Charge Transfer in Oxide Semiconductors from a Defect's Point of View

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Nontechnical description: All of today's electronic gadgets, from cell phones to LEDs, are based on a class of materials called semiconductors. From single element materials like Si, which initiated the computer revolution, to multi-element materials like GaN, which ushered in the lighting revolution, semiconductors form the heart of electronic technology today. A subset of electronic materials known as oxide semiconductors have traditionally been used as gas sensors and optical windows. However, recent studies demonstrate the potential of oxides for electronic device applications as well. For instance, they can relatively easily be made into large sized crystals, a key factor for bulk device manufacturing. On the other hand, controlling the conductivity, which is essential for a successful electronic material, is not so easy. Therefore, the bulk of the research activities focus on investigating the source and mechanisms of the conductivity inherent to the potentially exciting family of oxide semiconductors. In part, the high conductivity of as-grown oxides crystals is determined by various types of defects. Better understanding of the nature, evolution and interaction of the defects in these materials, help determine the appropriate steps needed to control the defects and, therefore, the resulting conductivity. By participating in this research, undergraduate and graduate students benefit from long-time collaboration with experts in the field located at, for instance, the Naval Research Laboratory and various industrial manufacturing facilities.Technical description: The relationship between the conductivity of as-grown oxide semiconductors and the numerous defects plaguing the material is an ongoing debate. The research project aims to minimize the debate by investigating the oxides utilizing a technique which can simultaneously identify the defects and their effect on conductivity. Specifically, the work investigates the link between charge transfer among point defects and the inherent conductivity in a subset of oxide semiconductors most pertinent to electronic applications, i.e. tin oxide, indium oxide and gallium oxide. The method employed, time-dependent photo-induced electron paramagnetic resonance (photo-EPR), combines the power of magnetic resonance with time evolution of an optical response to monitor charge transfer among defects uniquely identified by EPR. The analysis of the time-dependent data depends on a model developed from knowledge of the various impurities in the material as well as the crystal structure. Therefore, samples are characterized with techniques such as secondary ion mass spectrometry for impurity content and x-ray diffraction for crystal structure. Armed with the basic information provided by the characterization techniques, a unique model is developed and the EPR data are interpreted in terms of charge transfer parameters such as defect levels and thermal barriers. Significantly, because time-dependent photo-EPR probes a specific defect, the parameters may be directly assigned to that specific defect. Thus, the ultimate outcome of the work is the association of an identified defect with charge transfer parameters unique to that defect. The knowledge obtained is then used to tune growth conditions for desired conductivity.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.