The role of coherence for carrier relaxation in photo-excited semiconductors

The role played by the dynamics of the interband polarization for the correct description of the generation of carriers by a short laser pulse is investigated. The calculations are based on the semiconductor Bloch equations which are solved by means of a generalized Monte Carlo simulation. It turns out that some of the approaches commonly used for a microscopic modelling of the dephasing process fail in describing correctly the effect of carrier-carrier interaction in the low-density limit. By including terms which have the structure of “in-scattering” terms (vertex corrections) for the interband polarization, the experimentally observed features in the carrier dynamics are well reproduced in the whole density range.