X-ray induced hole trapping in electroradiographic plates

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Abstract
Changes in the electrical properties of pure a-Se and Cl-doped a-Se:0.35% As electroradiographic layers upon exposure to x-ray radiation has been examined using xerographic techniques based on the first residual potential VR1 and the cycled-up residual potential VR∞ Hole lifetime τ as determined from the Warter expression VR1=L2/2μτ has been found to decrease sharply with the x-ray dose, tending to saturate at higher dose levels in both pure a-Se and Cl-doped a-Se:0.35%As. The hole liftime following x-ray exposure recovers toward its equilibrium value at the anneal temperature. The recovery time is dependent on the dose and is accelerated appreciably by temperature, indicative of a thermally activated process. Cycled-up xerographic residual-potential measurements have indicated that the deep-hole-trap population increases with exposure to x rays. Comparison of cycled-up saturated residual potential and first-cycle residual-potential measurements have shown that as a result of x-ray exposure, the apparent capture coefficient increases, which can only be accounted for by introducing two x-ray-induced effects: (a) generation of deep hole traps and (b) formation of an equal number of trapped electrons and holes in the bulk due to irradiation. Recombination of holes with the trapped electrons in the bulk causes the apparent increase in the capture coefficient. A xerographic spectroscopy technique, based on the analysis of the rate of discharge of the saturated potential after the end of xerographic cycling, was used to map out the energy distribution of the occupied density of states in the mobility gap. It was found that the energy location of the peak of the occupied density of localized states remained unaffected by the exposure to x rays, but the peak value of the density of states increased with the exposure. The x-ray induced trap generation is thus simply an enhancement of the integrated concentration of inherent structural defects in the films located at their characteristic energies: 0.87 eV in pure a-Se and 0.90 eV in Cl-doped a-Se:0.35%As. The present results are consistent with the x-ray-sensitivity predictions of Kalade, Montrimas, and Rakauskas [Phys. Status Solidi A 25, 629 (1974)].