Phonon spectrum of a model of electronically excited silicon

With a view to clarifying the process proposed in the plasma-annealing hypothesis for laser-induced recrystallization, we have performed a simple calculation of phonon frequencies in electronically excited silicon. We make assumptions most favorable to the plasma-annealing hypothesis, i.e., we assume that the laser energy remains in the electronic system. We use a one-parameter interpolation of the electronic dielectric constant between the bond charge and the free-electron models in a computation of the phonon spectrum. The calculation yields a softening of the [111] zone-boundary transverse-acoustic phonons. Typically, such a softening would be thought to lead to a structural phase transition. The excited electron densities needed for this softening are in the range (6-10)×${10}^{21}$ ${\mathrm{cm}}^{-3\mathrm{}}$ which seems unreasonably large to us. Thus, on two counts, we have grounds for rejecting the plasma-annealing hypothesis.