Role of electronic-excitation effects in the melting and ablation of laser-excited silicon

We present a model of laser–solid interactions in silicon based on a previously-developed interatomic potential for silicon where the parameters describing the interactions depend on the temperature of the electronic subsystem Te, which is directly related to the density of electron–hole pairs and hence the number of broken covalent bonds. For 25 fs pulses, a wide range of fluence values are simulated resulting in heterogeneous melting, homogeneous melting, and ablation. The results presented here demonstrate that phase transitions can usually be described by ordinary thermal processes even when the electronic temperature Te is much greater than the lattice temperature TL during the transition. However, the evolution of the system and details of the phase transitions depend strongly on Te and corresponding density of broken bonds. Homogeneous melting appears to be an ordinary thermal phase transition, but occurring over very fast time scales (1–5 ps) before TL reaches the ordinary melting temperature of silicon. For high enough laser fluence, homogeneous melting is followed by rapid expansion of the superheated liquid and ablation. Rapid expansion of the superheated liquid occurs partly due to high pressures generated by a high density of broken bonds. As a result of rapid expansion of the superheated liquid, the system is readily driven into the liquid–vapor coexistence region which initiates phase explosion. These results strongly indicate that phase explosion, generally thought of as an ordinary thermal process, can occur even under strong nonequilibrium conditions when Te ≫ TL. Thus, the results both for melting and ablation processes suggest that for many cases there is no clear separation between thermal and nonthermal processes. Instead, ordinary thermal mechanisms are found to apply when Te ≫ TL, with the high density of broken bonds playing an important role in the detailed evolution of the system.