LAUSR

Abstract We report direct evidence for the characteristic behavior of single and bipolarons in chalcogenide glasses by using broad band impedance spectroscopy. The measured ac conductivity, σ(f, T) of Te based glasses exhibit distinct behavior from those reported in amorphous tetrahedral semiconductors depending on the temperature and frequency. At low temperatures, the measured conductivity is exclusively due to the bipolarons between two charged defect centers. On the other hand, at high temperatures we find in addition to bipolaronic contribution, the single polaron hopping between a charged and neutral defect centers. The behavior of both types of charge carrier (single and bipolaron) shows the distinct relaxation characteristics within the measured frequency window. Remarkably, we observe existence of neutral defect centers with large concentration in tellurium-based chalcogenide glass under dark (normal) conditions revealed by the electron spin resonance (ESR) spectroscopy, which is an essential component for the single polaron hopping process. The estimated neutral defect centers from the dc conductivity measurements are in agreement with those derived from the ESR results. Further, we found that the effective correlation energy (Ueff) becomes less negative in glasses which exhibit single polaron hopping than the glasses found to exhibit typical bipolaronic conduction. The present work unequivocally demonstrates the characteristic behavior of single and bipolarons in chalcogenide glasses and its interdependence on thermal history of the glass samples.