Conductivity and thermopower near the band edge in non-polaronic amorphous semiconductors: Theory and analysis

Abstract

We have been seeking a general formulation of transport in amorphous semiconductors, which takes into account contributions of the entire energy range extending from localized tail states to delocalized states above the mobility edge. Our description in terms of Kubo-Greenwood like formulae provides a natural explanation of the experimentally observed ‘discrepancy’ between ‘activation energies’ as deduced from the thermopower and those deduced from the conductivity. Moreover, there is a relation between thermopower and conductivity which allows one to separate unambiguously the statistical shift of the Fermi level from the temperature dependence of the average energy at which the carrier transport takes place. Finally the differential conductivity σ(ε) may be obtained by Laplace transformation from the temperature dependence of the ‘prefactor’ of the conductivity. As an application of our analysis we demonstrate for the case of a series of doped amorphous silicon samples that a clear distinction between doping induced changes of the transport mechanism and doping induced Fermi level shifts can be made.