Improved temporal performance and optical quantum efficiency of avalanche amorphous selenium for low dose medical imaging

Purpose: Active 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.

Approach: Two 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.

Results: The composite HBL has excellent temporal performance, with ghosting below 3% at 10 mR equivalent exposure. Furthermore, the composite HBL samples have dark current $<{10}^{-10}A/{\mathrm{cm}}^2$ 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.

Conclusions: The 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.