Optically-pumped NMR Enhancements Enable Studies of Semiconductor Interfaces

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Non-Technical Description: The basis of all modern electronics relies on multilayered materials composed of semiconductors. Here, the investigators are developing instrumentation to determine the specialized chemical structures and the 3-dimensional forms in interface regions where different semiconductor layers are in contact with one another. These methods combine laser excitation with nuclear magnetic resonance detection to provide atomic-level information critical to improving the performance of these modern electronic devices. This project includes collaboration with the National High Magnetic Field Lab and will impact semiconductor device fabrication industries. The research innovations are integrated with multidisciplinary courses offered at Washington University for educating and training students. The collaboration also provides new research networks for team members, enhancing the professional development of students and postdoctoral researchers. Informal science education activities for high school teachers are offered through the Institute for School Partnership.Technical Description: This research interrogates the structure of semiconductor interfaces by using enhanced signals that are created through optical pumping of conduction electrons. Optical pumping in the optically pumped nuclear magnetic resonance (OPNMR) localizes the detected signal to interfacial regions and greatly enhances the sensitivity, transforming NMR from a bulk to a surface technique. In collaboration with the National High Magnetic Field Lab, laser access and NMR sensitivity are improved by utilizing a new coil design based on high-temperature superconductors that permits unimpeded laser access and higher performance than conventional solenoids. Such spectroscopy can enable engineering of devices by revealing the underlying makeup of the interface, and examining strain induced by defects, lattice mismatch, and dopants. Model structures of GaAs coated with deposited metal oxide thin films, such as alumina and hafnium dioxide are probed. The OPNMR is developed to adapt these methods for studying other metal-oxide semiconductor devices to examine the role of defects in Fermi-level pinning at this interface. This information is critical to improving performance of metal-oxide semiconductor devices. The high polarization achieved is utilized to measure the magneto-optical penetration depth and ultimately the nuclear polarization. To date, these values have only been estimates but are crucial for understanding the observed spins and the effectiveness of the optical pumping, explicitly using nuclear spin temperature.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.