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

Abstract

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 modern 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.

Lead monoxide α-PbO: electronic properties and point defect formation

“…Similar behaviour of the dark current was observed by Mahmood and Kabir in amorphous selenium (a-Se) multilayer n − i − p structures [32] and by Street in hydrogenated amorphous silicon (a-Si:H) p − i − n structures [33,34]. Mahmood and Kabir explain dark current decay in a-Se multilayer structure by carrier trapping within comparatively thick (few µm [35]) n-and p-layers which induces screening of the electric field at the metal/n-or p-layer interfaces. The subsequent redistribution of the electric field suppresses carrier injection from metal contacts and reduces the dark current which is mainly controlled by the injection.…”

Charge transport in pure and stabilized amorphous selenium: re-examination of the density of states distribution in the mobility gap and the role of defects

“…With TOF and its variant, Interrupted Field TOF (IFTOF) measurements, one can identify clear transit times, extract drift mobilities and deep trapping times and hence obtain carrier ranges. These carrier range measurements played an important role in the design and development of multilayer photoconductor structures and their eventual commercialization in a-Se detectors [1,2].…”

“…Stabilized amorphous selenium (a-Se) based-multilayer photoconductive structures are currently used in flat panel X-ray imaging detectors (also called flat panel imagers) as well as avalanche solid state imaging devices (see, for example, [1][2][3][4][5]). The direct conversion detector principle is based on an absorbed X-ray photon generating a primary electron, which ionizes the medium and generates many electrons and holes.…”

Characterization of a‐Se p–i–n junction fabricated using bidirectional electrolysis in NaCl(aq)

“…By introducing an equivalent circuit, illustrated in the inset of the graph, the resistance after the electrolysis of region V R was calculated to be R B ¼ 3.2 Â 10 11 V, whereas before electrolysis it was R 0 ¼ 1.7 Â 10 12 V. The carrier concentration is expressed by n ¼ 1/qrm, where q is the elementary charge, r is the resistivity, and m is the mobility. The electron mobility of a-Se (m e ) was used from previous literature [5]. The electron concentration was calculated to be 4.5-6.3 Â 10 12 cm À3 before the electrolysis and 2.3-3.3 Â 10 13 cm À3 after the electrolysis, which is an increase of approximately one order.…”