO3/TiO2/ZnO multilayers deposited on silica glass substrates by RF sputtering as promising cap layers in optoelectronic applications

Abstract

This study employs radio frequency sputtering (RF) to fabricate a range of thin film structures on silica glass substrates. These structures encompass ZnO monolayers, (TiO2/ZnO and WO3/ZnO) bilayers, and (WO3/TiO2/ZnO) multilayers. Thorough investigations were conducted to analyze their structural evolution, surface morphology, and optical characteristics, affirming film crystallinity via X-ray diffraction. By Scherrer's formula, the crystallite size D is determined, the microstrain of films ε is extracted by the Williamson-Hall method, and then the dislocation density δ is computed. It is found that the crystallite size increases from 8.47 to 13.56 nm with increasing number of layers. FTIR spectra identified functional groups, while SEM provided cross-section views. The presence of a tungsten trioxide (WO3) cap layer significantly influences optical properties, particularly in multilayers, impacting key parameters such as optical bandgap and tail energy. As the number of layers increases, the bandgap decreases from 3.45 to 2.47 eV, while the tail energy increases from 0.48 to 0.96 eV. The optical dispersion parameters of the fabricated layers, including the single oscillator energy Eo (changing from 6.93 to 4.96 eV), dispersion energy Ed (ranging from 32 to 48 eV), and static refractive index no (varying from 2.36 to 3.26), are computed using the WWD (Wemple-Didomenico) model. As the number of layers rose, the sheet resistance (Rsh) of the films increased, accompanied by a decrease in coverage density (DC). Elevated sheet resistance contributes to enhanced electrical resistivity, potentially benefiting resistive devices. Simultaneously, reduced coverage density can improve transparency and promote coating uniformity, offering advantages in optoelectronic and display technologies. The study also assessed inter-band transition strength and examined energy loss functions, revealing improved optical properties with additional layers. These findings highlight the system's promising potential as a cap layer in optoelectronic applications.