Efficient sensing and imaging of low-light signals down to the single photon level with a true solid-state photomultiplier has been a long-standing quest with a wide range of applications in astronomy and spectroscopy, medical imaging, and the rapidly developing field of quantum optics and quantum information science. Low-light detection technology, which utilizes the avalanche phenomena for increasing the signal-to-noise ratio (SNR), is an extremely powerful tool that enables a deeper understanding of more sophisticated phenomena. The measurement under light-starved conditions offers the following unique advantages: nondestructive analysis of a substance, high-speed time-of-flight properties, and single-photon detectability. The most popular commercial detector for low-light detection with high dynamic range and linear mode operation is the vacuum photo multiplier tube (PMT). Although the main advantage of PMTs is high gain (typically 105-108) with low excess noise and room temperature operation, they are bulky and fragile, have poor quantum efficiency in the visible spectrum, are insensitive to infrared light and highly sensitive to magnetic fields. In comparison, the key advantages of solid-state technology are ruggedness, compact size, insensitivity to magnetic fields, and excellent uniformity of response. In practical avalanche detectors, the amount of enhancement in SNR is often severely limited by excess noise caused by the stochastic nature of the avalanche impact ionization process and the optimal SNR typically occurs at very low gain values. The magnitude of this noise enhancement depends on several factors such as the intrinsic material properties, detector device structure, and biasing conditions.
This project investigates a true solid-state alternative to the vacuum photomultiplier tube (PMTs) using amorphous selenium (a-Se) as the bulk avalanche i-layer, and a high permittivity/high-κ dielectric hole-blocking n-layer. Amorphous Selenium (a-Se) based solid-state detectors have some very distinct advantages. a-Se is readily produced uniformly over large area at substantially lower cost compared to crystalline semiconductors. It is the only amorphous material that produces avalanche at high fields. a-Se is the only exception to Webb’s criterion because only holes become hot carriers and undergo avalanche multiplication, and consequently, avalanche selenium devices are linear-mode devices with a negligibly small excess noise. Note that commercially, avalanche gain in a-Se enabled the development of the first optical camera with more sensitivity than human vision and, for example, capable of capturing astronomical phenomena such as auroras and solar eclipses. a-Se is a wide bandgap (2.1 eV) room-temperature semiconductor with ultra-low thermal generation of carriers even at high fields (lowest ever measured dark current density ~ 30 pA/cm2 at the onset of impact ionization ~ 70 V/µm as compared to any avalanche photodetectors). Thus, the material is highly stable at room temperature and devices utilizing a-Se as the active transport layer do not require cooling. Moreover, the a-Se layer can be deposited over thin-film-transistors (TFT) in the read-out electronics at temperatures that would not damage the underlying TFT-based active-matrix readouts (below ~ 200 oC).
Publications:
- L. Ho, A. Mukherjee, D. Vasileska, J. Akis, J. Stavro, W. Zhao, and A. H. Goldan, ”Modeling Dark Current Conduction Mechanisms and Mitigation Techniques in Vertically Stacked Amorphous Selenium-Based Photodetectors,” ACS Appl. Elec. Mat. 3 (8), 3538 (2021). Paper link
- A. Mukherjee, D. Vasileska, J. Akis, and A. H. Goldan, “Monte Carlo Solution of High Electric Field Hole Transport Processes in Avalanche Amorphous Selenium” ACS Omega 6, p.4574-4581 (2021). Paper link
- H. Kannan, J. Stavro, A. Mukherjee, S. Léveillé, K. Kisslinger, L. Guan, W. Zhao, A. Sahu, and A. H. Goldan, “Ultra-low Dark Currents in Avalanche Amorphous Selenium Photodetectors using Solution-processed Quantum Dot Blocking Layer” ACS Photonics, 7, p. 1367 (2020). Paper link