Far-Infrared Observation of D⁻ Band in n-Type Germanium

Abstract

Far-Infrared absorption spectra of As- or Sb-doped germanium have been investigated at 1.5∼20 K. Spectral shape and its concentration dependence in Ge(As) show good agreement with the theoretical calculation based on a D⁻ band model. Two types of thermally induced absorption are found in Ge(Sb). The first one shows an exponential dependence upon reciprocal absolute temperature and is ascribed to free carrier absorptions. The second one shows a different temperature dependence and is found to vanish under [111] uniaxial stress. These results give a direct evidence of the participation of a D⁻ band in the optical transitions. An absorption peak ascribed to the transition from D_1sD_1s to D⁺D⁻ in donor pairs is also found in the spectra of Ge(As) and Ge(Sb).