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.
Enhanced Detection Efficiency of Direct Conversion X-ray Detector Using Polyimide as Hole-Blocking Layer
In this article we demonstrate the performance of a direct conversion amorphous selenium (a-Se) X-ray detector using biphenyldisnhydride/1,4 phenylenediamine (BPDA/PPD) polyimide (PI) as a hole-blocking layer. The use of a PI layer with a-Se allows detector operation at high electric fields (≥10 V/μm) while maintaining low dark current, without deterioration of transient performance. The hole mobility of the PI/a-Se device is measured by the time-of-flight method at different electric fields to investigate the effect of the PI layer on detector performance. It was found that hole mobility as high as 0.75 cm2/Vs is achievable by increasing the electric field in the PI/a-Se device structure. Avalanche multiplication is also shown to be achievable when using PI as a blocking layer. Increasing the electric field within a-Se reduces the X-ray ionization energy, increases hole mobility, and improves the dynamic range and sensitivity of the detector.
Image quality of a digital chest radiography system based on a selenium detector.
A digital chest radiography system has been developed, with a detector based on the photoelectric properties of amorphous selenium. The selenium layer is deposited on a cylindrical aluminium drum, large enough to cover the full field of view for chest imaging. The electrostatic charge image which is formed on the selenium surface after x-ray exposure is read out by electrometer probes using fast drum rotation. For a physical evaluation of the attainable image quality, the characteristic curve, the modulation transfer function, and the noise spectra were measured. From these measurements, the signal-to-noise properties of the detector in terms of detective quantum efficiency (DQE) and noise equivalent quanta (NEQ) were derived. The results show that the selenium-based detector has a wide dynamic range and a significantly better DQE than screen-film and storage phosphor systems for spatial frequencies below the Nyquist limit (2.7 lp/mm). As a consequence, the detectability of small, low-contrast details is considerably improved.