PurposeActive matrix flat panel imagers (AMFPIs) with thin-film transistor arrays experience image quality degradation by electronic noise in low-dose radiography and fluoroscopy. One potential solution is to overcome electronic noise using avalanche gain in an amorphous selenium (a-Se) (HARP) photoconductor in indirect AMFPI. In this work, we aim to improve temporal performance of HARP using a novel composite hole blocking layer (HBL) structure and increase optical quantum efficiency (OQE) to CsI:Tl scintillators by tellurium (Te) doping.ApproachTwo different HARP structures were fabricated: Composite HBL samples and Te-doped samples. Dark current and optical sensitivity measurements were performed on the composite HBL samples to evaluate avalanche gain and temporal performance. The OQE and temporal performance of the Te-doped samples were characterized by optical sensitivity measurements. A charge transport model was used to investigate the hole mobility and lifetime of the Te-doped samples in combination with time-of-flight measurements.ResultsThe composite HBL has excellent temporal performance, with ghosting below 3% at 10 mR equivalent exposure. Furthermore, the composite HBL samples have dark current <10−10 A/cm2 and achieved an avalanche gain of 16. Te-doped samples increased OQE from 0.018 to 0.43 for 532 nm light. The addition of Te resulted in 2.1% first-frame lag, attributed to hole trapping within the layer.ConclusionsThe composite HBL and Te-doping can be utilized to improve upon the limitations of previously developed indirect HARP imagers, showing excellent temporal performance and increased OQE, respectively.
Active matrix flat panel imagers (AMFPIs) with thin-film transistor (TFT) arrays have become the dominant technology for digital x-ray imaging. However, their performance is degraded by electronic noise in low dose imaging applications. One potential solution is to overcome electronic noise using avalanche gain in an amorphous selenium (a-Se) photoconductor in indirect AMFPI, known as the scintillating high-gain avalanche rushing photoconductor AMFPI (SHARP-AMFPI). We previously developed two SHARP-AMFPI prototypes, however both have several areas of desired improvement. In this work, we fabricate and characterize HARP samples with a composite hole blocking layer (HBL) structure to reliably maintain avalanche fields while reducing temporal effects, as well as samples with tellurium (Te) alloyed a-Se to increase the optical quantum efficiency (OQE) to thallium doped cesium iodide (CsI:Tl) columnar scintillators. Our measurements show that the composite HBL has improved temporal performance over the original prototype, with ghosting below 3% at 10 mR equivalent exposure and no noticeable lag observed. We also show that the layer has comparable dark current to the previously used organic HBL and can reach an avalanche gain of 16. We aim to further reduce the dark current by improving the formulation of the n-type metal oxide layer using different deposition methods. Introducing Te-alloying to HARP samples shows an increase in OQE from 0.018 to 0.43 for 532 nm light. The addition of Te resulted in increased lag, attributed to charge trapping within the layer. Future work will investigate arsenic and chlorine co-doping to restore charge transport in this layer.
Active matrix flat panel imagers (AMFPIs) with thin film transistor (TFT) arrays are becoming the standard for digital x-ray imaging due to their high image quality and real time readout capabilities. However, in low dose applications their performance is degraded by electronic noise. A promising solution to this limitation is the Scintillator High-Gain Avalanche Rushing Photoconductor AMFPI (SHARP-AMFPI), an indirect detector that utilizes avalanche amorphous selenium (a-Se) to amplify optical signal from the scintillator prior to readout. We previously demonstrated the feasibility of a large area SHARP-AMFPI, however there are several areas of desired improvement. In this work, we present a newly fabricated SHARP-AMFPI prototype detector with the following developments: metal oxide hole blocking layer (HBL) with improved electron transport, transparent bias electrode for increased optical coupling, and detector assembly allowing for a back-irradiation (BI) geometry to improve detective quantum efficiency of scintillators. Our measurements showed that the new prototype has improved temporal performance, with lag and ghosting below 1%. We also show an improvement in optical coupling from 25% to 90% for cesium iodide (CsI) scintillator emissions. The remaining challenge of the SHARP-AMFPI is to reduce the dark current to prevent dielectric breakdown under high bias and further increase optical quantum efficiency (OQE) to CsI scintillators. We are proposing to use a newly developed quantum dot (QD) oxide layer, which shows to reduce the dark current by an order of magnitude, and tellurium doping, which could increase OQE to 85% to CsI at avalanche fields, in future prototype detectors.
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