Nonequilibrium Phase Transition in Highly Excited Semiconductors

Abstract

In the last ten years the properties of highly excited semiconductors have been studied thoroughly [1,2,3,4]. If the frequency of the exciting laser light is sufficiently high, electrons from the valence band are lifted into the conduction band. Thus, in the process of optical excitation one. generates electron-hole pairs (e-h pairs) i.e. electrons in the conduction band and holes or missing electrons in the valence band. In the low density regime electrons and holes form bound pair states, called excitons, due to their attractive Coulomb interaction. In an intermediate density regime one observes besides free carriers and excitons also higher bound states such as trions (e-e-h or e-h-h), biexcitons and even multiexcitons. In the high density regime bound states are no longer stable, because the attractive interaction between an electron and a hole is screened by the other electronic excitations. In this limit a plasma is formed.