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Role of the surface on electronic properties of GaN and ZnO

The wide-bandgap semiconductors of GaN and ZnO have gained unprecedented attention due to their unique applications in blue laser diodes and LEDs, optical detectors, high-power amplifiers, and chemical/gas sensing. Progress, however, is still challenged by the high density of defects in GaN and the lack of reliable p-type doping for ZnO. Another important issue, although sometimes neglected, is the role played by surfaces and interfaces in the electrical and optical properties of these semiconductors. Such surface-related effects can result in the reduced efficiency of emitters, shorter laser operation lifetimes, and earlier degradation of electronic devices. In particular, it is possible that the irreproducible and unstable p-type conductivity of ZnO is related to peculiarities of the surface conductivity in this material. Compared to the wealth of information now available on the bulk properties of GaN and ZnO, there is a relatively small amount of useful and reliable information on their surface properties. In this research program, an in-depth investigation will be undertaken to gain a comprehensive understanding of the processes at and near the surface for both GaN and ZnO. The electrical and optical properties of these systems will be probed under different ambient conditions, temperatures, and illumination. Band bending near the surface and its variation under illumination (photovoltage) will be studied using two unique methods: 1) a Kelvin probe combined with an optical cryostat; and 2) a microscopy technique combining local charge injection with subsequent imaging of the surface charge. As a result of these studies, the underlying mechanisms for surface band bending will be related to sample preparation, temperature, and ambient environment. Effective passivation schemes will also be explored to improve the performance of optical and electronic devices based on these wide-bandgap semiconductors.

Local electronic properties of quantum-confined semiconductor nanowires

In this research program, an in-depth investigation will be undertaken to gain a comprehensive understanding of the electrical behavior of ZnO nanowire systems. One-dimensional semiconductor nanostructures have attracted a great deal of attention primarily due to their potential applications as gas sensors, and as electronic and light-emitting nanodevices. In particular, ZnO nanowires are especially promising for the realization of novel nano-scale light emitting devices in the blue-ultraviolet spectral range. Although the optical properties of ZnO nanowires are beginning to receive attention, there is very little discussion in the literature concerning their electrical properties. Furthermore, many techniques for well-controlled nanowire growth are prohibitively expensive and lack scalability, which is a concern for future commercialization. Using conductive atomic force microscopy (CAFM), we will investigate the single-wire I-V behavior and photo-response of electrically isolated nanowires grown in porous anodic aluminum oxide (AAO) templates. We will also investigate the nature of the electronic states at the interface and charge transfer mechanisms using a novel technique that we have developed called electronic pump-probe AFM. We will synthesize ZnO nanowire arrays using low-cost synthesis techniques that are both scalable and accessible to industry. Specifically, nanowires will be deposited into AAO films via electrodeposition, direct RF sputtering, and a novel technique utilizing DC sputtering of Zn metal in a reactive gas atmosphere. These investigations will provide a fundamental understanding of confinement effects in the wide bandgap ZnO, which could lead to higher reliability, longer lifetime, and higher efficiency bright light sources, UV detectors, and gas sensors. Further development of our novel DC sputtering technique could result in highly efficient, controllable, and low-cost nanowire growth. Aspects of this research will also be incorporated in Longwood University's Modern Physics and Electronics labs, such as photoluminescence studies of quantum-confined systems, and fabrication and characterization of simple p-n junctions.

Construction of a low-cost scanning tunneling microscope

STM produces real-space images of a surface with atomic-resolution. Its operation relies on the quantum mechanical tunneling of electrons through a finite potential barrier. A sharp metal tip is brought to within a few tenths of a nanometer of a conducting sample surface. A potential is applied between the tip and surface, causing electrons to tunnel from the tip to the surface. The probability of electron transmission, and therefore the tunneling current, is exponentially dependend on the magnitude of the tip-surface gap. By manipulating the gap size with piezo-electric devices to maintain a constant current and rastering the tip across the surface, information about the topography and electric structure of the surface can be obtained. Essentially, the STM is a sort of "brail microscope" that allows us to "see" atoms on surfaces. Using principles of quantum mechanics, we can create a device that uses a sharp tip to "feel" what the surface of a conducting or semiconducting surface looks like with a resolution allowing for the determination of individual atom placements.