Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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Subject: search.filters.subject.Molecular Beam Epitaxy

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    OPTIMIZATION OF PLASMA ASSISTED MOLECULAR BEAM EPITAXY GROWN NbxTi1-xN FOR EPITAXIAL JOSEPHSON JUNCTIONS
    (2023) Thomas, Austin Michael; Richardson, Christopher; Takeuchi, Ichiro; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis is an investigation into the growth and characterization of NbNx and TiN transition metal nitrides, along with the alloy NbxTi1-xN. These materials are commonly used in many applications ranging from superconducting quantum computing, superconducting conventional computing, high kinetic inductance devices such as single photon detectors, and hard coatings for industrial applications. This thesis will begin with an overview of superconducting quantum computing and superconducting materials, then review the fabrication of Josephson junctions and highlight the need for material improvement. The goal of this work is to grow a superconducting nitride material which can be engineered to lattice match with AlN, the barrier layer in a hypothetical all-nitride, epitaxially grown superconducting quantum computing structure. The alloy NbxTi1-xN is chosen as the superconducting alloy of choice due to the range of lattice constants available, the high critical temperature of these nitrides, and the high quality of material able to be grown using PAMBE. The first aim of this thesis studies the binary transition metal nitrides NbNx and TiN to generate endpoints for various properties of the alloy NbxTi1-xN. This thesis is one of the first investigations of multi-phase growth of ε-NbN and γ-Nb4N3, and demonstrates control over the phase, crystal orientation, superconducting properties, and surface morphology by changing PAMBE growth parameters. The second aim of this thesis demonstrates the growth of NbxTi1-xN and is the first investigation of tunable material properties for this alloy by adjusting the composition. The last aim of this work is the development of a novel annealing scheme used to prepare NbxTi1-xN thin films for Josephson junction integration. The novel annealing scheme ensures excellent surface roughness of NbxTi1-xN thin films, increases the superconducting critical temperature of this alloy from approximately 14 K to 16.8 K, and improves the crystal quality by way of nitrogen incorporation and improvement of the crystal quality. The results from this work will be crucial in developing NbxTi1-xN / AlN / NbxTi1-xN Josephson junctions with smooth, uniform interfaces and low-loss, defect free nitride materials. Additionally, this thesis represents an investigation into the relationship between phases of NbNx and TiN, the role of nitrogen incorporation caused by in-situ annealing, and a useful record of control over this material using PAMBE growth conditions and alloy composition.
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    DIRECTED SELF-ASSEMBLY OF NANOSTRUCTURES AND THE OBSERVATIONS OF SELF-LIMITING GROWTH OF MOUNDS ON PATTERNED CRYSTAL SURFACE DURING EPITAXIAL GROWTH
    (2012) Lin, Chuan-Fu; Phaneuf, Raymond J; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    In this thesis I describe an approach toward investigating moving interfaces, surface stabilities and directing self assembly of nanostructures, using lithographic patterning to perturb a flat crystalline surface over a range of spatial frequencies, followed by epitaxial growth. GaAs(001) shows a transient instability toward topographical perturbations. We model this behavior using an Ehrlich-Schwoebel (ES) barrier which impedes the diffusion of atoms across steps from above. We show via both kinetic Monte Carlo (kMC) simulations and molecular beam epitaxial (MBE) growth experiments that patterning in the presence of an ES barrier can be used to direct the self assembly of mounds. Second, as we track the time evolution of mound formation, we find the evidence of "Self-Limiting Growth" on surfaces - we find that in the initial stage of growth, the pattern directs the spontaneous formation of multilayer islands at 2-fold bridge sites between neighboring nanopits along [110] crystal orientation, seemingly due to the presence of an Ehrlich-Schwoebel barrier and the effect of heterogeneous nucleation sites on the surfaces. However, as growth continues, the height of mounds at 2-fold bridge sites "self-limits": the mounds cease to grow. Beyond this point an initially less favored 4-fold bridge sites dominate, and a different pattern of self assembled mounds begins. The observation of self-limiting behavior brings us new understanding of mechanism for crystal growth. We also find that the transient amplification of pattern corrugation during growth is correlated with self-limiting behavior of mounds. We propose that a minimum, `critical terrace size' at the top of each mound is responsible for the observed self-limiting growth behavior. Finally, the observation of the sequence of the mounds forming on the patterned surfaces gives us rather direct evidence that the formation of growth mounds on the surface is a nucleated process, rather than an instability.
