The mechanism of sensitivity reduction in selenium layers irradiated by X-rays

The process of sensitivity reduction (fatigue) in amorphous selenium layers irradiated by X-rays has been considered taking into account carrier photogeneration, trapping, and recombination. Analytical expressions describing the kinetics of carrier trapping at the levels responsible for a fatigue have been obtained. Formulas to determine a sensitivity of both fatigued and unfatigued layers have been derived. Computed and experimental dependences of sensitivity upon the layer thickness and the hardness of X-rays are in a good qualitative agreement. Theoretical dependences of fatigue vs. irradiation dose, layer thickness, electric field, and density of traps correctly reflect analogous experimental regularities. It has been established that the fatigue dependence on the irradiation dose rate is determined by the carrier lifetime at the traps responsible for a fatigue process. Comparing theoretical results with experimental data on fatigue, the density of traps responsible for a fatigue process have been determined as well as values of recombination and trapping coefficients in electroradiographic selenium layers.