The metal-semiconductor transition in amorphous Si_1−x Cr _x films:T ^0.19-contribution to the metallic conductivity

Abstract

A detailed phenomenological re-analysis of previously published conductivity data, σ(T, x), is presented. It is based on the investigation of differences, σ(T, x ₁)−σ(T, x ₂). In this way, the cusp-like low-temperature term is amplified against the other temperature dependent contributions. This term can be described by$\bar \beta (x) \cdot T^p$ wherep=0.19±0.03. It is present, if σ(4.2 K,x) exceeds 260 Ω⁻¹ cm⁻¹, at least up to σ(4.2 K,x)≈1350 Ω⁻¹ cm⁻¹ and forT≲60 K. But it is absent, if σ(4.2 K,x)≲180 Ω⁻¹ cm⁻¹. The disappearance of this contribution should be related to the metal-semiconductor transition, taking place atx _c ≈0.14. On the other hand, the presence of a term proportional toT ^1/2, as predicted by Altshuler and Aronov, seems unlikely.

It is argued that the term$\bar \beta \cdot T^p$ should be related to the interplay of electron-electron interaction and disorder. The comparison with data from the literature shows that this contribution might also be present in heavily doped crystalline semiconductors.