Growth dynamics and surface scaling of air-oxidized NiO thin films from sputtered Ni

Nickel oxide (NiO) thin films have emerged as pivotal materials for next-generation optoelectronic applications. In this work, we present a comprehensive investigation of the correlation between surface fractality, optical, electrical properties in NiO thin films, deposited by the magnetron sputtering at varying deposition times (50, 70, and 90 min). The evolution of surface morphology, quantified through power spectral density (PSD) based fractal analysis, revealed a progressive enhancement in surface complexity with deposition time, characterized by the fractal dimension (D_f), ranges from 2.06 ± 0.02 to 2.24 ± 0.03. This trend reflects a transition from a kinetically limited to a diffusion-dominated growth regime. Concurrently, the roughness exponent (α) decreased from 0.94 to 0.76, whereas the growth exponent (β ≈ 0.27) remained nearly invariant, suggesting a self-affine surface evolution governed by competitive aggregation and relaxation mechanisms. Optical spectroscopy confirmed a strong interplay between the fractal scaling parameters and the optical properties. The optical band gap (E_g) exhibited a systematic redshift from 3.79 eV to 3.68 eV as D_f increased, indicating that enhanced surface irregularity and nanoscale disorder facilitate localized states within the band structure. Electrical measurements further revealed a monotonic reduction in conductivity from 9 × 10⁻⁴ S cm⁻¹ to 4 × 10⁻⁴ S cm⁻¹ with increasing deposition time, consistent with charge carrier scattering induced by increased morphological irregularities. The relationship between fractal scaling parameters and electrical conductivity was validated using the Fal'ko–Efetov relation. The integrated structural, optical, and electrical analysis provides a mechanistic understanding of how fractal morphology governs the functional performance of NiO films. These findings highlight that controlled fractal growth in sputtered NiO enables the rational tuning of band gap and conductivity, offering a robust pathway to optimize p-type transparent electrodes and hole transport layers for high-performance optoelectronic, photovoltaic, and photoelectrochemical applications.