Physics Theses and Dissertations

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https://hdl.handle.net/1794/2048

This collection contains some of the theses and dissertations produced by students in the University of Oregon Physics Graduate Program. Paper copies of these and other dissertations and theses are available through the UO Libraries.

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Browsing Physics Theses and Dissertations by Author "Aleman, Benjamin"

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    A Tailored Approach to Engineering Solid State Single Photon Sources
    (University of Oregon, 2024-01-09) Klaiss, Rachael; Aleman, Benjamin
    Integrated quantum information technologies such as photonic circuits, quantum transducers, and magnetic sensors require robust single-photon sources in precise locations. Solid-state single photon emitters (SPEs) hosted by mid-bandgap defects in 2D material hexagonal boron nitride (hBN) are bright and stable at room temperature and demonstrate strong coupling to external fields, making them desirable candidates for quantum device applications. However, the specific atomic structure of hBN SPEs remains unidentified, making deterministic engineering a challenge. While recent studies have narrowed the range of possible defect candidates by demonstrating the role of carbon in hBN SPEs, the methods to engineer carbon-based defects in hBN either produce randomly located emitters or require bottom-up crystal growth on structured substrates. We achieved patterned arrays of SPEs via focused ion beam (FIB) milling followed by chemical vapor deposition (CVD) of nanocrystalline graphite source for carbon diffusion, and found that both techniques are necessary for significant and repeatable creation of SPEs. This technique creates localized emitters with ten times the yield of carbon annealing alone. Furthermore, by adjusting the parameters of FIB exposure time and carbon annealing time, we found multiple different parameter combinations that successfully created SPEs, demonstrating the adjustability of this technique based on device application requirements. Additionally, we performed atomic force microscopy to characterize the surface morphology of hBN regions patterned by Ga+ FIB to create SPEs at a range of ion doses and found that material swelling is strongly correlated to successful SPE creation. Furthermore, we simulated vacancy and impurity profiles to elucidate how Ga+ FIB patterning induces lattice damage in the form of vacancies, structural voids, and amorphous layers, creating a diffusion barrier to control the introduction of carbon impurities to engineer isolated SPEs with high resolution of process control. Our results provide novel insight into the formation of hBN SPEs created by high-energy, heavy-ion FIB that can be leveraged for monolithic hBN photonic devices and a wide range of low-dimensional solid-state SPE hosts. This dissertation includes previously published and unpublished coauthored material.
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    The Effect of Copper on the Defect Structure of Cadmium Telluride Thin-Film Solar Cells
    (University of Oregon, 2016-02-23) Warren, Charles; Aleman, Benjamin
    Transient photocapacitance (TPC) and transient photocurrent (TPI) spectroscopy have been used to examine the defect structure in the upper-half of the bandgap of CdTe solar cells, with an emphasis on understanding the effect of copper. TPC spectra reveal two defects in the CdTe devices at optical energies of 1.2eV and 0.9eV with respect to the valence band. The origin of the 1.2eV defect could not be associated with a particular element, although copper and zinc were ruled out as sources. TPI spectra were used to observe that the density of the 1.2eV defect was dramatically reduced by thermally annealing the devices, suggesting that the defect itself is annealed during the treatment. The set of CdTe samples examined used a rapid thermal processing treatment to control the amount of copper that diffused into the CdTe layer from the Cu:ZnTe interfacial layer at the back of the device. Comparison of devices with varying amounts of copper in the CdTe layer revealed that the 0.9eV defect seen in TPC was associated with the presence of copper in the absorber layer. TPI spectra confirmed the association of the 0.9eV with copper and showed that the magnitude of the 0.9eV defect signal increased as more copper was diffused into the CdTe layer. A proportional link between the density of the 0.9eV defect observed in TPI spectra and the amount of copper in the absorber layer observed via ToF-SIMS further established that copper is responsible for the existence of the defect. Numerical modeling of the CdTe devices was used to confirm that the spatial distribution of copper observed in ToF-SIMS was consistent with the relative variation of defect magnitudes observed in TPI. The fact that the copper-associated 0.9eV defect lies close to mid-gap suggests that it will act as an efficient recombination center in CdTe. Therefore, it is suggested that this work has detected the deep defect that is responsible for the decreased minority carrier lifetime that has been previously associated with the amount of copper in the CdTe layer