Thermally induced multiscale reorganisation of SnOx thin films: reciprocal-space ordering and real-space surface coarsening

Thermal processing reorganises thin films across several length scales, while conventional descriptors such as crystallinity and average roughness capture only part of this evolution. Spray-pyrolysed SnO x thin films were investigated before and after annealing at 450 °C for 5 h using X-ray diffraction (XRD), atomic force microscopy (AFM), real-space autocorrelation and power spectral density (PSD). The predominantly diffuse XRD response of the as-deposited film evolved into resolved maxima superimposed on a broad background, indicating increased reciprocal-space selectivity and finite-range ordering within a partly disordered scattering-density field. The root-mean-square height increased from 15.864 to 40.305 nm and the maximum height range from 127.670 to 318.050 nm, while kurtosis decreased from 4.407 to 3.106. The AFM correlation length increased from approximately 0.26 to 0.32 μm. Consistently, the spectral centroid decreased from 1.59 to 1.24 μm –1 and the fraction of normalised spectral power below 1 μm –1 increased from 0.554 to 0.680. These results show a concurrent multiscale transformation combining finite-range atomic ordering, vertical amplification and lateral surface coarsening. Integrating diffraction, real-space correlation and spatial-frequency analysis provides a more complete physical description than scalar roughness or binary structural classifications alone.