Doctoral Defense

Development of High-Resolution Amorphous Selenium Detectors for X-Ray Imaging

Zhihang Han

August 8, 2024
2:00 PM
Light Engineering, Room 250
Advisor:  Amir Goldan
Co-Advisor: Petar Djuric

Amorphous selenium (a-Se) is a promising material for large-area imaging detectors, especially in applications requiring low-light and low-dose radiation detection. Its direct conversion property and wide bandgap provide excellent spatial resolution and sensitivity for high-energy X-ray detection.

To meet the demands of high-energy and high spatial resolution X-ray detection in synchrotron light source applications, we have successfully fabricated a-Se detectors by thermally evaporating a-Se onto a Mixed-Mode Pixel Array Detector (MM-PAD) Application Specific Integrated Circuit (ASIC). This work encompasses the entire process from detector fabrication to performance evaluation, including Modulation Transfer Function (MTF) measurements and simulations. To achieve a low dark current device, we incorporated a high-k, non-insulating strontium titanate layer. This layer significantly reduces the electric field at the HBL/high-voltage-metal-electrode interface, limiting Schottky injection from the high-voltage electrode and preventing Joule heating from crystallizing the amorphous selenium layer. This structure achieves reliable and repeatable amplification gain without irreversible breakdown. Initial results demonstrate a low dark current density of approximately 0.1 nA/cm(^2). a-Se, due to its low-cost deposition process and predominantly single-carrier multiplication process without memory, has the potential to be a true solid-state photomultiplier.

Additionally, we have fabricated a novel structure known as the field-shaped multiwell avalanche detector (SWAD). The SWAD is designed for unipolar time-differential charge sensing. Incorporating a high-k blocking layer in a-Se photodetectors is crucial for achieving excellent performance. Our primary focus is on developing and evaluating a SWAD device with a high-k blocking layer to enhance the capabilities of the a-Se photodetector for advanced X-ray imaging applications