Relaxation of the electrical properties of vacuum-deposited a-Se1−xAsx photoconductive films: Charge-carrier lifetimes and drift mobilities

Abstract

The authors have examined the relaxation of the electrical properties of vacuum-deposited x-ray photoconductor-type a-Se1−xAsx (x=0%–0.5%) films in terms of the time evaluation of the deep trapping time τ, i.e., carrier lifetime, and the drift mobility μ from the time the samples were brought to 23 °C after they had been annealed at 55 °C (above the glass-transition temperature Tg) for 30 min. The changes in the lifetime and drift mobility arise from structural-relaxation processes and have been modeled using a stretched exponential-relaxation process, i.e., τ=τ∞+(τo−τ∞)exp[−(t/τsr)β], where τ∞ is the lifetime when the sample is fully relaxed (the final “equilibrium” value), τo is the initial lifetime, τsr is the characteristic structural-relaxation time that controls the relaxation of the observed property, and β is the stretching factor. The authors have examined the relaxation of τ and μ as a function of composition. Within experimental errors, the structural-relaxation time associated with electron and hole lifetimes were the same, indicating that the same structural changes must be influencing both electron and hole deep traps. τsr was 7–8 h for pure a-Se and increased linearly with the As content to about 40 h for a-Se:0.5%As. The stretching factor β was in the range 0.6–0.7 for all the samples. The relaxation of the electron-drift mobility could also be fitted to a stretched exponential as well, using the same structural-relaxation time as for the relaxation of the electron lifetime. The increase in the carrier range (μτ) was used to estimate the ratio of the final to initial hole and electron deep-trap concentrations. This ratio was about 0.5 for both hole and electron deep traps for the present conditions that involved equilibration at 55 °C (above Tg) and relaxation at 23 °C. The significant increase in the carrier range during structural-relaxation leads to marked improvements in the performance of a-Se based x-ray detectors from the instant they are manufactured. As a corollary, one can conclude that the electrical properties of a-Se photoconductors will always relax toward their equilibrium values, reaching stable (relaxed) equilibriumlike values within a few days at room temperature in the worst case.