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.
New development of large-area direct conversion detector for digital radiography using amorphous selenium with a C60-doped polymer layer
We have developed a novel direct conversion detector for digital radiography by using a fullerene (C60)-doped polymer layer added on a thick amorphous selenium (a-Se) layer coupled to an amorphous silicon thin-film transistor (a-Si TFT) array. This detector exhibits considerable improvement in the lag characteristics and durability in high ambient temperatures. The C60-doped polymer layer, which is directly and uniformly solution cast on the a-Se layer and followed by an inorganic electron-transporting layer, smoothly changes the electronic junction between the a-Se layer and the inorganic layer. It lubricates the emission of photocurrents from the a-Se photo-conversion layer and leads to the improved lag characteristics. Another merit of using a C60-doped polymer is that it is stabile in high-temperature ambient conditions and is not degraded by humidity or a large amount of X-ray exposure. The polymer layer prevents the crystallization of a-Se, which otherwise occurs on exposure of a-Se to high temperature not only during the deposition of the inorganic layer or the metal electrode layer in the manufacturing process but also in actual use. A prototype detector, with a size of 17 in × 17 in and a pixel pitch of 150 μm, exhibited a good resolution; its DQE is approximately 48% at 1 cy/mm in 258 μC/kg (RQA5). This new development can simplify cooling apparatus and detector modules and also make a wide range of operational environments available. In addition, the improved lag characteristics make it possible to reduce the exposure intervals for static imaging, tomosynthesis, and other various exposure techniques.
Dark current in multilayer stabilized amorphous selenium X-ray photoconductors
Commercially available flat panel, direct conversion, digital X-ray image detectors use a photoconductive layer of stabilized amorphous selenium (a-Se) sandwiched between two electrodes to convert incident X-ray radiation into stored electrical charge. These detectors use thin n -like and p -like blocking layers to trap charge carriers injected from the contacts and thus reduce the dark current by limiting the rate of carrier injection. The effect of each of these blocking layers is determined by measuring the dark current in 200 μm thick mammographic detector-like samples of a-Se with three different structures: a single intrinsic layer and n-i and i-p double layers. Dark current is measured as a function of time over 20,000 s and as a function of applied electric field over the range of −10 V/μm to +10 V/μm. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)