Percolation-controlled conduction in chalcogenide glass semiconductors

The applicability of effective-medium percolation theory (EMPT) to the composition dependence of the electrical conductivity is discussed for binary and multicomponent chalcogenide glasses. Calculations assuming a random mixture of atoms, which are applied to explain the experiment for As-Te films by Ast, do not agree with experiments on all the systems considered. By assuming a strong chemical ordering which forms microscopic molecular species, the conductivity over the measured compositional range can be well described with EMPT in the As-Se, As-S, and ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$-${\mathrm{Sb}}_2$ ${\mathrm{Se}}_3$ systems, except for Te-Se, ${\mathrm{As}}_2$ ${\mathrm{Se}}_3$-${\mathrm{As}}_2$ ${\mathrm{Te}}_3$, and ${\mathrm{As}}_2$ ${\mathrm{S}}_{2.63}$-Te which contain the isomorphous Te, Se, and S atoms. Chalcogenide glasses can be classified roughly into three groups: a "random mixture of atoms" type, a "strong chemical ordering" type, and an "isomorphous-atom" type. It is suggested that EMPT has broad application in structural studies of chalcogenide glasses.