Theory for the instability of the diamond structure of Si, Ge, and C induced by a dense electron-hole plasma

The effect of a dense electron-hole plasma on the stability of the diamond lattice of the crystalline group-IV elemental semiconductors C, Si, and Ge is examined with use of a tight-binding model. Such a plasma may result, for example, from a short, intense laser pulse. We find that the transverse-acoustic phonons of Si become soft if about 9% of the electrons are excited from the valence band into the conduction band. At higher densities of the electron-hole excitations the cubic symmetry of the diamond lattice is destroyed within less than 100 fs after the creation of the electron-hole plasma. This is much shorter than the time needed for the crystal to melt. The instability of the lattice then leads directly to a very rapid melting of the crystal structure. Our results are in agreement with recent experiments using pulsed lasers to induce disorder in crystalline Si surfaces. We obtain for C and Ge essentially the same theoretical results as for Si.