Engineering of a Blocking Layer Structure for Low-Lag Operation of the a-PbO-Based X-Ray Detector

Abstract

Direct conversion flat panel detectors are of great significance to the field of medical X-ray imaging since they offer imaging performance and diagnostic capabilities not achievable with other methods. Currently, mammographic direct conversion detectors employ a layer of amorphous selenium (a-Se) photoconductor. Although its properties ideally fit the requirements of mammography, where "soft" X-rays are used, a-Se cannot be used in high-energy X-ray procedures. To extend the diagnostic capabilities of the direct conversion detectors, amorphous lead oxide (a-PbO) is proposed as an alternative photoconductor. It is a high effective atomic number material and thus has a higher X-ray stopping power over the wide X-ray energy range. a-PbO is, therefore, a suitable candidate for applications in radiography, fluoroscopy, and digital tomosynthesis. Here, we report on the development of a blocking structure with a polyimide (PI) layer needed to maintain low dark current at high electric fields. We demonstrate that a 1-µm-thick PI blocking layer allows the operation of the detector at strong electric fields (≥10 V/µm) while suppressing the dark current to an innocuous level (<1 pA/mm 2 ). It also improves temporal performance by reducing signal lag. No ghosting effect was observed at exposure rates up to 1 R/s; however, at high radiation levels, the detector's sensitivity degraded. This degradation is not permanent as the detector restores its original sensitivity after several hours of rest in the dark without bias applied.

The X-ray Sensitivity of an Amorphous Lead Oxide Photoconductor

“…Generally speaking, the carriers can be trapped at localized states within the mobility gap of a-PbO, in either shallow or deep traps. However, a previous investigation of the ghosting effect [21] suggested that no deep trapping occurs in PI/a-PbO photoconductive structures, at least at the relatively low exposures used in this study. Ghosting is caused by deep bulk trapping of photogenerated carriers, which subsequently recombine with the drifting carriers of the opposite sign, resulting in sensitivity degradation.…”

“…Finally, a top Au contact (readout electrode) 1.1 mm in diameter was sputtered atop of the a-PbO, which provided an effective detector area of 0.95 mm 2 . Detailed descriptions of the PI application and a-PbO deposition can be found in [21,56].…”

“…The requirement for a relatively low deposition temperature makes single-crystalline photoconductors unsuitable for use in direct conversion imaging detectors. As for the disordered (polycrystalline and amorphous), high-Z semiconductors that can produce large-area detectors, polycrystalline layers of PbI 2 [8][9][10], HgI 2 [10][11][12], CdTe [13], Cd 1-x Zn x Te [14], BiI 3 [15], ZnO [16], PbO [17,18], perovskites [19], and amorphous PbO (a-PbO) [20,21] are considered promising. However, at the current stage of their technology, the majority of the materials in this list exhibit signal lag-a residual current that continues to flow after X-ray exposure [17,18,22].…”

“…In the continuous advancement of a-PbO technology, we recently reported on the development of a blocking structure for a-PbO detectors, in which a thin layer of polyimide (PI) is introduced between the electrode and a-PbO layer, thereby preventing their interaction. This blocking structure is needed to maintain low dark current (<1 pA/mm 2 ) in a strong operational electric field (≥10 V/µm), while preserving the temporal performance suitable for real-time imaging with lag values down to 0.9% at 30 fps read-out [21].…”

“…Since no detectable ghosting effect was observed at relevant exposure rates, deep trapping can be ruled out as a primary cause for W ± degradation. Additionally, the quasi-rectangular shape of the X-ray response indicates the unrestricted flow of the photogenerated carriers from the a-PbO photoconductive layer through the PI blocking layer into the ITO electrode [21], meaning that no accumulation of carriers in shallow states at the PI/a-PbO interface are expected as well. Therefore, a trapping mechanism can be excluded from the reasons for the carrier loss in a-PbO and will not be discussed further.…”