Amorphous selenium and its alloys from early xeroradiography to high resolution X-ray image detectors and ultrasensitive imaging tubes

We describe the progress in the science and technology of stabilized a-Se from its early use in xerography and xeroradiography to its present use in commercial modem flat panel X-ray imagers and ultrasensitive video tubes which utilize impact ionization of drifting holes. Both electrons and holes can drift in stabilized a-Se, which is a distinct advantage since X-ray photogeneration of charge carriers occurs throughout the bulk of the photoconductive layer. An a-Se photoconductor has to be operated at high fields to ensure that the photogeneration efficiency is sufficiently large to provide reasonable X-ray sensitivity. However, at high fields, the dark current is unacceptably large in simple metal/a-Se/metal devices, and special multilayer device structures need to be designed. The dark current decays with time and increases with the nominal applied field. The reduction of the dark current to a tolerable level was one of the key factors that lead to the commercialization of a-Se X-ray detectors. We discuss the origin of the dark current, and highlight some of the current challenges in the design of next generation detectors. We also discuss the origin of impact ionization in a-Se, and its fruitful utilization in ultrasensitive imaging devices, including the Harpicon, which are likely to lead to new high detective quantum efficiency detectors. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim