Far-Infrared Magneto-Spectroscopy of Shallow Impurities and Excitons in Germanium by Using Lasers

Abstract

Optical transitions between excited states of impurities in germanium, Sb or As, in a magnetic field have been investigated using HCN, DCN and H20 lasers. The energy levels of states with even parity and M= +1, i.e., 3D+l, (113) and (115), are determined experimentally and are compared with the values obtained by the variational calculation. In pure germanium under photo-excitation, the Zeeman effect of the far-infrared absorption of an indirect exciton has been. observed. An attempt is presented to interpret the origin of the strong absorption peaks of the exciton on the basis of the adiabatic approximation for the Coulomb potential. The present work is concerned with optical transitions between the excited states of group V impurities in germanium and with transitions of an exciton from its ground state to an excited state in undoped germanium. Both the impurity and exciton in germanium have a series of hydrogen-like states; the optical transitions occur in the far-infrared region. The s- and d-like excited states can be observed only by the transitions starting from some excited states. On excitons, we have little information on their excited states, though the photo-luminescence experiments reveal the nature of the ground state. Since the densities of excited impurities and of excitons are rather small, certain intense light sources are required for the spectroscopic observation of these weak absorption. Such light sources are provided by backward tubes and far-infrared lasers.