Dynamics of New Phase Formation in Silicon during Femtosecond Laser Ablation

Abstract

Experimental results, obtained through micro-Raman spectroscopy and transmission electron microscopy, along with numerical simulations, demonstrate that under intense (10¹³–10¹⁴ W/cm²) femtosecond (∼100 fs) laser impact on the (111)Si surface, new polymorphic phases Si-III and Si-XII are formed both on the surface and in the bulk of the material. These phases are localized in lattice defects and at the periphery of the ablation crater. This localization is attributed to the multistage nature of laser-induced phase transitions in silicon. Specifically, these transitions are initiated by a shock wave, resulting in a cascading transformation process on subnanosecond time scales: Si-I → Si-II → Si-III/Si-XII. The phase transition Si-I → Si-II occurs at the front of the shock wave, whereas at the rear of the shock wave a field of dynamic stresses arises, facilitating the phase transition from Si-II to Si-III/Si-XII. On sub-microsecond time scales, most of the new phases disappear as the material relaxes back to its initial state.