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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 V,, and the cycled-up residual potential V,, Hole lifetime r as determined from the Warter expression VRI=L2/ 2~~7 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:O.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 localizid 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)]. 7087

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Currently there is much interest in stabilized amorphous selenium (a-Se alloyed with 0.2–0.3% As and doped with 5–20 ppm Cl) as an x-ray photoconductor in various x-ray imaging systems. Although optical photogeneration in a-Se is well characterized, experiments on its x-ray sensitivity have been limited and, further, have resulted in widely differing conclusions especially for the electron hole pair (EHP) creation energy EEHP. In this work, we first introduce absolute and incremental sensitivity (S and s, respectively) concepts within the electrostatic readout mode of operation and then establish models linking these two sensitivities to the energy EEHP absorbed to create a free EHP. X-ray irradiation induced xerographic discharge experiments have been carried out on vacuum deposited stabilized a-Se x-ray photoconductor films to examine how the sensitivity depends on the charge carrier schubwegs (μτF where μ is the drift mobility, τ is the lifetime, and F is the electric field), the mean photon energy, and the detector thickness L. Experiments indicate that unless both electron and hole schubwegs are much longer than the photoconductor thickness, the sensitivity is reduced. The efficiency of EHP generation per absorbed x-ray photon, i.e., the apparent energy absorbed per free electron hole pair EEHP has been determined as a function of electric field and mean photon energy. EEHP decreases with increasing applied field and energy. By examining EEHP vs 1/F and extrapolating to 1/F=0, a saturation value EEHP0 for EEHP is predicted and estimated (4 to 6 eV) which, within experimental errors, is close to the theoretical expectation of ∼2.2Eg and substantially smaller than previously reported values for EEHP0 (18–29 eV).

Time-of-fiight (TOF) hole drift mobility (Pd) measurements have been carried out on vacuum deposited and identically aged a-Se photoreceptor type films over a temperature range encompassing the glass transformation region to study the nature of mobility controlling shallow traps in this elemental chalcogenide semiconductor. Differential scanning calorimetry (DSC) experiments using both heating and cooling scans have also been carried out on the same films to correlate the enthalpy relaxation phenomenon with the TOF drift mobility dependence on temperature. By considering the heating rate dependence of the minimum peak in thefld vs Tbehavior and the heating and cooling rate dependence of the DSC glass transition temperature, it is shown that the mean retardation times, T,l and Tn. associated with the relaxation of the shallow traps and the enthalpy, respectively, have similar temperature dependencies, essentially Vogel-Tammann-Fulcher type, with negligible structural contribution. Correlation is also established with the relaxation of the mechanical properties of a-Se, viz., microhardness. Moreover, the rate of equilibration of shallow traps in a-Se is found to be inversely proportional to the 'viscosity, 1], i.e., (aNJath~ 1/1], or Til ~rl. The present work provides further experimental evidence that the shallow traps in a-Se are structural defects which are thermodynamic in origin and it also shows that their equilibration involves atomic motions similar to those which control the viscosity.