Effect of thermal treatment on thin films of NiO:WO3 for optoelectronic applications

Nickel oxide (NiO), a semiconducting metal oxide, with its exceptional optical, electrical, and magnetic properties make it highly sought after for use in optical, optoelectronic, and electrochromic devices. Recent advancements in NiO-optical devices relies on the optical parameters like intrinsic optical nonlinearities. The current study involves the deposition of NiO:WO₃ thin films utilizing radio frequency (RF) sputtering at various substrate temperature (T_s) of 100, 200, and 300 °C. An investigation was carried out to analyse the significance of T_s on the structural, optical, morphological, & vibrational characteristics. The X-ray diffraction investigation revealed the amorphous nature of the films. Scanning electron microscopy confirms the compact pinhole free surface of the films. The results of energy dispersive X-ray spectroscopy (EDS) spectra verified the existence of nickel (Ni), oxygen (O), and tungsten (W) components in the films. XPS analysis confirms the presence of both Ni²⁺ and Ni³⁺ states. The optical transmittance increased, and the band gap decreased with increase in T_s. The impact of T_s on various optical parameters such as refractive index, extinction co-efficient, dielectric constants, optical non-linear susceptibility, and dispersion energy parameters obtained by the Wemple-DiDomenico theory has been calculated and extensively investigated. The results indicate that NiO:WO₃ films can be used to fabricate electrochromic and optoelectronic devices.