Many-body aspects of the optical spectra of bulk and low-dimensional doped semiconductors

Abstract

We discuss the many-body aspects of the optical spectra of doped semiconductors, for both bulk and low-dimensional systems. At low doping concentrations the spectra are dominated by excitonic effects, consistent with the single-particle band structure of these systems. With increasing doping atomic excitons lose their identity and eventually unbind, while the spectral weight moves continuously to the Fermi level. Depending on the electron-mass to hole-mass ratio, the spectra display a broadened singularity as well as Auger-like indirect transitions. We calculate the onset of absorption in this regime to second order in the effective interaction. We stress that a calculation of the spectra near the Fermi level is outside the scope of conventional perturbation theory and we comment on the validity of various previously proposed calculational schemes. We conclude by discussing the problem in terms of Fadeev-like equations, which describe up to three-particle correlations exactly.