THERMOPOWER OF TRANSMUTATION-DOPED GE-GA IN THE REGION FOR HOPPING CONDUCTIVITY

The low-temperature thermopower of transmutation-doped Ge:Ga is invest igated experimentally and theoretically. The large values of the therm opower observed in the region for epsilon(1) conduction and its sharp drop upon the transition to conduction between impurities are interpre ted as manifestations of the phonon drag of free holes and its suppres sion in the region for hopping transport. The positive sign of the the rmopower and its magnitude in the hopping-conduction saturation region can be explained theoretically under the assumption that the classica l epsilon(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 ep silon(1) conduction to hopping conduction. After the transition to var iable-range hopping (T less than or equal to 2 K), the thermopower dec reases sharply and takes anomalous, vanishingly snail values. They can be explained within the standard theory of hopping thermopower only u nder the condition that the contribution caused by the asymmetry of th e density of states of the impurity band in the vicinity of the Fermi level and the correlation contribution ape compensated. (C) 1997 Ameri can Institute of Physics. [S1063-7826(97)00610-8].