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20 April 2005 Ghosting mechanisms in a-Se based direct conversion x-ray image sensors
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Abstract
The dependence of the x-ray sensitivity of a-Se based x-ray image detectors on repeated x-ray exposures is studied by considering deep trapping of charge carriers, trapped charges due to previous exposures, trap filling effects, recombination between trapped and drifting carriers, x-ray induced new deep trap center generation, space charge effects, and electric field dependent electron-hole pair creation energy. We simultaneously solve the continuity equations for both holes and electrons, trapping rate equations, and the Poisson’s equation across the photoconductor for a pulse x-ray exposure by the finite difference method. We also perform Monte Carlo Simulations of carrier transports and obtain almost identical results. The change in relative sensitivity (ghosting) as a function of cumulative x-ray exposures for different levels of trapping and different detector operating conditions are examined. The relative sensitivity decreases with increasing cumulated x-ray exposure. The amount of ghosting in a-Se detectors increases with decreasing applied electric field. The sensitivity reduction at negative bias is greater than at positive bias. The theoretical model shows a very good agreement with the experimental relative sensitivity vs. cumulative x-ray exposure characteristics. The comparison of the model with the experimental data reveals that the recombination between trapped and the oppositely charged drifting carriers and x-ray induced new deep trap centers are mainly responsible for the sensitivity reduction in biased a-Se-based x-ray detectors.
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M. Zahangir Kabir, M. Yunus, S. O. Kasap, Olivier Tousignant, Habib Mani, and Philippe Gauthier "Ghosting mechanisms in a-Se based direct conversion x-ray image sensors", Proc. SPIE 5745, Medical Imaging 2005: Physics of Medical Imaging, (20 April 2005); https://doi.org/10.1117/12.594692
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