Doping and annealing effects on persistent photoconductivity in doping-modulated amorphous-silicon superlattices

Abstract
The effects of doping concentration and annealing on the persistent photoconductivity (PPC) in doping-modulated npnp. . . hydrogenated-amorphous-silicon (a-Si:H) multilayer structures and in compensated a-Si:H films have been studied. When the conductivity activation energy of the p-type layers was kept fixed at 0.53 eV and the activation energy of the n-type layers was varied from 0.31 to 0.52 eV by doping, the ratio of the excess conductivity to the annealed-state conductivity (σE/σA) decreased from ∼30 to nearly zero in multilayer films. This result contradicts the usual models of E and AX centers. This argument is confirmed by the effect of annealing on the PPC in multilayer structures: the PPC for the multilayer structures deposited at 250 °C is greatly reduced after 300 °C annealing for 30 min. This annealing shows little change of the conductivity activation energy for the p-type layers. The annealing behavior of the PPC also rules out the dangling-bond model. We suggest that special centers of phosphorous, boron, and hydrogen complexes in the p-type layers near the n-p interfaces may explain the experimental results for the PPC of the multilayer structures.