Photoconductivity studies of the mobility edge in amorphous silicon

Abstract

In this paper the photoconductivity of undoped a-Si specimens, prepared by the glow-discharge technique, is investigated in the weakly absorbed spectral region between 0·45 and 1·1 eV, which is appreciably less than the optical gap of the material (≃ 1·65 eV). The aim of the ‘photoconductive probing’ technique is to determine the threshold energies Δε₁ and Δε₂ for the transitions from localized initial states just below the Fermi level into final extended and localized tail states respectively. The method of analysis whereby Δε₁ and Δε₂ are determined from the spectral dependence of the photocurrent are considered in some detail. It is concluded that the matrix element associated with transitions into extended tail states is three to five times larger than that for transitions into localized tail states. The approach is then applied to investigate the temperature dependence of the threshold energies between 80 and about 400 K. The results lead to unexpectedly large values (9 × 10⁻⁴ to 12 × 10⁻⁴ eV K⁻¹) for the temperature coefficient which describes the combined shift of ε_f and ε_c towards the bottom of the tail states with increasing T. As there is little evidence for an appreciable temperature coefficient associated with ε_f in undoped a-Si, it appears that the position of the mobility edge is strongly temperature dependent. This tentative conclusion is discussed in the light of recently published transport experiments on a-Si which independently point in the same direction. The related problem of the distribution and recombination of photogenerated electrons has also been investigated in the paper.