Hall effect and impurity conduction in substitutionally doped amorphous silicon

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Abstract
Hall mobility and conductivity measurements have been made as a function of temperature on a series of n- and p-type a—Si specimens prepared by the glow discharge technique and doped with phosphorus or boron atoms. The Hall effect exhibits an interesting double reversal in sign : n-type samples show a Hall potential in the direction normally expected for holes and the opposite applies to p-type specimens. In lightly doped n-type samples, the observed Hall mobility μmH has a magnitude of about 0.1 cm2V−1see−1, essentially independent of temperature; this is in agreement with theoretical predictions for transport in the extended states. On increasing the donor density, μmH begins to show a temperature dependence which varies systematically with the doping level. The conductivity and Hall data for n-type specimens have been analysed in terms of a model with two parallel conduction paths, one in the extended electron states and the other through the localized impurity levels where transport takes place by hopping. The results show that impurity conduction depends strongly on the donor density; it becomes the predominant transport mechanism, even at room temperature, for specimens containing 5 × 1018 cm−3 donors or more. The analysis also leads to a value of α−1 ≃ 15 Å for the spatial extension of the localized state wavefunction.