ECE Deparmental Seminar
Sensing Hot Carriers in Glass
Prof. Amir H. Goldan
Stony Brook University, Department of Radiology
Friday, 11/10/17, 11:00am
Light Engineering 250
Abstract: Amorphous solids have been subject of international interest for a century and wide-spread research in this subject area, ushered by Leningrad experiments that showed semiconducting behavior in various chalcogenide glasses, has been stimulated by commercial applications such as Ovonic switching and memory devices, Xerography, Xeroradiography, thin-film devices, solar cells, and digital radiography. Recently we showed that amorphous (or glassy) selenium (a-Se) also shows promise for picosecond time-of- flight applications such as positron emission tomography or PET. This is because of the presence of avalanche multiplication gain in a-Se due to impact ionization of holes at high electric fields. One major problem in amorphous devices, which are easier and less expensive to develop than their crystalline counterparts, is low carrier mobility and consequently, poor temporal performance. To overcome this problem, we proposed the concept of unipolar time-differential (UTD) charge sensing in non-dispersive amorphous semiconductors. Inspired by Charpak’s Nobel prize winning invention, we recently fabricated the first UTD detector using a multi-well Frisch grid substrate and a-Se as the photoconductive film. The time-of- flight experimental results showed more than 2 orders-of- magnitude improvement in impulse response time. Our goal is now to combine UTD charge sensing and avalanche multiplication gain in one device. For a-Se operating in the avalanche mode at high electric fields, charge drift occurs via band transport in extended states with non-activated microscopic mobility, and thus, photocarriers experience negligible interruption by capture and thermal-release events due to shallow traps. The implication of (1) non-activated microscopic band-mobility, (2) avalanche gain, and (3) UTD charge sensing are the realization of a detector that can achieve picosecond time-resolution with a material that is low-cost and uniformly scalable to large-area. In addition, we are trying to prove that avalanche multiplication gain in a-Se (due to impact ionization of hot hole carriers) is noise-free and Non-Markovian. To prove this, we require a Monte Carlo solution to the Semi-classical Boltzmann transport equation and need to show that as each hole carrier is accelerated toward the pixel, phonon scattering processes delay its attainment of the ionization threshold.
Bio: Amir H. Goldan is currently an assistant professor of Radiology at Stony Brook University where he is working on the development and fabrication of medical imaging detectors for time-of- flight positron emission tomography (PET) and photon-counting mammography. Dr. Goldan received his Ph.D. in electrical and computer engineering from the University of Waterloo in CANADA. During his Ph.D. studies, he introduced the novel concept of unipolar time-differential charge sensing to overcome the problem of poor charge transport in low-mobility amorphous detectors and to enable them achieve temporal performance similar to their high-mobility crystalline counterparts. Dr. Goldan has recently been awarded his first NIH-R21 grant for the feasibility study of picosecond noise-free avalanche selenium detectors for time-of- flight PET.