Thermal and nonthermal melting of silicon under femtosecond x-ray irradiation

Abstract

As is known from visible-light experiments, silicon under femtosecond pulse irradiation can undergo so-called "nonthermal melting" if the density of electrons excited from the valence to the conduction band overcomes a certain critical value. Such ultrafast transition is induced by strong changes in the atomic potential energy surface, which trigger atomic relocation. However, heating of a material due to the electron-phonon coupling can also lead to a phase transition, called "thermal melting." This thermal melting can occur even if the excited-electron density is much too low to induce nonthermal effects. To study phase transitions, and in particular, the interplay of the thermal and nonthermal effects in silicon under a femtosecond x-ray irradiation, we propose their unified treatment by going beyond the Born-Oppenheimer approximation within our hybrid model based on tight-binding molecular dynamics. With our extended model we identify damage thresholds for various phase transitions in irradiated silicon. We show that electron-phonon coupling triggers the phase transition of solid silicon into a low-density liquid phase if the energy deposited into the sample is above ∼0.65 eV per atom. For the deposited doses of over ∼0.9 eV per atom, solid silicon undergoes a phase transition into high-density liquid phase triggered by an interplay between electron-phonon heating and nonthermal effects. These thresholds are much lower than those predicted with the Born-Oppenheimer approximation (∼2.1 eV/atom), and indicate a significant contribution of electron-phonon coupling to the relaxation of the laser-excited silicon. We expect that these results will stimulate dedicated experimental studies, unveiling in detail various paths of structural relaxation within laser-irradiated silicon.

Simulating electronically driven structural changes in silicon with two-temperature molecular dynamics

“…[2,13]. The steady and rapid rise of the RMSD after the structure has melted is indicative of a liquidlike phase rather than an amorphous solid, as previously established to be true [2,13,14].…”

“…Based on the shape of the w function that we employed, must satisfy the constraint (T e ) 1 2 dE e dT e (T e ), per Eq. (14). It should also be small enough that C e 2 [and thus W = 0 per Eq.…”

“…These studies do predict bond softening at high excitations but they predict melting to occur at higher excitations than expected based on experiment. More recent methods [13,14] have avoided these two assumptions and, consequently, predicted melting at lower excitations. However, it is unclear to what ex-tent this is attributable to the more realistic electronic distributions versus the nonadiabatic effects.…”

“…Silicon is a widely studied material in the context of strongly driven phase transitions, both experimentally [1][2][3][4][5][6] and theoretically [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29], where it is found to melt on a subpicosecond timescale at high excitations. Most theoretical studies of nonthermal melting in silicon have employed ab initio simulation methods.…”

Periodic surface structure creation by UV femtosecond pulses on silicon

“…Femtosecond laser nano-and microstructuring has been investigated for the past decade [5][6][7][8][9]. The formation of laser-induced periodic surface structures (LIPSS) on silicon and on other materials is extensively investigated both theoretically and experimentally [10][11][12][13][14][15][16][17]. Irradiation of the surface with multiple linearly polarized femtosecond laser pulses can lead to the formation of periodic surface structures in two different ways; the high spatial frequency and the low spatial frequency LIPSS.…”