Electron and hole transport in the band-tail states of amorphous silicon at low temperatures

Abstract

In this paper a detailed investigation is presented of the new low-temperature transport regime in amorphous silicon (a-Si) recently reported by the authors. Electron and hole drift mobility results on reverse-biased Cr-i-n⁺ barriers and p⁺-i-n⁺ junctions were extended to 10 K. Activation energies below 30 K are found to lie between 1 and 2meV. Both carrier mobilities, μ_e and μ_b are systematically dependent on the excitation intensity and this aspect has been further investigated. The discussion of these results leads to the conclusion that the observed electron hopping transport takes place near the bottom of the tail states where the thermalization of excess electrons has become sufficiently slow for well defined transit signals; transport is constrained to hopping paths in a region of sufficiently strong wavefunction overlap between sites. These conclusions are tested by model calculations of thermalization and hopping transport in two tail-state distributions. An exponential distribution cannot account for the experimental results, but a distribution partly based on field-effect measurements in a-Si leads to a steep thermalization limit, an essential feature for the interpretation of both the low-temperature transient mobility and the multitrapping transport above 150 K. Calculated μ_e values are in quantitative agreement with experiment. The above distribution is also used in an analysis of steady electron conduction between 400 and 30 K. The δ (ε) distribution shows a peak in the extended states at all temperatures down to 150 K. The tail state contribution to δ increases rapidly with decreasing T and at 295 K is about 30%. Below 100 K, electron conduction in a-Si takes place entirely in the tail states.