High gain lateral amorphous selenium (a-Se) detector for medical imaging
Authors: Shiva Abbaszadeh, Kai Wang, Nicholas Allec, Feng Chen and Karim Karim.
Abstract:
Amorphous selenium (a-Se) is a well known photoconductor and has been used in both indirect and direct conversion x-ray detectors for a variety of medical imaging modalities such as mammography. It goes without saying that interest for having a photodetector with higher gain never ceases. There has been a lot of research on taking advantages of the avalanche multiplication phenomenon inside a vertical a-Se structure to produce high internal gain in the photodetector (e.g. HARP camera for low-light settings). The fast response time and high gain of the a-Se avalanche photodetector makes it a promising candidate to replace photomultiplier tubes (PMT) or silicon avalanche photodiode (APD) based photomultipliers (SiPM) in applications such as positron emission tomography (PET) detectors. Recently, a lateral metal-semiconductor-metal (MSM) a-Se photodetector has been reported as a competitive alternative in terms of ease of fabrication and integration, speed and low dark current. Thus, we believe the lateral structure is also promising for high gain photodetector applications like PET. In this paper, we intend to investigate the effect of increasing electric field on the lateral a-Se structure and compare the results with the modified lucky drift model which presents a good agreement with experimental data on avalanche multiplication in vertical a-Se structures. Our study shows that a gain of ~100 can be achieved in a lateral structure under a modest field strength of 40 V/um. Even though the observed dark current of ~ 800 nA is still far beyond the requirement of a practical detector, the achievable high gain allows us to design better detectors for high-end applications such as PET with a unique lateral approach.