Raman scattering from electronic excitations in Ge

Abstract

Polarized Raman spectra of pure, gallium-doped, and arsenic-doped germanium have been measured as a function of impurity concentration and temperature, using the output of a 2.1-μm laser to which germanium is transparent. The impurity concentration ranged from 6.3×${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$ (semiconducting regime) to 5.5×${10}^{17}$ ${\mathrm{cm}}^{-3\mathrm{}}$ (metallic regime). The acceptor spectrum has been resolved into two lines whose polarization characteristics have been measured and deduced group theoretically. The donor spectrum consists, at low impurity concentration, of a single valley-orbit Raman line whose polarization selection rules have been found to be, in good agreement with group theory, identical to those of the zone-center optic phonon of the host crystal. The effects of wave-function overlap at higher impurity concentration on the electronic states have been systematically studied. In particular, the donor spectra have been studied as the crystal undergoes the Mott transition. Polarization studies of the light scattered from metallic samples show that intervalley fluctuations are responsible for the observed single-particle excitations.