Enhancing Temporal Performance of a-Se Detectors using a Low-temperature Hole-Blocking Bilayer Design

Over the past few decades, amorphous selenium (a-Se) X-ray detectors have gained widespread use in mammography due to their remarkable spatial resolution capabilities. However, these devices encounter challenges in applications requiring lower radiation exposures and dynamic imaging, such as dynamic mammography tomosynthesis. The potential enhancement of sensitivity and temporal performance by increasing the applied voltage across the selenium layer is counteracted by concerns about dark current. Furthermore, certain applications necessitate the placement of a low-temperature hole-blocking layer on the top surface of a-Se to enable high-voltage mode in hole collecting mode. Although the use of SU-8 as a top layer has demonstrated satisfactory temporal performance, there remains room for improvement. Additionally, the use of SU-8 at the bottom has revealed interface compatibility issues. In this research, we address the compatibility challenges associated with the positioning of the SU-8 layer at the bottom by introducing a novel bilayer configuration. This bilayer setup was evaluated in both top and bottom positions. Our results indicate that when the bilayer configuration is located at the top, it combines the strengths of both layers, merging the high signal level in a Cs-doped a-Se device with the low dark current characteristics of the SU-8 layer. Notably, among the samples incorporating hole-blocking layers, the bilayer positioned at the top exhibits the most favorable lag performance, measuring below 0.5% after the 7th exposure, and a sensitivity reduction of 14% after 20 exposures (equivalent of 0.175 Gy). Moreover, employing this bilayer arrangement at the bottom enhances sensitivity by 16.1% compared to devices utilizing only the SU-8 hole-blocking layer. This improvement underscores the effective mitigation of interface challenges through the utilization of Cs-doped a-Se when SU-8 is placed at the bottom.