Abstract
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. Athree-parameter model was employed using an exponential bandtail widthΔE, the band mobilityμ0, and the attempt-to-escapefrequency ν. Low-temperature measurements indicate a value aroundμ0 = 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 of7 cm2 V−1 s−1. These properties and those of the frequencyν are discussed as possible attributes of a mobility edge.
Highlights
Over the last three decades, experimental characterizations of electrical transport in disordered semiconductors and insulators have accreted substantially
The main result is that the electron and hole band mobilities μ0 in a wide range of disordered silicon materials are reasonably consistent with a universal value around 100.5 cm2 V−1 s−1; while this result has been proposed or suspected before, we find that this value describes a surprisingly large range of materials: amorphous silicon (a-Si):H of several varieties, a-Si:H with Ge (a-SiGe):H, and at least some samples of μc-Si:H
We were unable to obtain satisfactory three-parameter fits to the a-SiGe:H alloy sample studied by Longeaud and Vanderhagen (1990); as for the work of Liu et al the electron drift mobility itself was reasonably consistent with the other work on Ge alloying that we have summarized in figure 1
Summary
Over the last three decades, experimental characterizations of electrical transport in disordered semiconductors and insulators have accreted substantially. For disordered semiconductors the time dependence of carrier drift is often anomalous, in the sense that the displacement x(t) is not proportional to the time interval since the carriers were generated This type of transport could only be analysed properly once Scher, Lax, and Montroll introduced the ideas of ‘dispersive transport’ in the mid-1970s (see Scher et al 1991). This paper is a re-examination of the trapping parameters that emerge from many of the previous measurements of electron and hole drift mobilities in disordered silicons. The main result is that the electron and hole band mobilities μ0 in a wide range of disordered silicon materials are reasonably consistent with a universal value around 100.5 cm V−1 s−1; while this result has been proposed or suspected before, we find that this value describes a surprisingly large range of materials: a-Si:H of several varieties, a-SiGe:H, and at least some samples of μc-Si:H. We discuss some of the possibilities in the concluding section of this paper
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