Tuned ionic mobility by ultrafast laser pulses in black silicon

Highly nonequilibrium conditions in femtosecond-laser excited solids cause a variety of ultrafast phenomena that are not accessible by thermal conditions, such as subpicosecond solid-to-liquid or solid-to-solid phase transitions. In recent years, the microscopic pathways of various laser-induced crystal rearrangements could be identified and led to novel applications and/or improvements in optoelectronics, photonics, and nanotechnology. However, it remains unclear what effect a femtosecond-laser excitation has on ionic impurities within an altered crystal environment, in particular on the atomic mobility. Here, we performed ab initio molecular dynamics simulations on laser-excited black silicon, a promising material for high-efficiency solar cells, using the code CHIVES. By computing time-dependent Bragg peak intensities for doping densities of 0.16% and 2.31% we could identify the overall nonthermal weakening of the crystal environment with increasing impurity density. The analysis of Si-S bond angles and lengths after different excitation densities, as well as computing interatomic forces, allowed one to identify a change in ion mobility with increasing impurity density and excitation strength. Our results indicate the importance of impurity concentrations for ionic mobility in laser-excited black silicon and could give significant insight for semiconductor device optimization and materials science advancement.