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    Silicon-Germanium Photodetectors for Optical Telecommunications
    (2012) Ali, Dyan; Goldhar, Julius; Richardson, Christopher J.K.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis investigates the design and growth of silicon-germanium p-i-n photodetectors for optical telecommunications applications. Two types of heterostructures are considered: strained silicon-germanium layers grown directly on silicon substrates, and strain-balanced silicon-germanium/silicon superlattice grown on relaxed buffer layers. The heterostructures are designed using existing band structure models and are grown using solid source molecular beam epitaxy (SS-MBE). To facilitate these growths, an atomic absorption spectroscopy- based flux monitor for the silicon source is developed and calibrated. In addition, the development of a substrate preparation procedure for relaxed buffer layers that is compatible with SS-MBE is developed and allows the growth of epitaxial films with low defect densities. P-i-n diodes processed from these films are shown to have low reverse leakage currents densities compared to other competing devices. Photocurrent spectroscopy is used to characterize these structures. A clear reduction in the bandgap of the heterostructures over that of the constituent alloys due to exploitation of the Type-II band offsets in the silicon-germanium material system is demonstrated in both, the strained and strain-balanced photodetectors. Finally, the low leakage current densities are exploited to fabricate devices with noise equivalent powers comparable to or better than competing approaches based on the growth of germanium on silicon substrates.
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    TEMPERATURE DEPENDENCE OF THE GROWTH MODE DURING HOMOEPITAXY ON PATTERNED GALLIUM ARSENIDE (001); ATOMIC-SCALE MECHANISMS FOR UNSTABLE GROWTH.
    (2006-12-11) tadayyon-eslami, tabassom; Phaneuf, Raymond j; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    ABSTRACT Title of Document: TEMPERATURE DEPENDENCE OF THE GROWTH MODE DURING HOMOEPITAXY ON PATTERNED GALLIUM ARSENIDE (001); ATOMIC-SCALE MECHANISMS FOR UNSTABLE GROWTH. Tabassom Tadayyon-Eslami, Doctor of Philosophy, 2006 Directed By: Professor Raymond J. Phaneuf, Department of Materials Science and Engineering In this thesis we present an extensive investigation of instability in molecular beam epitaxial growth of GaAs(001) over a range of pattern periods, cell sizes, growth temperature and As2 flux. We find very good agreement with predictions of the continuum models of Sun, Guo and Grant [Phys. Rev. A 40, 6763(1989)] for the growth above ~540ºC and Lai and Das Sarma [Phys. Rev. Lett. 66, 2348 (1991)] for the growth below this temperature. Changing the growth temperature to lower than 540 ºC leads to the formation of ring-like protrusions in the [110] direction around pits patterned on the initial substrate, which are absent for growth at higher temperature. This change in growth mode occurs in the temperature range within in which both pre-roughening transition and surface reconstruction transition (β2(2x4) to c(4x4)) also occur. We rule out the possibility of preroughening and the change in surface reconstruction as the reason for this growth mode change, based on the As2 flux dependence of the growth mode transition temperature. Based on our atomic force microcopy characterization of the surface morphology during early the stage of growth, we propose a physically based model for the growth, which involves a competition between decreased adatom collection efficiency during growth on small terraces and a small anisotropic multiple step Ehrlich-Schwoebel barrier at the pit edge. This provides a physical basis for the nonlinear term in the continuum models proposed by Sun et. al., and Lai and Das Sarma, whose predictions qualitatively describe our experimental observations.