Metastabilities in doping-modulated amorphous silicon superlattices

Abstract

Bias- and photo-induced metastabilities in the electrical conductance of doping-modulated npnp hydrogenated amorphous silicon (a-Si:H) superlattices have been studied. During the application of a bias-voltage, the conductance in both coplanar and transverse directions changes with time and reaches a steady-state value specific to each bias-voltage. These changes under bias proceed faster at higher temperatures. The electronic transport takes place through the diffusion over the potential barrier in the transverse direction for the npnp multilayer with moderate n- and p-layer doping concentration. The tunneling current through the localized states in the gap is dominant for the multilayer with a p-layer of high doping concentration. The currents through the localized gap states in the transverse direction increase after high voltage-bias annealing. These bias-induced metastable states in both coplanar and transverse conductances are recovered by zero-bias annealing indicating that the density of dangling bonds in p and n-layers increases during the bias annealing. Light-induced metastable excess conductance, called persistent phtoconductivity(PPC), is also observed in both compensated and multilayered a-Si:H films. In compensated a-Si:H films, single carrier injection was performed using Schottky barrier structures. Only hole injection can give rise to the PPc in compensated films, with the result that the PPC is not a recombination-enhanced effect. A correlation between the magnitude of the PPC and the defect density measured by the constant photocurrent method is observed. On the basis of the experimental results, a hole-induced doping efficiency change is proposed as the origin of the PPC in compensated films. In a-Si:H npnp multilayers, the decay of the PPC exhibits a stretched-exponential form, arising from the dispersive diffusion of hydrogen. The annealing temperature of the PPC is similar to the thermal equilibrium temperature in players. And only the ...