The metal-semiconductor transition in amorphous Si1-x Crx films: phenomenological model for the metallic region

A detailed phenomenological re-analysis of previously published conductivity data, (T, x), is presented. It was shown in [1] that the cusp-like low-temperature contribution can be described by wherep=0.19±0.03. Starting from this result, two furtherT dependent contributions are separated: The high-temperature region is dominated by a positive contribution _ht (T, x), which is approximately independent ofx, nearly linear inT above 100 K and nearly quadratic inT below 30 K. ForT 4 K, there is a small deviation, increasing withT, from the superposition of the above mechanisms. The relation between, being negative, and theT independent part,, exhibits a singularity, where and=0.68±0.05 –(p–0.19). This singularity should be related to the metal-semiconductor transition, taking place atT_c0.14. The quantity should be interpreted as minimum metallic conductivity. The limitingT dependences asxx_c+0 agrees quantitatively with that one obtained previously for the activated region,xx_c–0. Extrapolation of the phenomenological model obtained leads to the hypothesis that the interplay of and _ht could be the main origin of the temperature coefficient changing its sign in the Mooijregion, at _tc=0. The model enables several trend predictions concerning the value of _tc=0.