Browsing by Subject "Epitaxy"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Data for Room-Temperature Valence Transition in a Strain-Tuned Perovskite Oxide
    (2022-11-09) Chaturvedi, Vipul; Ghosh, Supriya; Gautreau, Dominique; Postiglione, William M; Dewey, John E; Quarterman, Patrick; Balakrishnan, Purnima P; Kirby, Brian J; Zhou, Hua; Cheng, Huikai; Huon, Amanda; Fitzsimmons, Michael R; Korostynski, Caroline; Jacobson, Andrew; Figari, Lucca; Barriocanal, Javier G; Birol, Turan; Mkhoyan, K Andre; Leighton, Chris; leighton@umn.edu; Leighton, Chris; Leighton Electronic and Magnetic Materials Lab
    Cobalt oxides have long been understood to display intriguing phenomena known as spin-state crossovers, where the cobalt ion spin changes vs. temperature, pressure, etc. A very different situation was recently uncovered in praseodymium-containing cobalt oxides, where a first-order coupled spin-state/structural/metal-insulator transition occurs, driven by a remarkable praseodymium valence transition. Such valence transitions, particularly when triggering spin-state and metal-insulator transitions, offer highly appealing functionality, but have thus far been confined to cryogenic temperatures in bulk materials (e.g., 90 K in Pr1-xCaxCoO3). Here, we show that in thin films of the complex perovskite (Pr1-yYy)1-xCaxCoO3-delta, heteroepitaxial strain tuning enables stabilization of valence-driven spin-state/structural/metal-insulator transitions to at least 291 K, i.e., around room temperature. This dataset contains all digital data published in the Nature Communications paper of the same name.
  • Thumbnail Image
    Item
    Recognition and assembly at multiple length-scales.
    (2010-05) Olmsted, Brian Keith
    Many molecular materials capable of crystallizing into an ordered solid state may assume multiple packing arrangements. This behavior is called polymorphism and is common among organic molecules such as pharmaceuticals and dyes. Controlling the nucleation of specific polymorphic crystals is not well understood, but is tantamount to the development and manufacture of new industrial products. One phenomena that has been observed to influence crystal orientation, growth rate, and morphology is epitaxy. Epitaxy refers to a condition by which a crystalline substrate presents a similar two-dimensional lattice to a crystalline plane of a nucleating species, resulting in a condition that lowers the energy barrier to nucleation and results in a preferential orientation of crystal growth on the substrate. Therefore, epitaxial nucleation may provide routes to selectively nucleate polymorphs and attain control over otherwise unpredictable crystallization events. The literature provides several examples of epitaxial relationships between a substrate and a crystal overlayer in fields involving inorganic crystals as well as organic crystals, and because epitaxy relies on geometric comparisons between lattice parameters, computational prediction of epitaxy is an active area of research. Our laboratories have developed software; named GRACE, to attempt to predict epitaxial relationships and this software has been used to verify epitaxy reported in the literature. One particularly useful feature of GRACE is its ability to handle a library of substrates and screen them against a corresponding database of crystal structures available as candidate crystal overlayers. In this capacity GRACE allows large libraries of substrates and crystals to be reduced to an experimentally manageable size, whereby combinatorial crystallizations can be tested for selective nucleation arising from epitaxial interfaces. This research also focuses on other aspects of nucleation that are not yet fully understood. Epitaxial interfaces are by definition, abrupt. However, a specialized class of crystals involving a domain that completely overgrows a core crystal by epitaxial mechanisms has revealed a zone of intermixing spanning close to a micron. In situ Atomic Force Microscopy (AFM) reveals the mechanisms for these observations and provides insight into how epitaxial interfaces behave mechanistically. Notably, it was revealed that process conditions between phases of growth in the formation of core-shroud heterocrystals may yield controllable interfacial thicknesses between crystalline domains, It was also discovered that the propensity for abrupt, epitaxial interfaces may be limited by the thermodynamic behavior of specific crystal interfaces under conditions of near-equilibrium. Although the use of in situ AFM is excellent for the study of crystal growth, the mass-transfer limitations at crystallizing interfaces inside an (AFM) fluid cell are not directly discernable and the assumption is typically made that conditions in the bulk solution are the same inside the cell. By implementing computational fluid dynamic (CFD) simulations for flow and mass transport, in situ AFM was studied to determine how the different conditions at the crystal surface are in comparison to the bulk solution outside the cell. The geometry of the internal volume of the AFM fluid cell imparts specific fluid flow and mass transport limitations on the environment directly at the area of investigation for crystal growth and in some cases may have significant ramifications for the appropriate correlation of bulk solution variables to crystal growth variables. The results of the CFD calculations indicate that differences are significant, though usually minor and these results may prove useful for future fluid cell design. Finally, photolithographic techniques were employed to produce millions of micron-sized particles with shapes mimicking molecular contours and other crystallographically significant contours to study how symmetry and packing originates at the micron length-scale. Although much is known about assembly at the molecular level for symmetry and packing, the assembly of anisotropic particles at longer length scales, which involve different interactive forces, has not been studied. This work concludes by performing preliminary work in elucidating the general behavior towards symmetry and packing in two-dimensions of micron-sized particles by using gravitational gradients and dielectrophoresis.
