Pulsing frequency induced change in optical constants and dispersion energy parameters of WO₃ films grown by pulsed direct current magnetron sputtering

In this work, we present the pulsing frequency induced change in the structural, optical, vibrational, and luminescence properties of tungsten oxide (WO₃) thin films deposited on microscopic glass and fluorine doped tin oxide (SnO₂:F) coated glass substrates by pulsed dc magnetron sputtering technique. The WO₃ films deposited on SnO₂:F substrate belongs to monoclinic phase. The pulsing frequency has a significant influence on the preferred orientation and crystallinity of WO₃ film. The maximum optical transmittance of 85% was observed for the film and the slight shift in transmission threshold towards higher wavelength region with increasing pulsing frequency revealed the systematic reduction in optical energy band gap (3.78 to 3.13 eV) of the films. The refractive index (n) of films are found to decrease (1.832 to 1.333 at 550 nm) with increasing pulsing frequency and the average value of extinction coefficient (k) is in the order of 10⁻³. It was observed that the dispersion data obeyed the single oscillator of the Wemple-Didomenico model, from which the dispersion energy (E_d) parameters, dielectric constants, plasma frequency, oscillator strength, and oscillator energy (E_o) of WO₃ films were calculated and reported for the first time due to variation in pulsing frequency during deposition by pulsed dc magnetron sputtering. The E_o is change between 6.30 and 3.88 eV, while the E_d varies from 25.81 to 7.88 eV, with pulsing frequency. The Raman peak observed at 1095 cm⁻¹ attributes the presence of W-O symmetric stretching vibration. The slight shift in photoluminescence band is attributed to the difference in excitons transition. We have made an attempt to discuss and correlate these results with the light of possible mechanisms underlying the phenomena.