Technical: The proposed work will experimentally study Zinc oxide (ZnO) semiconductor that has attracted resurgent interest as a material for optoelectronic applications. While ZnO has many inherent advantages, the lack of control over dopants and defects presents an obstacle to the realization of practical devices. Reliable p-type conductivity, necessary for widespread applications, has been elusive. A key challenge is to introduce acceptors without being overwhelmed by compensating donors. Hydrogen plays a central role in acceptor doping of ZnO as well as unintentional n-type conductivity. The proposed work will focus on different forms of hydrogen in ZnO and related materials. Specifically, interstitial hydrogen donors, substitutional hydrogen donors, acceptor-hydrogen complexes, and vacancy-hydrogen complexes will be investigated with a range of experimental techniques. In this renewal proposal, research will be extended from ZnO to other oxide crystals, including tin oxide and strontium titanate. The proposed work will shed light on the origins of n-type conductivity in transparent conducting oxides, an open question with technological ramifications. The overall goal of the project is to investigate the behavior of hydrogen in ZnO and other oxide crystals. The research goals will promote a fundamental understanding of hydrogen in semiconductors. By extending the research to oxide crystals, the PI will investigate the possibility of universal behavior across different materials systems. The most "high-risk" goal is the achievement of p-type ZnO. Reliable p-type conductivity in ZnO would be transformative, opening up a range of novel applications. This renewal project builds on significant work by the PI's research group that resulted in high-impact publications, invited talks, Ph.D. dissertations, and undergraduate research experiences.
The project addresses basic research issues in a topical area of materials science with technological relevance, and is expected to provide unique opportunities for graduate and undergraduate training in an interdisciplinary field. This research project is also expected to have broader impacts through the training of scientists in this research field, through the wide dissemination of the findings of this research through publications. The proposed educational activities will also benefit a range of students from elementary to Ph.D. In conjunction with the research part of this proposal, educational and outreach activities will encourage students from diverse backgrounds to enter fields related to materials science and solid-state physics.
Oxide materials, once thought mainly as insulators, are now known to exhibit a dizzying array of properties, from superconductivity to ferromagnetism. In this NSF-funded project, researchers performed experiments to elucidate the role of defects in oxide materials. One material is zinc oxide (ZnO), a promising semiconductor for optoelectronic applications. For a practical device to work, manufacturers must be able to dope the material with impurity atoms called donors and acceptors. Donors contribute negative electrons to the semiconductor, making it n-type. Acceptors take electrons away, leaving positive "holes" that make the semiconductor p-type. While making n-type ZnO is easy, researchers have been continually frustrated in their attempts to make p-type ZnO. In the present work, nitrogen was shown to be a "deep" acceptor. This means that a huge amount of energy must be given to an electron to place it onto the nitrogen atom–much more than the thermal energies found at room temperature. A deep acceptor therefore cannot be used to make p-type material. While this result is "negative," the researchers pointed to semiconductor alloys, which are composed of 3 or more elements, as a possible way to obtain the desired properties. A second important oxide material is strontium titanate (STO), a popular substrate for superconductors. The researchers achieved a 400-fold increase in the electrical conductivity of a crystal simply by exposing it to light. Remarkably, this increase persisted for days at room temperature. The phenomenon they witnessed—"persistent photoconductivity"—is a far cry from superconductivity, the complete lack of electrical resistance pursued by other physicists, usually using temperatures near absolute zero. But the fact that they’ve achieved this at room temperature makes the phenomenon more immediately practical. And while other researchers have created persistent photoconductivity in other materials before, this is the most dramatic display of the phenomenon yet. This research provided excellent experience for graduate students. They disseminated their results by writing articles and giving presentations at international conferences. Recent Ph.D. students have gone on to industrial and postdoctoral research positions.
