Improved Temporal Performance and Optical Quantum Efficiency of Avalanche Amorphous Selenium for Low Dose Medical Imaging

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.