Mr. Tristen T. Thibault Profile

Tristen T. Thibault

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Proceedings Article | 4 April 2022 Poster + Presentation + Paper

Charge recombination and electron-hole pair creation energy in amorphous lead oxide x-ray photoconductor

Oleksandr Grynko, Tristen Thibault, Emma Pineau, Giovanni DeCrescenzo, Alla Reznik

Proceedings Volume 12031, 120314M (2022) https://doi.org/10.1117/12.2612937

KEYWORDS: X-rays, Photoresistors, Sensors, X-ray detectors, Lead, Oxides, Photons, Electric field sensors, X-ray imaging, Diagnostics

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The X-ray sensitivity is one of the most important parameters for X-ray photoconductors; it is usually described in terms of the electron-hole pair creation energy W± – the average energy required to generate a single detectable pair of electron and hole. This parameter accounts only for the detected carriers, reflecting that only a fraction of the X-ray–generated charges are collected, while the rest are lost through the recombination processes. The charge recombination results in an effective W± that is not a material parameter but rather a characteristic of the detection system, which remains larger than its theoretical value 𝑊± ⁰ and depends on the applied electric field F, X-ray energy E and exposure X. In this work, we characterize the X-ray sensitivity of amorphous lead oxide (a-PbO) photoconductor, which is considered as one of the most promising replacements for amorphous selenium (a-Se) for radiographic, fluoroscopic, and tomosynthesis applications, and examine the underlying mechanisms responsible for charge recombination. W± was measured in a wide range of electric fields (1–20 V/μm), X-ray exposures (0.02–2 R) and energies (29–51 keV). W± decreases with both the electric field and X-ray energy, saturating at 18–32 eV/ehp depending on the energy. W± increases with radiation exposure, especially at lower fields and higher energies. The particular dependencies of W± on these parameters, especially X-ray energy-dependent W±, conclude that the columnar recombination mechanism dominates in the a-PbO photoconductor with a secondary contribution of the bulk recombination.

Proceedings Article | 4 April 2022 Poster + Presentation + Paper

Investigation of dark current mechanisms in a polyimide blocking layer utilized in an amorphous lead oxide-based x-ray detector

Tristen Thibault, Oleksandr Grynko, Emma Pineau, Alla Reznik

Proceedings Volume 12031, 120314J (2022) https://doi.org/10.1117/12.2613179

KEYWORDS: Sensors, Mathematical modeling, Interfaces, X-ray detectors, Data modeling, X-rays, Dielectrics, Amorphous semiconductors

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Amorphous lead oxide (a-PbO) is considered a promising photoconductor to replace amorphous selenium (a-Se), extending the diagnostic capabilities of direct conversion detectors to dynamic and high-energy applications. A blocking layer structure was required to maintain dark current (DC) to tolerable levels in a-PbO detectors (below 1 – 10 pA/mm² ). Adopting an approach utilized in the development of a-Se technology, here, a polyimide (PI) blocking layer is used to suppress DC in an a-PbO detector. DC was measured as a function of time (DC kinetics) on a single-pixel PI/a-PbO detector prototype. DC decayed below 1 pA/mm2 within two hours of bias application. To optimize the PI/a-PbO detector’s operation, a mathematical model was derived to simulate experimental observed DC decay and probe DC suppression mechanisms. The model shows that DC decay results from an accumulation of charge within the bulk of PI and at the PI/aPbO interface. This screens the electric field inside the PI layer and reduces injection from the ITO. The results of the DC kinetics modelling indicate that the conduction mechanism responsible for DC decay in PI/a-PbO detectors is different from that in the similar PI/a-Se detectors. The Poole-Frenkel model best describes the DC-field dependence in PI/a-Se detectors. In contrast, the Schottky model more accurately describes DC-field dependency in PI/a-PbO detectors. A unique distribution of the trapping states within the PI/a-PbO structure is suggested to cause this difference. In addition, the model reveals that the field redistribution causes the field within a-PbO to increase with time, and therefore charge collection efficiency and temporal performance improve.

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