Phonon sidebands of excitons bound to isoelectronic impurities in semiconductors.

Abstract

The configuration coordinate (CC) and momentum conservation (MC) models have been widely used to explain the phonon sidebands of impurity spectra in semiconductors. In this paper, the distinction between the CC and MC models is discussed. We conclude that the MC model only applies to shallow Coulombic impurities; in other cases, such as isoelectronic traps, the CC model is more appropriate. We show that the Huang-Rhys parameters for bulk phonon modes coupling to a bound electron or exciton can be calculated from the bound-state wave function in k space if the phonon-induced intervalley and intravalley electron scattering processes of the pure crystal are known. We study in detail the phonon sidebands of nitrogen-bound excitons in GaP, giving the selection rules for electron-phonon coupling in the CC model, and showing that their strength can be well accounted for by the CC model. The apparently anomalous ``X'' peak of the LO-phonon sideband in GaP:N is shown to be associated with intervalley scattering in the conduction band. The MC model, which has been used in an attempt to explain the phonon sidebands of GaP:N in some previous work, is shown to be inapplicable to this case.