Abstract
Amorphous selenium (a-Se) is a glass-former capable of deposition at high rates by thermal evaporation over a large area. It was chosen as a direct conversion material due to its appealing properties for imaging in both low and high X-ray energy ranges (<30 keV and <30 keV, respectively). It has a bandgap of 2.2 eV and can achieve high photodetection efficiency at short wavelengths less than 400 nm which makes it appealing for indirect conversion detectors. The integration of a-Se with readout integrated circuits started with thin-film transistors for digital flat panel X-ray detectors. With increasing applications in life science, biomedical imaging, X-ray imaging, high energy physics, and industrial imaging that require high spatial resolution, the integration of a-Se and CMOS is one direct way to improve the high-contrast visualization and high-frequency response. Over the past decade, significant improvements in a-Se/CMOS technologies have been achieved with improvements to modulation transfer function and detective quantum efficiency. We summarize recent advances in integrating and photon-counting detectors based on a-Se coupled with CMOS readout and discuss some of the shortcomings in the detector structure, such as low charge conversion efficiency at low electric field and high dark current at high electric field. Different pixel architectures and their performance will be highlighted.
Highlights
Amorphous selenium is one of the best photoconductors that was used in the photocopy industry
Technologies, guided by theoretical understanding and experimental verification of device limitations, and include reduced pixel size, architectural and circuit designs, which have led to improvements to modulation transfer functions (MTFs) and detective quantum efficiency (DQE)
Advances have largely been driven by the improvements to complementary metal-oxide-semiconductor (CMOS) technologies, which continue at a rapid pace
Summary
Amorphous selenium (with a bandgap of approximately 2.2 eV) is one of the best photoconductors that was used in the photocopy industry. Understanding the junction of metal-amorphous selenium and its interface with blocking layers for increasing the electric field with a-Se to utilize avalanche multiplication has led to the emergence of a wide spectrum of new applications. It has potential for use in life sciences and biomedical engineering applications [26] and various indirect and direct conversion medical X-ray imaging applications (e.g., fluoroscopy [27], positron emission tomography [28], and single-photon emission computed tomography [29]). We will summarize the progress made in a-Se on CMOS platforms over the last decade, covering low-energy (~10 to 30 keV) X-ray detection for applications such as mammography and a few applications at higher energy (>30 keV), such as synchrotron imaging
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