  • Thumbnail Image
    Item
    Solid Source Metal-Organic Molecular Beam Epitaxy Toward All-Epitaxial Ferroelectric Capacitors
    (2021-07) Nunn, William
    Great strides have been made in the area of thin film synthesis of complex materials. Among these, perovskite oxides have been identified as an immensely important multi-functional class due to exhibiting a large variety of materials properties, including ferroelectricity. Much progress has been made in the development of ferroelectric perovskite oxides but, unfortunately, the model electrode materials desired for many devices mostly contain difficult to work with or “stubborn” elements due to their ultra-low vapor pressures, in evaporation techniques, or low oxidation potentials, in general. Despite the construction of ferroelectric-metal heterostructures having a large impact on device fabrication, deposition of these electrode materials with atomic precision remains challenging in techniques like molecular beam epitaxy (MBE) and has not progressed much past using electron-beam evaporation.To deposit metals and metal oxides in a simpler, more cost-effective, and safer manner, a modification of MBE is developed for the first time here in this work and henceforth referred to as solid source metal-organic MBE. The growth of the simple metal Pt, binary oxide RuO2, and complex perovskite oxide SrRuO3 are shown using metal-organic source temperatures less than 100°C. Furthermore, the metals in these solid metal-organic precursors are in a pre-oxidized state, come bonded with an additional source of oxygen, are air stable, non-toxic, and can be used directly in-vacuum instead of requiring complicated external gas inlets. The growth results from this novel technique introduce it as another advancement in the long history of MBE. Additionally, with regards to the ferroelectric material, control over complex oxide stoichiometry has remained one of the largest issues within oxide MBE synthesis. Here, a different but rapidly expanding metal-organic-based MBE approach, hybrid MBE, is employed for the growth of ferroelectric and dielectric perovskite oxides with great control over the cation stoichiometry and, therefore, the structure and properties. The prototypical ferroelectric BaTiO3 is studied as well as the consequence of substituting Sn for Ti in the growth of the complete BaTiO3 – BaSnO3 alloy system for the first time in MBE. Together, these two approaches are utilized and developed for the goal of creating all-epitaxial in-situ-grown ferroelectric capacitors.
  • Thumbnail Image
    Item
    Structure and Transport in Epitaxial BaSnO3: Doping, Mobility and the Insulator-Metal Transition
    (2018-08) Ganguly, Koustav
    The recent discovery of high room temperature electron mobility in wide band gap BaSnO3 (BSO) has generated exceptional interest in this perovskite oxide for electronic devices. Outstanding issues with regards to epitaxial films include understanding transport mechanisms, determining the optimal dopant, and understanding the role of structural defects (like dislocations) in limiting mobility. Here, we discuss detailed temperature and field-dependent electronic transport in both oxygen vacancy and La-doped BSO films grown via high pressure oxygen sputter deposition. High-resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM) confirm phase-pure, close to stoichiometric, smooth, epitaxial BSO(001). Film thickness, growth rate, deposition temperature, and substrate (i.e., lattice mismatch) have all been systematically varied and related to mobility. Detailed transport accompanied with STEM has been used to understand the structure-electronic property relationships and reveal the correlation between misfit and threading dislocations in BSO thin films. As-grown undoped, insulating films can be made conductive with controllable n-type doping by vacuum reduction, resulting in 300 K Hall mobilities up to 35 cm2V-1s-1 (on LaAlO3(001)) at 5×1019 cm-3. The mobility-electron density relation has been probed in this manner, down to 2×1017 cm-3, the lowest electron density probed in BSO till date. 2% La-doped BSO films, on the other hand, demonstrate 300 K electron mobilities up to 70 cm2V-1s-1 at ~2 ×1020 electrons per cm3. With increasing film thickness a clear insulator-metal transition is observed with both dopants, likely related to defect density near the substrate. The low temperature upturn in resistivity observed in metallic-like BSO has been analyzed using out-of-plane and in-plane magnetoresistance (MR) measurements. Two-dimensional weak localization (WL) has been identified as the underlying mechanism behind this low temperature quantum correction. Overall, the results not only validate the technique of high-pressure oxygen sputtering as a viable approach to produce high quality BSO films, but also provide insight into the mobility-electron density relation, and mobility-limiting factors in these films. The mobility values reported in this thesis are record values for sputtered films and are comparable to that obtained via pulsed laser deposition (PLD) in previous studies.