Radiation detectors are critically important for environmental controls and monitoring of hazardous radioactive materials at airports and many others safety critical locations. Advanced nuclear detection is a timely issue for national security, nuclear power plants, and for military security. This project aims to develop highly sensitive, compact nuclear detectors using aluminum gallium nitride crystalline films produced using the same technology which was used for blue-green-white light emitting diodes and it was awarded the 2014 Nobel Prize. More recently an innovative extension of this technology using Aluminum Gallium Nitride materials has led to ultraviolet LEDs for air and water purification and power electronics for electric vehicles and advanced military radars. These developments offer low-cost and high sensitivity performance, with the potential of integrating functionalities such as lighting, ultraviolet detection, as well as radio transmission on a single microchip. This project proposes material and device innovations, to explore the use of Aluminum Gallium Nitride technology for low-cost, compact and highly sensitive nuclear radiation detectors. The challenge is the production of high quality material, which can withstand high temperatures and harsh environments, such as in a nuclear power plant. The team proposes to produce these materials and working electrical devices to benchmark against existing higher cost, bulkier legacy technology. The proposed work will lead to the education of at least 2 PhD students, 1 African American and 1 military veteran currently in the team's group, who will go into jobs in either government research or advanced manufacturing. The proposed research will further cement University of South Carolina?s track record of excellence in Aluminum Gallium Nitride materials for harsh environment electronics. During the PI's sabbatical at Morgan State University, a historically black university in Baltimore, MD, the devices produced in this work will be integrated into senior design projects.
The team proposes to develop a low-noise, high speed, ionizing radiation detector using ultra-wide bandgap aluminum gallium nitride epitaxial layers on aluminum nitride/sapphire templates. This ternary material is radiation hard and leads to devices with very low leakage currents even in harsh environments. It allows for monolithic integration with readout and power conditioning electronics, as well as other functionalities such as ultraviolet light sources. The proposed detector, a 2-5?m thick channel field effect phototransistor with a high internal current gain and low dark current will be grown by metalorganic chemical vapor deposition. It will be ideal for detecting pulses of radiation in Geiger mode, and eventually higher penetration radiation using thicker absorbing layers. The program exploits the shallow penetration deep ultraviolet light to improve materials development for thicker layers for soft beta radiation from Nickel-63. The ability to use monochromatic light enables characterization with spectral selectivity to the bandgaps, not possible with broadband beta-illumination. The team's initial experiments showed noise equivalent power <5fW, although these transistors had slow response times ~20s. Through a noise study, this was attributed to charge trapping at the aluminum nitride template/channel growth interface. The high current gain was partially a consequence of trapping induced photoconductivity. The growth solutions consist of electrically isolating this interface from the transistor channel, either with a thick strain engineered layer and/or a graded back barrier layer. Thus, any crystal growth strategy or device architecture that speeds up the device will lower photocurrent, but the Lorentzian noise arising from slow traps will also be reduced. Thus the tradeoff between current gain and speed, endemic to all detectors, is complicated by noise considerations, leading to the central question: How far can NEP and response time be decreased simultaneously by eliminating the influence of traps Initial analyses indicate that Nano-second to micro-second response times are possible, consistent with recombination times in direct gap semiconductors. The capability of engineering thick channel transistor layers directly translates to power electronics as well, as it enables the ability to block high voltages.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
