Transport in a-Si:H and Its Alloys

Abstract

Basic information on the carrier transport in a-Si:H is obtained from the temperature dependence of the conductivity. In Fig. 3.1 we show an Arrhenius plot of the dark and photoconductivity in undoped a-Si:H. In the region 250 K < T < 350 K, the dark conductivity shows activated behavior, given by σ = σ₀ exp(-E _a/kT). The activation energy, E _a, can be taken as an approximate measure of the energy difference between the Fermi level, E _F, and the band edge, E _c. An accurate determination of E _c — E _F is more complicated, since the Fermi energy, the mobility edge and the band-gap are also temperature-dependent and contribute to the conductance—temperature dependence [2, 119]. Further complications arise from the fact that the activation process involves a broadened distribution of states [120]-[122] and that multi-phonon processes are involved in the transitions [123]. In undoped material the activation energy is usually slightly less than half of the band gap, and the material is naturally n-type. In defect-rich material the activated behavior levels off towards lower temperatures, and gives rise to a Tdependence, given by σ(T) = σ₀ exp[(- A/T)^1/4], which is usually attributed to variable-range-hopping.