ELECTRICAL-PROPERTIES OF NICKEL-DOPED ARSENIC TRISULFIDE

Results of temperature and frequency dependent a.c. conductivity 0 pur e an nickel-doped a-As2S3 are reported. The a.c. conductivity of pure As2S3 obeys a well-known relationship: sigma(ac) is-proportional-to om ega(s). Frequency exponent s is found to decrease with increasing temp erature. Correlated barrier hopping (CBH) model successfully explains the entire behaviour of a.c. conductivity with respect to temperature and frequency for pure As2S3. But a different behaviour of a.c. conduc tivity has been observed for the nickel doped As2S3. At higher tempera tures, distinct peaks have been observed in the plots of temperature d ependence of a.c. conductivity. The frequency dependent behaviour of a .c. conductivity (sigma(ac) is-proportional-to omega(s)) for nickel-do ped As2S3 is similar to pure AS2S3 at lower temperatures. But at highe r temperatures, In sigma(ac) vs In f curves have been found to deviate from linearity. Such a behaviour has been explained by assuming that nickel doping gives rise to some neutral defect states (D0') in the ba nd gap. Single polaron hopping is expected to occur between these D0' and D+ states. Furthermore, all D+, D0' pairs are assumed to be equiva lent, having a fixed relaxation time at a given temperature. The contr ibution of this relaxation to a.c. conductivity is found to be respons ible for the observed peak in the plots of temperature dependence of a .c. conductivity for nickel-doped As2S3. The entire behaviour of a.c. conductivity with respect to temperature and frequency has been explai ned by using CBH and ''simple pair'' models. Theoretical results obtai ned by using these models, have been found to be in agreement with the experimental results.