Theory of electron two-band self-trapping in atomic soft configurations: Hybridization of states, formation of negative-U centers, and anharmonic atomic dynamics

A consistent theory of two-band self-trapping of electrons (holes) in semiconductors is presented, in which negative-U centers in the interband (mobility) gap are formed as singlet electron (hole) pairs self-trapped in atomic soft configurations. Both the configuration softness and hybridization of the bare single-electron state with extended band states are essential for formation of the self-trapped states and negative-U centers. The hybridization significantly changes appropriate soft configurations, introducing additional anharmonic features in the related atomic dynamics. Examples are mobility-gap states and negative-U centers and the related atomic dynamics in glassy semiconductors, the properties of which are discussed.