Density-matrix approach to nonequilibrium free-carrier screening in semiconductors

We address the problem of how to derive quantum kinetic transport equations for electrically or optically excited semiconductor systems, where the electron dynamics is dominated by Coulombic interparticle collisions. In particular for the density-matrix approach we show how, in contrast to the frequently used approximation of a (statically) prescreened Hamiltonian, to obtain consistently the screening of the Coulomb interactions. In this connection we present a critical comparison between the generalized Boltzmann equations as obtained within the frameworks of density matrices and nonequilibrium Green's functions. Particular emphasis is given to a general formulation of the problem that allows an extension of our approach to a wide range of different systems.