Preparation and characterization of transparent NiO thin films deposited by spray pyrolysis technique

Nickel oxide thin films were successfully fabricated with various deposition time (td = 5, 10, and 15 min) on glass substrates using spray pyrolysis technique. The deposited films undergo thermal treatment at 350 °C for various annealing time (ta = 0, 15, 30 and 60 min). In this study, the effect of td and ta on film thickness was observed and their influence on structural, morphological and optical properties were investigated. The films deposited with td = 5 min showed amorphous structure while the films grown at higher deposition time became partially crystallized with preferred growth along (1 1 1) direction. Heat treatment carried out in air allowed us to tune the polycrystalline structure and the diffraction intensity at preferred peak increases with the increase in ta which is a consequence of better crystallinity. This was reflected in the AFM micrographs of the films which suggested that the thermal annealing (or increasing ta) facilitates the process of grain-growth, and improves the crystalline microstructure. The optical transmission of the films was found to vary with td and ta and thus film thickness. The thinner films show higher transparency in the UV–vis spectral region. The optical band gap was blue-shifted from 3.35 eV to 3.51 eV depending on ta. The effect of ta on the various optical constants of the NiO films has also been discussed.