Correlated topographic and structural modification on Si surface during multi-shot femtosecond laser exposures: Si nanopolymorphs as potential local structural nanomarkers

High-pressure Si-XII and Si-III nanocrystalline polymorphs, as well as amorphous Si phase, appear consequently during multi-shot femtosecond-laser exposure of crystalline Si wafer surface above its spallation threshold along with permanently developing quasi-regular surface texture (ripples, microcones), residual hydrostatic stresses and subsurface damage, which are characterized by scanning and transmission electron microscopy, as well as by Raman micro-spectroscopy. The consequent yields of these structural Si phases indicate not only their spatially different appearance, but also potentially enable to track nanoscale, transient laser-induced high-pressure, high-temperature physical processes – local variation of ablation mechanism and rate, pressurization/pressure release, melting/resolidification, amorphization, annealing – versus cumulative laser exposure and the related development of the surface topography.