Transit-time measurements of charge carriers in disordered silicons: Amorphous, microcrystalline and porous

We summarize published hole transit-time measurements for hydrogenated amorphous silicon, microcrystalline silicon, and light-emitting nanoporous silicon in terms of drift mobilities and dispersion parameters. For amorphous and microcrystalline silicon, the anomalously dispersive measurements are broadly consistent with multiple-trapping by bandtail traps with an exponential distribution of energy depths. One unexplained result has been that the trap emission prefactor frequency is about 1000 times smaller in microcrystalline silicon than in amorphous silicon. We present a model incorporating both detailed-balance effects and a previously proposed Meyer-Neldel variation of with trap-depth; the model accounts for the factor 1000. We discuss general trap distributions incorporating variations of both trap depth and prefactor frequency; a model for which dispersion is due entirely to prefactor variation accounts for measurements on nanoporous silicon.