Spray pyrolysis-deposited Cu-doped iron oxide thin films: structure, optics, and nonlinear properties
摘要
Cu-doped hematite (Fe2O3) thin films with 0–10 wt% Cu were deposited on glass and examined to link surface microstructure, out-of-plane texture, and optical response. X-ray diffraction confirms single-phase rhombohedral hematite without secondary phases. Halder–Wagner analysis gives crystallite sizes of 13.5–13.9 nm. Plan-view FE-SEM shows faceted grains composed of hexagonal/pseudo-hexagonal plates and elongated/prismatic grains; number-weighted grain-size distributions are unimodal and narrow (mean ≈ 0.23 μm; spans below 2) for every composition, demonstrating normal grain growth with no coarse-grain tails. Texture quantified from integrated XRD peak areas indicates a composition-dependent enhancement of the –110– family that maximizes at 5 wt% Cu, evidencing orientation selection without microstructural coarsening. Indirect Eg = 1.81–1.96 eV and direct Eg = 2.94–3.00 eV across 0–10 wt% Cu, with a slight indirect gap narrowing at higher Cu. The Urbach energy rises with Cu content—from 0.39 eV (1 wt%) to 0.44 eV (5 wt%)—signaling an increased tail-state density. The refractive index exhibits normal dispersion, decreasing from n = 2.09 at 600 nm to n = 1.81 at 1500 nm for the most dispersive film. Using the measured absorption, the optical skin depth (penetration depth) lies in the sub-wavelength to few-hundred nanometer range across the visible–NIR and varies systematically with Cu. Third-order nonlinear parameters estimated via Miller’s rule show composition-dependent changes: |χ(3)|= (2.45–6.71) × 10–15 (esu) and n2 of (1.26–3.16) × 10–16 m2 W−1, with a local maximum at 1 wt% Cu. Overall, Cu enables tuning of texture and linear/nonlinear optics while preserving narrow grain-size distributions—useful for oxide film optoelectronic and photoelectrochemical devices.