Green's Function Approach to Nonequilibrium Charge Carriers in Direct Gap Semiconductors Generated by Short Light Pulses

Abstract

In the present paper the Green's function approach to nonequilibrium quasi-free charge carriers in highly excited semiconductors is applied. I n doing this a system of kinetic equations for generalized Wigner distributions of renormalized quasi-electrons and holes is obtained, that takes into account (additionally to well-known published results) the interaction of the longitudinal optical phonons and the intraband interaction of the radiation field with the electron-hole plasma. Thus, utilizing the corresponding field equations and the suitable boundary conditions, relaxation processes in the semiconductor after excitation by a short laser pulse and a laser probe beam experiment may be described.I n der Arbeit wird die Methode der Green'schen Funktionen fur das Nichtgleichgewicht auf das Verhalten quasifreier Ladungstrager in hochangeregten direkten Halbleitern im nichtstationiiren Bereich angewendet. Dabei wird ein System von kinetischen Gleichungen fur verallgemeinerte Wigner-Verteilungen renormierter Quasi-Elektronen und Locher erhalten, welches gegenuber den aus der Literatur bekannten auch die Wechselwirkung mit longitudinalen optischen Phononen und die Intraband-Wechselwirkung mit dem Strahlungsfeld einschliel3t. Zusammen mit den entspreclienden Feldgleichungen und unter geeigneten Randbedingungen konnen somit Relaxationsprozesse im Halbleiter nach Anregung durch einen kurzen Laserimpuls und deren Austesten mittels eines Teststrahlexperimentes beschrieben werden.

Screening and exchange in the theory of the femtosecond kinetics of the electron-hole plasma

“…Three main theoretical approaches prevail in the literature describing the EHP dynamics in semiconductors. In a first approach, investigations start from the very general equations of the non-equilibrium quantum statistical mechanics (Balescu-Resibois formalism [25], time-dependent Green's functions [22][23][24][26][27][28][29]). This ambitious approach may in principle describe both the coherent and incoherent interaction of electrons with subpicosecond optical pulses.…”

An approach to the many-exciton system

“…Nowadays using short-pulse technique one may generate a high electron-hole density in a semiconductor. At very high excitation or on a very short time scale the carrier distributions turn out to differ radically from the thermal distributions and the theoretical tool that seems most adapted to these circumstances is the non-equilibrium Green function method first developed by Keldysh (1965) and then widely used, for example, by Haug (1985), Henneberger (1986), May (1986), Schafer and Treusch (1986) and Glaeske and Schubert (1988). At moderate excitation below the Mott transition the electron-hole attractive Coulomb potential is still strong enough to form bound pair states called excitons.…”