Evidence for insulator–metal transition in amorphous chalcogenide Se–Ge–Te films

Abstract

The temperature dependence of the dc conductivity and thermoelectric power was determined for five different amorphous chalcogenide Se–Ge–Te films, with Ge = 3.0–22 at.%, Se = 0–97 at.% and Te = 0–97 at.%. The films were prepared by thermal evaporation of GeSe₄, GeTe₄ and GeSe₂Te₂ quenched bulk materials. Values of the activation energy calculated from the temperature dependence of both electrical conductivity and thermoelectric power showed a decrease with increasing Ge content in the Se–Ge films as well as with replacement of Te for Se in the Se–Ge–Te films. The results showed an Anderson transition, with the conductivity showing insulating behaviour on the Ge–Se side to metallic behaviour at the binary composition Ge–Te. The radius of localization was obtained for the different compositions investigated. The wave function associated with the charge carriers at the composition Ge_3.3Te_96.7 is non-localized. A minimum metallic conductivity of 237 ± 5 (Ω cm)⁻¹ was found.