Technical. This project makes use of a variety of optical and magnetic resonance techniques to determine fundamental behavior of atomic-scale defects in two wide-band-gap semiconducting materials, zinc oxide (ZnO) and aluminum nitride (AlN). Five specific research areas are tar-geted, four in ZnO and one in AlN. In ZnO, these include the trapping of hydrogen in oxygen va-cancies, the release of hidden hydrogen, the formation of defect complexes involving Zn vacan-cies, and the wavelength dependences of photoinduced changes in charge state. In AlN, the pri-mary donors, acceptors, and their simpler complexes will be identified and characterized. The proposed investigations in ZnO and AlN emphasize the use of EPR/ENDOR techniques to iden-tify models of specific defect complexes. At the same time, correlations with optical absorptions and emissions will be established. Sample modifications will include high-temperature anneals and electron irradiations. Magnetic resonance, optical, and electrical characterization tools will be employed; magnetic resonance techniques include electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR), and solid-state nuclear magnetic resonance (NMR). Optical techniques include photoluminescence (PL), photoluminescence excitation (PLE), Raman, and absorption. Temperature-dependent Hall effect measurements will provide electrical information. Non-Technical. The project addresses fundamental research issues in a topical area of elec-tronic/photonic materials science having technological relevance. This project includes graduate and undergraduate student mentoring, visits to non-PhD institutions in the region where the co-PIs will present talks, and the integration of wide-band-gap semiconductor-based research exam-ples (including demonstrations) into the materials physics courses at West Virginia University. Students will gain valuable experience in a wide variety of advanced spectroscopic materials characterization techniques. These students are also provided opportunities, through project col-laborators, to work with scientists at both government labs and other universities. Such interac-tions will ensure a practical applications-oriented side to our fundamental studies. On a broader scale, an increased understanding in the fundamental defect properties of these semiconductors may have significant societal impacts: improved light emitters in the ultraviolet and visible spec-tral regions, more efficient and cost-effective solid-state lighting, improved gas sensors, im-proved scintillators for portable nuclear detectors, photocatalysis of hydrogen, and emerging ar-eas--magnetic semiconductors and spintronics.
This NSF Grant supported a series of fundamental experimental studies of point defects in technologically important single crystals. These defects control the device-relevant properties of these crystals and allow their use as optical and electronic sensors and detectors and as generators of ultraviolet light. Among the practical applications of these materials are neutron detectors used to search for contraband nuclear materials, light emitting diodes and lasers for monitoring biochemical molecules, and photocatalysis where sunlight converts water into hydrogen and oxygen. In this study, the electrical conductivity of zinc oxide crystals was related to specific impurities and it was shown how these impurities control the light emitted by the crystals. The device-limiting role of oxygen vacancies was demonstrated in titanium dioxide crystals. For the first time, the role of hydrogen was clarified in titanium dioxide crystals. Also, it was shown how fluorine and lithium impurities greatly change the electrical properties of titanium dioxide crystals. Based on the understanding of defects generated by this project, lithium tetraborate crystals (with their constituent lithium and boron ions) can now be developed as a neutron detector and a scintillator material. Collaborators in this research included faculty at William Jewell College and scientists at national laboratories and in a defense industry. As a result of this project, three participating graduate students received their PhD degrees in Physics and entered the scientific work force in the USA. Fifteen papers based on research results obtained from this project were published in refereed scientific journals. These results are now available for other scientists and engineers to use as they develop devices based on these materials.
