X-ray sensitivity of photoconductors: application to stabilized a-Se

Abstract

The x-ray sensitivity of a high-resistivity photoconductor sandwiched between two parallel plate electrodes and operating under a constant field is analysed by considering charge carrier generation that follows the x-ray photon absorption profile and taking into account both electron and hole trapping phenomena but neglecting recombination, bulk space charge and diffusion effects. The amount of collected charge in the external circuit due to distributed generation of electrons and holes through the detector is calculated by integrating the Hecht collection efficiency with Ramo's theorem across the sample thickness. The results of the model allow the x-ray sensitivity to be calculated as a function of the applied field, detector thickness and electron and hole ranges (µτ), given the field and energy dependence of the electron and hole pair creation energy, W±, and the energy spectrum of incident x-ray radiation. The sensitivity model was applied to stabilized a-Se that is currently used as a successful x-ray photoconductor in the recently developed flat panel x-ray image detectors. Recent free electron-hole pair creation energy versus electric field data at room temperature and appropriate electron and hole drift mobilities were used to calculate the sensitivity for monoenergetic x-rays at 20 and at 60 keV. For the 20 keV radiation, it was shown that a typical detector thickness of 200 µm (4 × attenuation depth at 20 keV) with currently attainable electron and hole trapping parameters in a-Se was operating optimally, the sensitivity of which can only be increased by further increasing the applied field. With the receiving electrode positively biased, the sensitivity was much more dependent on the hole lifetime than electron lifetime. The absence of hole transport results in a reduction in sensitivity by a factor of about 4.4, whereas the absence of electron transport results in a sensitivity degradation of only 22%. The ratio of hole trapping limited sensitivity to electron trapping limited sensitivity is about 0.3. For a detector of thickness 200 µm operating at 10 V µm-1, the maximum sensitivity is about 220 pC cm-2 mR-1, and this sensitivity degrades by more than 10% when either the electron lifetime falls below ~20 µs or the hole lifetime falls below ~5 µs. When the hole lifetime is very short so that the sensitivity is substantially reduced, the sensitivity versus thickness dependence at a given field exhibits a maximum (an optimal thickness) that is less than that for full absorption. In the case of 60 keV x-ray photons, it is more useful to examine the sensitivity as a function of detector thickness given the practical bias voltage limit. The sensitivity versus thickness behaviour for a given bias voltage exhibits a maximum, that is an optimal thickness, that is less than that for nearly full absorption. Electron lifetimes longer than ~200 µs and hole lifetimes longer than ~10 µs do not significantly affect the sensitivity.