X-RAY DOSE DEPENDENCE OF DARK CURRENT IN AMORPHOUS SELENIUM-ALLOY X-RAY PHOTOCONDUCTORS

Abstract

The dark current is an important characteristic of a photoconductive X-ray detector, and can impact the dynamic range of the detector and its detective quantum efficiency. It is therefore essential that the dark current and its behavior with time and x-ray irradiation are well characterized and understood in amorphous selenium (a-Se) X-ray detectors for the future enhancement of these detectors. Throughout the course of this work, the dark current in practical a-Se multilayer photoconductors were studied as function of time and x-ray dose delivered to the detector material. The dark current in these multilayer structures has been measured as a function of different rest time periods, sample structure, single X-ray irradiation on the sample and multiple irradiation on the sample. Experiments were performed by resting the sample in dark for a period of time (24 hours) and then samples were exposed to X-ray radiation. It has been observed that most of the trapped charge carriers in the bulk of the material are discharged after resting the sample in dark for 24 hours. It was observed that multilayer sample structures p-i-n and n-i-p exhibit much less dark current compared to other samples with single layer and double layer structures, that is, i-layer only, n-i and p-i structures. The experiments support that the dark current is controlled by injection of charge carriers from contacts. Single X-ray irradiation and multiple irradiation experiments were performed on multilayer a-Se photoconductors at a dose rate of 0.51 Gy s-1 with an exposure duration of 3 s. Samples were exposed to single irradiation at 100 s and 400 s. The dark current following the photocurrent was recorded. Multiple irradiation experiments were also performed on these multilayer samples. With different reverse bias voltages, samples were irradiated 10 times from 200s to 2000s. It was found that the dark current tends to increase with repeated X-ray irradiation but the increase depends on the applied reverse bias; the increase is negligible at a field of 10 V μm-1. After the cessation of the irradiation, the dark current decays and tends to reach a steady state value at t = 4000s. After 24 hr of resting in the dark, the dark current was nearly as low as the original dark current before the X-ray irradiation