Drift-mobility measurements and mobility edges in disordered silicons

Published electron and hole drift-mobility measurements in hydrogenated amorphous silicon (a-Si:H), amorphous silicon alloys (a-SiGe:H and a-SiC:H), and microcrystalline silicon (muc-Si:H) are analysed in terms of the exponential bandtail trapping model. A three-parameter model was employed using an exponential bandtail width DeltaE, the band mobility mu0, and the attempt-to-escape frequency nu. Low-temperature measurements indicate a value around mu0 = 1 cm2 V-1 s-1 for both the conduction and valence bands over the entire range of materials. High temperature-measurements for electrons in a-Si:H suggest a larger value of 7 cm2 V-1 s-1. These properties and those of the frequency nu are discussed as possible attributes of a mobility edge.