Stress‐Driven Nanoprotrusion Formation in Amorphous Silicon via Sub‐Ablation Femtosecond Laser Irradiation

ABSTRACT

Nanoscale control of surface morphology is crucial for emerging technologies such as strain‐engineered optoelectronics, photonic devices, and functional interfaces. However, achieving deterministic nanoscale surface morphology without inducing melting, ablation, or defects remains a major challenge. This work demonstrates a sub‐ablation femtosecond laser strategy that generates nanoscale surface protrusions on amorphous silicon via a stress‐induced solid‐state process. Within the fluence window of 0.03–0.09 J cm ⁻² , which is below the experimentally measured multi‐pulse ablation threshold of silicon, protrusions with heights ranging from 0.37 to 121.1 nm are produced. Raman spectroscopy reveals that laser irradiation induces nanocrystalline silicon with a crystallinity of 32%–45%, generating gigapascal‐scale residual stress within the surface layer. The accumulated stress relaxes through out‐of‐plane displacement, leading to deterministic nanoprotrusion formation rather than oxidation‐ or ablation‐driven surface nanostructure. Low‐temperature annealing further amplifies the structures while preserving the lateral geometry, confirming that the laser‐fabricated structures correspond to a metastable stress configuration whose relaxation drives additional deformation. These results highlight stress‐induced nanostructuring as a promising route for laser nanofabrication, offering a powerful strategy for low‐damage and highly controllable nanoscale manufacturing based on intrinsic material responses rather than material removal.