Enhanced Dark Current Suppression of Amorphous Selenium Detector With Use of IGZO Hole Blocking Layer
We examined the potential application of indium gallium zinc oxide (IGZO) as a hole-blocking layer for an amorphous selenium (a-Se)-based detector to reduce the dark current and improve the sensitivity of the detector. By employing a thin layer of IGZO (375 nm), the dark current of an a-Se detector remains below 1 pA/mm2 up to electric fields as high as 60 V/μm. The measured dark current at different electric fields is comparable to the thermal generation currents in a-Se, thus demonstrating the good hole-blocking properties of IGZO. The detector's photo response was characterized using a blue light-emitting diode at different electric fields. A factor of three improvement in external quantum efficiency was observed by increasing the electric field of the detector from 10 to 50 V/μm.
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 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.
Low Dark-Current Lateral Amorphous-Selenium Metal–Semiconductor–Metal Photodetector
We report a lateral amorphous-selenium (a-Se) metal-semiconductor-metal detector with a blocking contact. The blocking contact, a polyimide layer, is shown to significantly reduce the dark current even at high applied biases that result in high photo-current-to-dark-current ratios, thus leading to wide dynamic range and high signal-to-noise ratio. The use of the polyimide blocking contact prevents the injection of both holes and electrons and improves considerably upon the high dark current of previously reported lateral a-Se detectors. The proposed detector demonstrates the feasibility of low-cost lateral a-Se devices for indirect conversion digital X-ray imaging applications such as chest radiography, fluoroscopy, and computed tomography.