On the temperature dependence of a.c. conduction in chalcogenide glasses

Although it is now fairly well established that defects in amorphous chalcogenide semiconductors are normally charged (D+ and D−), it is demonstrated that neutral defects D0, generated by the reverse reaction of 2D0 → D+ + D−, can be important for a.c. conduction. The contribution to [sgrave](ω, T) from correlated barrier hopping of single polarons exceeds that of bipolarons at high temperatures (T). An expression for [sgrave](ω, T), which includes terms due to electrons hopping between D0 and D+ and holes between D0 and D−, as well as two electrons hopping between D− and D+ (bipolarons), is able to account for all the features observed in chalcogenide glass semiconductors. Effects on the a.c. conductivity of the addition of impurities to chalcogenides are also discussed in terms of the present model. The energy levels and densities of charged defects are estimated for given materials, and these are supported by values deduced from photoluminescence, drift mobility and d.c. conductivity studies.