RF Sputtering Power-Driven Growth and Structure–Property Relationships in NiOx Thin Films Deposited at Room Temperature

Nickel oxide (NiOx) thin films are promising p-type transparent semiconductors for optoelectronic and energy-conversion devices. This study investigates the role of radio-frequency (RF) sputtering power in the growth evolution, microstructural development, and functional properties of NiOx thin films deposited at room temperature. The RF sputtering power was varied from 75 to 150 W, while the deposition time was adjusted to maintain an approximately constant thickness of ~110 nm. The films were characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), X-ray diffraction (XRD), optical transmittance, and electrical measurements. AFM and SEM analyses revealed an evolution from relatively smooth surfaces at low RF sputtering powers toward increased surface roughness at higher powers. XRD confirmed polycrystalline NiOx with a face-centered cubic structure, showing a preferential orientation shift from (111) to (200) and a maximum crystallite size of 14.73 nm at 125 W. Electrical resistivity showed a non-monotonic dependence on RF sputtering power, ranging from 25.39 to 309.85 kΩ·cm, indicating that the electrical response cannot be explained solely by crystallite size. Thickness-dependent analysis revealed an evolution from early-stage nucleation toward more developed columnar growth. Finally, comparison with dip-coating and AACVD revealed distinct structure–property relationships among the deposition routes.