Development of High-performance Flexible Poly-Si Thin Film Transistors and GaAs PIN Photodiode for Advanced X-ray Imaging
Digital X-ray imaging techniques are widely adopted for medical diagnosis and industrial inspections. However, the existing flat panel detectors 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 photodiodes. Unlike traditional flat panel detector with the passive pixel sensor (PPS) design, the APS design allows charge amplification before passing charge onto external readout circuit. A charge gain > 30 can be easily achieved which effectively eliminates the post-amplification noise. The overall pixel noise of 608 e is well below that of the PPS circuit. MILC TFTs were fabricated on a flexible metal foil substrate. The outstanding thermal stability of the substrate allows high-temperature processes for TFT fabrication, which is critical to improving device performance by enlarging grain size and reducing defect densities of the active film. The TFTs demonstrate an effective mobility (μeff) of 135 cm2/V∙s, a threshold voltage (VTH) of 2.9 V, an ION/IOFF close to 105, and a subthreshold swing (SS) of 890 mV/dec. Flexible GaAs photodiode was achieved by using a novel water-assisted epitaxial liftoff (H2O-ELO) technology. The heteroepitaxial growth of GaAs by metal organic chemical vapor deposition (MOCVD) is conducted on a fluoride buffer. The water-soluble fluorides enable the GaAs film to be liftoff and transferred to a foreign substrate. This strategy allows large area and flexible GaAs photodiode array to be manufactured at a lower cost. The GaAs photodiode devices demonstrate a solar conversion efficiency of 12 % with an open circuit voltage (Voc) of 567 mV, a short circuit current (Jsc) of 27 mA/cm2, and a fill factor (FF) of 0.77 when operating in photovoltaic mode. Operating in photodiode mode, the devices demonstrate a low dark current (Io) of 10-6 A/cm2 which leads to low photodiode shot noise. Finally, the photodiodes also show fast response to light signal with both rise time (trise) and fall time (tfall) close to 60 μs which increases overall the APS circuit speed.
Networked Digital Library of Theses and Dissertations