Electronic Transport Properties of Stabilized Amorphous Selenium X-ray Photoconductors

by Fogal, Bud J

Amorphous selenium (a-Se) and its alloys are important photoconductor materials used in direct conversion flat panel digital x-ray detectors. The performance of these detectors is determined, in part, by the electronic transport properties of the a-Se photoconductor layer namely, the charge carrier mobility m and the deep trapping lifetime t. The product of the mobility and the lifetime mt, referred to as the charge carrier range, determines the average distance that photo-generated charge will travel before being removed from the transport band by deep localized states in the mobility gap of the semiconductor. The loss of carriers to these deep states reduces the amount of charge collected per unit of x-ray exposure, and, hence, limits the x-ray sensitivity of the detector. Two experimental techniques that may be used to measure the transport properties of holes and electrons in high resistivity semiconductors are described in this thesis. The Time-of-Flight (TOF) transient photoconductivity technique is used to evaluate the charge carrier mobility by measuring the time required for the charge carriers to transit a fixed distance under the influence of an applied electric field. The Interrupted-Field Time-of-Flight (IFTOF) technique is used to determine the charge carrier deep trapping time; the drift of the injected carriers is temporarily interrupted at a position in the sample by removing the applied field. When the field is reapplied the number of charge carriers has decreased due to trapping events. The carrier lifetime is determined from the dependence of the fraction of recovered charge carriers before and after the interruption with the interruption time.

TOF and IFTOF measurements were carried out on a number of samples of vacuum deposited selenium alloy x-ray photoconductors. Device quality photoconductor films are fabricated by evaporating a-Se source material that has been alloyed with a small quantitiy of As (~0.3 at. %) and doped with a halogen (typically Cl) in the p.p.m. range. The dependence of the carrier range on the composition of the photoreceptor film was accurately measured using both TOF and IFTOF measurements. It was found that the transport properties of the film could be controlled by suitably adjusting the composition of the alloy. Combined IFTOF and TOF measurements were also performed on several samples to examine the effects of trapped electrons on the hole transport properties in a-Se films. It was found that drifting holes recombine with the trapped electrons, and that this process could be described by a Langevin recombination process. This finding is important for the correct modeling of amorphous selenium digital x-ray detector designs. Finally, the effects of x-ray exposure on a-Se films were examined. A temporary reduction in the effective hole lifetime was observed due to an increase in the number of hole capture centers following an x-ray exposure. The capture coefficient between free holes and the x-ray induced hole capture centers was measured using combined TOF and IFTOF measurements. It was shown that this capture process was governed by the Langevin recombination mechanism. From these observations it was concluded that trapped electrons from a previous x-ray exposure act as recombination centers for subsequently generated holes, thereby reducing the effective hole lifetime in the sample.