X-ray sensitivity of photoconductors: application to stabilizeda-Se

Abstract

The x-ray sensitivity of a high-resistivity photoconductor sandwichedbetween two parallel plate electrodes and operating under a constant field isanalysed by considering charge carrier generation that follows the x-ray photonabsorption profile and taking into account both electron and hole trappingphenomena but neglecting recombination, bulk space charge and diffusion effects.The amount of collected charge in the external circuit due to distributedgeneration of electrons and holes through the detector is calculated byintegrating the Hecht collection efficiency with Ramo's theorem across thesample thickness. The results of the model allow the x-ray sensitivity to becalculated as a function of the applied field, detector thickness and electronand hole ranges (µτ), given the field and energy dependence of theelectron and hole pair creation energy, W±, and the energy spectrum ofincident x-ray radiation. The sensitivity model was applied to stabilized a-Sethat is currently used as a successful x-ray photoconductor in the recentlydeveloped flat panel x-ray image detectors. Recent free electron-hole paircreation energy versus electric field data at room temperature and appropriateelectron and hole drift mobilities were used to calculate the sensitivity formonoenergetic x-rays at 20 and at 60 keV. For the 20 keV radiation, it was shownthat a typical detector thickness of 200 µm (4 × attenuation depth at20 keV) with currently attainable electron and hole trapping parameters in a-Sewas operating optimally, the sensitivity of which can only be increased byfurther increasing the applied field. With the receiving electrode positively biased,the sensitivity was much more dependent onthe hole lifetime than electron lifetime. The absence of hole transport resultsin a reduction in sensitivity by a factor of about 4.4, whereas the absence ofelectron transport results in a sensitivity degradation of only 22%. The ratioof hole trapping limited sensitivity to electron trapping limited sensitivity isabout 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 thissensitivity degrades by more than 10% when either the electron lifetime fallsbelow ~20 µs or the hole lifetime falls below ~5 µs. When thehole 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 caseof 60 keV x-ray photons, it is more useful to examine the sensitivity as afunction of detector thickness given the practical bias voltage limit. Thesensitivity versus thickness behaviour for a given bias voltage exhibits amaximum, that is an optimal thickness, that is less than that for nearly fullabsorption. Electron lifetimes longer than ~200 µs and hole lifetimeslonger than ~10 µs do not significantly affect the sensitivity.