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 (μc-Si:H) are analysed in terms of the exponential bandtail trapping model. A three-parameter model was employed using an exponential bandtail width ΔE, the band mobility μ0, and the attempt-toescape frequency v. Low-temperature measurements indicate a value around μ0 = 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 v are discussed as possible attributes of a mobility edge.