Modeling of dark current and ghosting in multilayer amorphous selenium X-ray detectors

A theoretical model for describing the bias-dependent transient behavior of dark current in multilayer amorphous selenium (a-Se) detectors has been developed by solving the trapping rate equations and Poisson's equation in the a-Se layer. The transient dark currents in these detectors are measured and the proposed dark model is compared with the measured data. The model shows a very good agreement with the experimental results. It has been found that the dark current is mainly controlled by the Schottky emission of holes from the metal/a-Se contact. The space charge build-up due to the hole injection and trapping in the blocking layer reduces the internal field at the metal/a-Se interface of positive side and thus the dark current eventually is limited by the space charge. It has been found that the electric fields at the metal contacts reduce to 20-30% of the applied field (applied voltage/thickness). The comparison of the model with the experimental data estimates some important properties (e.g., trap center concentrations, space charges, and effective barrier heights) of the blocking layers of the multilayer detectors. The dependence of the X-ray sensitivity of multilayer a-Se X-ray imaging detectors on repeated X-ray exposures is studied by considering accumulated trapped charges and their effects (trap filling, recombination, electric field profile, electric field dependent electron-hole pair creation), the carrier transport in the blocking layers, X-ray induced metastable deep trap center generations, and the effects of dark current. We simultaneously solve the continuity equations for both holes and electrons, trapping rate equations, and the Poisson's equation across the photoconductor for a step X-ray exposure by the Backward Euler finite difference method. The theoretical model shows a very good agreement with the experimental relative sensitivity versus cumulative X-ray exposure characteristics. The electric field distribution across the multilayer detector and the dark current density under repeated exposures are also estimated.