DRIFT MOBILITY RELAXATION IN A-SE

Time-of-flight (TOF) hole drift mobility (μd) measurements have been carried out on vacuum deposited and identically aged a-Se photoreceptor type films over a temperature range encompassing the glass transformation region to study the nature of mobility controlling shallow traps in this elemental chalcogenide semiconductor. Differential scanning calorimetry (DSC) experiments using both heating and cooling scans have also been carried out on the same films to correlate the enthalpy relaxation phenomenon with the TOF drift mobility dependence on temperature. By considering the heating rate dependence of the minimum peak in the μd vs T behavior and the heating and cooling rate dependence of the DSC glass transition temperature, it is shown that the mean retardation times, τμ and τH, associated with the relaxation of the shallow traps and the enthalpy, respectively, have similar temperature dependencies, essentially Vogel-Tammann-Fulcher type, with negligible structural contribution. Correlation is also established with the relaxation of the mechanical properties of a-Se, viz., microhardness. Moreover, the rate of equilibration of shallow traps in a-Se is found to be inversely proportional to the viscosity, η, i.e., (∂Nt/∂t)T ∼1/η, or τ μ∼η. The present work provides further experimental evidence that the shallow traps in a-Se are structural defects which are thermodynamic in origin and it also shows that their equilibration involves atomic motions similar to those which control the viscosity.