Thermopower of transmutation-doped Ge:Ga in the region for hopping conductivity

Abstract

The low-temperature thermopower of transmutation-doped Ge:Ga is investigated experimentally and theoretically. The large values of the thermopower observed in the region for ɛ 1 conduction and its sharp drop upon the transition to conduction between impurities are interpreted as manifestations of the phonon drag of free holes and its suppression in the region for hopping transport. The positive sign of the thermopower and its magnitude in the hopping-conduction saturation region can be explained theoretically under the assumption that the classical ɛ 2 conduction channel, which is not manifested explicitly in the electrical conductivity, makes a contribution to the thermopower in the narrow temperature range associated with the transition from ɛ 1 conduction to hopping conduction. After the transition to variable-range hopping (T⩽2 K), the thermopower decreases sharply and takes anomalous, vanishingly small values. They can be explained within the standard theory of hopping thermopower only under the condition that the contribution caused by the asymmetry of the density of states of the impurity band in the vicinity of the Fermi level and the correlation contribution are compensated.