In our experimental work, nickel oxide (NiO) thin films were deposited onto silicon substrates using RF magnetron sputtering at a fixed power of 200 W. A key observation was the evolution of the film's microstructure with increasing oxygen flow: higher oxygen fluxes consistently produced smaller grain sizes while maintaining a distinct granular morphology. This tuning of oxygen content during growth yielded significant changes in the material's fundamental properties. Interestingly, the optical bandgap energy showed a clear upward trend, widening from 2.6 eV to 3.4 eV as more oxygen was introduced. Equally striking was the electrical response, where the film's resistivity plummeted dramatically – falling from 78 Ω·m down to just 0.6 Ω·m. Crucially, we also investigated the impact of post-deposition annealing at 400 °C. This thermal treatment proved beneficial, further reducing both the bandgap energy and the resistivity compared to the as-deposited films. The study reveals that post-annealing improvements in NiO thin films are primarily due to enhanced crystallinity and reduced electron scattering at grain boundaries. This reveals a powerful tool for tailoring NiO thin films by controlling the oxygen flow rate during sputter deposition, enabling the engineering of optical transparency and electrical conductivity. This is particularly useful for next-generation energy conversion and optoelectronic devices.
Bukhari et al. (2025) studied this question.