TECHNICAL EXPLANATION This project addresses fundamental materials science issues associated with defects and impurities in zinc oxide (ZnO). The overall goal of the research is to obtain a complete understanding of the nature and behavior of donors and acceptors in ZnO, so that improvements in the growth and doping of ZnO crystals can be realized. Optical, electrical, and magnetic properties of bulk ZnO crystals will be studied using a variety of techniques including electron paramagnetic resonance, photoluminescence, optical absorption, and Hall effect. ZnO is a wide-band-gap semiconductor with excellent prospects to be a versatile and low-cost ultraviolet light emitter and detector. It is also a major participant in the emerging field of room-temperature spintronics. It has a large exciton binding energy of 60 meV and can easily be grown as large single crystals suitable for use as substrates in homoepitaxial film growth. However, before ZnO is able to reach its full potential, a better understanding of the shallow donors and acceptors must be developed. It is easy to produce n-type ZnO, but very difficult to produce p-type material. Significant progress in p-doping depends on minimizing the concentrations of unwanted donors and, at the same time, incorporating isolated acceptors that result in the desired electrical behavior. This is the motivation for investigation of donors that occur in ZnO without deliberate doping (e.g., native defects such as oxygen vacancies or zinc interstitials, or perhaps trace impurities in the starting materials). It is well known that the conductivity of ZnO is easily increased by annealing crystals in zinc vapor at temperatures near 1100C. The approach will be to determine whether the increase is caused by oxygen vacancies or zinc interstitials. A parallel set of experiments will explore the possibility that annealing ZnO in vacuum, nitrogen, or other reducing atmospheres increases the number of zinc interstitials in the material when a zinc-rich layer forms at the surface due to the removal of oxygen. In a second part of the project, large concentrations of oxygen vacancies and zinc vacancies will be produced by irradiation with high-energy electrons and protons. Electron paramagnetic resonance provides clear identification of such defects, and allows their associated absorption bands and luminescence features to be established. This information will then be used to monitor these defects in ZnO grown by different techniques and subjected to various anneal conditions. The third and fourth parts of the project address the electronic structure of acceptors such as nitrogen and lithium and the role of hydrogen in ZnO (i.e., to assess whether it is a shallow donor or simply a passivator of acceptors). The final two parts of the project address the properties of isolated transition-metal ions in ZnO (e.g., Mn2+, Co2+, V2+, Ni3+, Fe3+ and Cu2+) and the origin of an unstructured green luminescence band. Through this approach the project expects to identify the fundamental mechanisms that control the electrical conductivity of ZnO bulk crystals and thin films. NON-TECHNICAL EXPLANATION The project addresses fundamental materials research with strong technological relevance to electronics and photonics, and effectively integrates research and education. Project activities include campus visits and presentations at non-Ph.D. institutions in the region (Marshall University, Frostburg State University, West Virginia University Institute of Technology, West Virginia Wesleyan College, Fairmont State College, and Wheeling Jesuit University), the integration of ZnO-based research examples (including demonstrations) into the undergraduate solid-state physics course at West Virginia University, and the selection of an undergraduate physics or engineering major from an underrepresented group to work on the project (during the academic year and in the summer).
