Substrate-mediated laser annealing for depth-dependent structure regulation

Precision control of material properties in-depth in thin films is increasingly critical for advanced optoelectronic devices. Laser annealing offers the spatial selectivity required for such modification. However, achieving depth-selective modification remains challenging due to the incomplete understanding of transient heat transfer mechanisms and the narrow processing window. Herein, we elucidate a substrate-mediated “bottom-up” thermal pathway using a prototype of indium tin oxide (ITO) on silicon under 355 nm nanosecond laser irradiation. Unlike direct optical absorption, the substrate (silicon) acts as a localized heating source, establishing a sharp vertical thermal gradient in the thin film above that drives phase evolution from the interface upwards. Systematic optimization of the laser fluence increased the electrical conductivity from 1.480 × 10 3 S/cm in the as-deposited film to a peak value of 3.035 × 10 3 S/cm, corresponding to an enhancement of more than twofold. Furthermore, comprehensive characterization of the film evolution revealed four distinct microstructural and compositional regimes. These findings demonstrate a precise thin film modification technology for material systems involving a transparent layer on a light-absorbing substrate, which provides a valuable depth-selective structural engineering in development of advanced optoelectronic systems.