We propose an experimental procedure for testing the Einstein relation for carrier drift and diffusion in semiconductors exhibiting non-Gaussian or dispersive transport. We present corresponding hole time-of-flight and steady-state photocarrier grating measurements in hydrogenated amorphous silicon (a-Si:H). For a range of mobilities 10 25 10 22 cm 2 ͞V s we find that our estimates of hole diffusion are approximately twice as large as predicted by the Einstein relation and the mobility measurements. We consider the deviation to represent an upper bound to any true failure of the Einstein relation for hole transport in a-Si:H.
The mobility-lifetime products of electrons and holes [(μτ)e and (μτ)h] in undoped hydrogenated amorphous silicon samples have been studied by photoconductivity and ambipolar diffusion length measurements. The density of dangling bonds Nd in the samples is changed over a range of 3×1015–2×1018 cm−3 by annealing at high temperatures. Nd and the Urbach tail slope Eov have been determined by the constant photocurrent method. In addition, the optical gap, the activation energy of dark conductivity, and the exponent governing the intensity dependence of σpc have been measured. The results show that there is a correlation between Nd and Eov which is consistent with equilibrium theory. (μτ)e and (μτ)h change in quite different ways as Nd increases, namely, (μτ)e decreases as a linear function of the inverse of Nd. However, (μτ)h remains almost constant when Nd≤5×1016 cm−3, then decreases fast for higher Nd. The asymmetric dependence of transport properties of electrons and holes on Nd suggests that for electrons recombination through dangling bond states is dominant; but, for holes, recombination mainly proceeds through deep band tail states, especially when Nd is relatively low.
To analyse the influence of the grain boundaries (gb) on the transport of carriers in hydrogenated microcrystalline silicon (μC-Si:H) the ambipolar diffusion length (LLMB) was measured by SSPG. In addition, the films were characterised by photo-conductivity, dark conductivity activation energy, Urbach energy (determined by CPM), hydrogen effusion, Raman spectroscopy, X-ray scattering and optical transmission.The sample series was prepared by PECVD of SiH4 diluted with increasing H2 content. Taking the structural information by Raman spectra and X-ray into account, we explain our optical and activation energy measurements within a three-phase-model (amorphous phase, crystalline phase, gb) and a Fermi level pinning in μc-Si:H.
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