Materials Science & Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2792
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Item Plasma Oxidized AlOx Tunnel Barriers and Nb/Al Bilayers Examined by Electrical Transport(2022) Barcikowski, Zachary Scott; Cumings, John; Pomeroy, Joshua; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Results are reported for two related projects: the examination of material stability of plasma oxidized, free energy confined aluminum oxide and the evolution of the electronic structure in Nb/Al bilayers as a function of Al thickness. Al/AlOx and Nb are critical materials for solid-state quantum computing, mostly driven by the relatively large superconducting gap of Nb (1.5 meV) and ∼ 2 nm diffusion limited oxide formed on Al with room temperature thermal oxidation. Plasma oxidation and free energy confinement of AlOx with Co electrodes is used to produce homogeneous tunnel barriers with an O/Al ratio approaching Al2O3. The weeks long time stability of resulting metal-insulator-metal tunnel junctions is found to greatly improve, as resistance measured over ≈ 8 months increases by 34.0 ± 5.4 % in the confined devices (Co/AlOx/Co) compared to an increase of 95.4 ± 7.8 % in unconfined devices (Co/Al/AlOx/Co). In the second experiment, normal metal-insulator-superconductor (NIS) tunnel junctions are used to study the interplay of superconducting properties in Nb/Al bilayers as a function of Al thickness. The performance of superconducting quantum information devices is sensitive to thedetailed nature of the superconducting state in the materials, which is drastically altered through proximity in the case of dissimilar materials. I extract the effective Nb/Al quasiparticle DOS from the conductance spectra of NIS tunnel junctions with Nb/Al superconducting electrodes. The conductance spectra evolve from a primarily single-gapped structure for thin Al (< 20 nm) to a dual gapped structure at thicker Al. I present a modified Blonders-Tinkham-Klapwijk (BTK) based model interpreting the conductance spectra as a steady-state convolution of the Al-like DOS and the Nb-like DOS in the bilayer. These results inform future device design for quantum information by providing additional grounding to current proximity effect theory.Item EFFECTS OF DOPING AND DEFECTS IN BaSnO3 AND COVETIC ALLOYS(2018) Jaim, H. M. Iftekhar; Takeuchi, Ichiro; Salamanca-Riba, Lourdes G.; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Doping and defects have played major roles for optimizing the structural, electrical, optical and mechanical properties of materials over the centuries. With the advent of modern fabrication and characterization tools, we are engineering materials by modifying the fundamental structure at the nano-scale. Such research and innovations are necessary to find alternatives of the known materials to meet technological, economic and environmental challenges. In this dissertation, we will discuss two classes of materials to identify the effects of atomic level engineering on the enhanced properties of Covetic alloys and BaSnO3 perovskites. The first part of the thesis is based on the nanoscale and surface characterizations of the carbon doped metal alloys of aluminum and silver produced by the Covetic process. We have found the presence of sp2 bonded 3D epitaxial carbon in graphene nano-ribbon form on the aluminum and silver atomic planes. Such directional presence of sp2 carbon in the crystalline form along with other allotropes will be studied. Here, we have detailed the bonding, strain, defect concentrations, and oxidation characteristics of these compositions, and distinguished Covetic materials from other carbon based composites. Covetic process is significant as it defies the traditional metal-carbon phase diagrams under non-equilibrium conditions. The second part of the dissertation focuses on the BaSnO3, a major candidate in s-band electronics and transparent conducting oxide industry. Here, we will demonstrate the role of oxygen vacancies for inducing high conductivity in the BaSnO3 thin films from insulating state and their stability on single crystal substrates deposited by pulsed laser deposition. To further investigate the role of multi-valent rare earth element doping on optical and electronic properties, results of Lead (Pb), Bismuth (Bi) and Strontium (Sr) substitution by combinatorial synthesis of BaSnO3 are presented. We have tuned the bandgap from 3 to 4 eV based on the substitutions, and observed clustering and in-gap states for Pb and Bi-substitutions, respectively. Lastly, experiments regarding the search of the BaSnO3 based superconductors by carrier injection, multi-valent states and strain engineering will be discussed.Item 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.