SUBJECT: Ph.D. Dissertation Defense
BY: Arith Rajapakse
TIME: Tuesday, November 2, 2021, 11:00 a.m.
PLACE: Boggs, 3-47
TITLE: Sensing the Interactions of Ionizing Radiation in a Semiconductor Substrate using Carbon Nanotubes
COMMITTEE: Dr. Anna Erickson, Chair (NRE/MP)
Dr. Steven Biegalski (NRE/MP)
Dr. Chris Wang (NRE/MP)
Dr. Lei Raymond Cao (The Ohio State University)
Dr. Ilia Ivanov (Oak Ridge National Lab)


The production and collection of charge carriers or photons in a bulk medium is the basis behind the detection of ionizing radiation. This core operating principle established the requirements of acceptable detection efficiency and complete energy deposition which, in turn, set volume and atomic number restrictions on these bulk mediums. As a result, the application of nanomaterials in radiation detection is often hindered by their nanoscale dimensions material composition. This work investigates the use of vertically aligned carbon nanotubes (CNTs) as a sensing volume for ionizing radiation. Specifically, CNTs are integrated into a global gate field effect transistor geometry where the gate voltage controls the conductance of the device by modulating the Schottky barrier between the CNT channel and the source and drain electrodes. Similar to a p-channel metal-oxide-semiconductor field effect transistor (MOSFET), the conductance of the CNT-based detector increases at negative gate voltages. In the presence of diagnostic range x-rays, the current flowing through the device increases from the baseline noticeably due to the radiation induced electron-hole pairs adding to the electric field felt by the CNT/electrode interface. Despite the scale of the device, it is sensitive to x-rays of varying energy and flux. The device also demonstrates considerable inherent photoconductive properties which can be utilized to further increase the detection efficiency of ionizing radiation or be used for new photodetection applications entirely. This novel device demonstrates the promise of integrating nanostructured materials into traditional semiconductor substrates for radiation detection. (