Persistent photoconductance in doping-modulated and compensateda-Si:H

We present experimental results and numerical calculations in support of the following model to explain the origin of the persistent-photoconductivity effect (PPC) in p-n multilayers of hydrogenated amorphous silicon (a-Si:H): Small light exposures create Staebler-Wronski defects in the p-type regions of the multilayer, making these regions more intrinsic. This brings the equilibrium Fermi level of the multilayer closer to the conduction band in the depletion zones of the n-type regions, causing an increase in the conductance of the layered structure when the conductance is electron dominated. At large light exposures, the Staebler-Wronski defects created in the n-type regions pull the Fermi level away from the conduction band, decreasing the conductance of the film. Our experimental results show that, for a given light intensity, the creation rate and annealing kinetics of the PPC in multilayers are correlated with the creation rate and annealing kinetics of the light-induced conductance changes in unlayered p-type and n-type a-Si:H films having the same dopings as the p-type and n-type regions in the multilayer. The PPC follows a stretched-exponential time relaxation with the same parameters describing the decays of other metastable conditions in a-Si:H. Our computer calculations can reproduce the dark conductivity and magnitude of the PPC in a multilayer (doped at 100 ppm) as a function of sublayer thickness d, except for d20 nm.The calculated values can be made to agree with the measured ones at d20 nm by letting the defect structure of the doped a-Si:H in the multilayer change with decreasing d for d20 nm. Photothermal-deflection-spectroscopy measurements show that indeed the concentration of defects in the multilayer decreases with decreasing d for d20 nm. We find that the rate of creation and annealing kinetics of the PPC in multilayers with d20 nm is correlated with the rate of creation and annealing kinetics of the light-induced conductance changes in unlayered a-Si:H films having smaller gas-phase doping levels than the ones used in depositing the multilayers. Compensated films appear to behave similarly to multilayers with small d. The model proposed for PPC in multilayers can explain all the experimental facts associated with the effect and is extended to account for the light-induced conductance changes in compensated and other layered films, like n-i-n-i, p-i-p-i, and n-i-p-i multilayers.