The properties of crystalline silicon under laser irradiation

The initial effect of laser irradiation is the excitation of electron-hole pairs. It has not, however, been established whether the excited electrons transfer their energy to the lattice rapidly (leading to thermal annealing), or retain it for long times (leading to annealing caused by the softening of the lattice resulting from the presence of the excited electrons). In this paper, we use parameter-free, self-consistent calculations to investigate the consequences of large concentrations of excited electrons in the absence of energy transfer to the lattice. We assume that the excited electrons reach thermal equilibrium among themselves. We find that the valence-band density of states remains essentially unchanged and conclude that the observed large changes in the valence photoemission spectra cannot be attributed to the presence of excited electrons, but are more likely due to surface reconstruction. We also find that the lattice constracts for all but extremely high concentrations of excited electrons. This results supports Gauster's analysis of his experimental data, according to which electronic contraction occurs at low levels of excitation, whereas, at higher levels of excitation, the observed dilation is due to transfer of energy from the electronic system to the lattice. Finally, we discuss the relevance of our results to more recent suggestions about the role of the excited electrons during laser annealing.