Effect of substrate temperature on structural and optical properties of nickel tungsten oxide thin films

Abstract

In this work, we present the substrate temperature induced change in the structural, optical, vibrational and luminescence properties of mixed NiO:WO3 (95:5) thin films deposited on glass substrates by rf magnetron sputtering technique. X-ray diffraction analysis revealed the onset of crystallization of the films occurred at 300 °C. The average optical transmittance of the films varied between 91 and 97 % in the visible region. The refractive index and extinction coefficient of films are found to decrease with increasing substrate temperature. It was observed that the dispersion data obeyed the single oscillator of the Wemple-DiDomenico model, from which the dielectric constants, ratio between free carrier density and free carrier effective mass, plasma frequency, oscillator energy, oscillator strength, and dispersion energy parameters of NiO:WO3 films were calculated and reported for the first time due to variation in substrate temperature during deposition by rf magnetron sputtering. The micro-Raman result shows two broad peaks corresponding to one-phonon LO mode at 570 cm−1 and two-phonon LO mode at 1,100 cm−1 due to the vibrations of Ni–O bonds and the peak found at 870 cm−1 belongs to the W–O mode. Room temperature photoluminescence (RTPL) study exhibits two characteristic emission peaks at 3.32 eV (374 nm) and 2.93 eV (423 nm), which corresponding to the transition of 3d8 Ni2+ ions. We have made an attempt to discuss and correlate these results with the light of possible mechanisms underlying the phenomena.