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

Abstract

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, μe and μb 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 cann...