[CCoE Notice] PhD defense announcement

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[Dissertation Defense Announcement at the Cullen College of Engineering]
Development of High-performance Flexible Poly-Si Thin Film Transistors and GaAs PIN Photodiode for Advanced X-ray Imaging

Bo Yu

November 29, 2022; 12:00 PM - 2:00 PM (CST)
Location: Mechanical Engineering Big Conference Room (Eng 1, #202)


Committee Chair:
Venkat Selvamanickam, Ph.D.

Committee Members:
Jae-Hyun Ryou, Ph.D. | Bo Zhao, Ph.D. | Wanda Zagozdzon-Wosik, Ph.D. | Jinghong Chen, Ph.D.

Abstract

Digital X-ray imaging techniques are widely adopted for medical diagnosis and industrial inspections. However, the existing flat panel detectors (FPD) fail to meet the challenges posed by large-area and curved surface imaging. They are also inadequate for low radiation dose and high spatial resolution applications such as digital breast tomosynthesis (DBT) and fluoroscopy. In this work, a flexible imager which can conform to non-flat surface and potentially be manufactured at lower costs is designed with active pixel sensor (APS) circuit architecture which is enabled by metal induced laterally crystallized (MILC) thin film transistors (TFTs) and GaAs PIN photodiode (PD). Unlike traditional FPD with PPS design, the APS design allows charge amplification before passing charge onto external readout circuit. Charge gain > 100 can be easily achieved which effectively eliminate the post-amplification noise. The overall pixel noise of 984 e is well below that of PPS circuit.
MILC TFTs were fabricated on flexible metal foil substrate. The outstanding thermal stability of the substrate allows high temperature processes for TFT fabrication which is critical to improving the device performance by enlarging grain size and reducing defect densities of the active film. The TFTs demonstrate an effective mobility of 135 cm^2/V∙s, a threshold voltage of 2.9 V, an ION/IOFF close to 10^5, and a subthreshold swing of 890 mV/dec.
Flexible GaAs PD was achieved with a novel water-assisted epitaxial liftoff technology. Heteroepitaxial growth of GaAs by MOCVD is conducted on fluoride buffer. The water-soluble fluorides enable the GaAs film to be liftoff and further transferred to a foreign substrate. This strategy allows large area and flexible GaAs PD array to be manufactured at lower cost. The GaAs PD devices demonstrate a solar cell efficiency of 12 % with a Voc of 567 mV, an Isc of 27 mA/cm^2, and an FF of 0.77 when operating in photovoltaic mode. Operating in PD mode, the devices demonstrate a low dark current of 10^-6 A/cm^2 which leads to low PD shot noise. Finally, they also show fast response to light signal with both rise time and fall time close to 60 μs which increases overall APS circuit speed.

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