X-ray-induced recombination effects in a-Se-based x-ray photoconductors used in direct conversion x-ray sensors

Stabilized amorphous selenium (a-Se) is currently used as an x-ray photoconductor in direct conversion flat-panel digital x-ray image detectors. Therefore, there is much interest in x-ray-induced effects in a-Se, especially changes in charge carrier lifetimes that result from x-ray exposure. We have observed that the exposure of an a-Se x-ray detector sample to x rays induces negative capture centers in the bulk and thereby reduces the hole lifetime. By using conventional and interrupted field time-of-flight (IFTOF) transient photoconductivity techniques in a TOF-IFTOF-TOF sequence, we were able to develop a technique that allows the measurement of the capture coefficient C-r between free holes and x-ray-induced negative centers, which we believe to be trapped electrons. We find that the capture process follows the Langevin recombination mechanism, the same recombination mechanism that has been observed in the case of recombination between free holes and free electrons in a-Se. We have shown that the concentration of x-ray-induced negative centers increases almost linearly with the x-ray exposure. As a corollary, in terms of fundamental physics of amorphous semiconductors, we can also conclude that the influence of potential fluctuations in the noncrystalline structure in shielding a charged center in a-Se is relatively small. (C) 2004 American Vacuum Society.