Browsing by Author "Nunn, William"

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    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.
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    X-ray Diffraction and Atomic Force Microscopy of RuO2 thin films
    (2022-08-18) Nunn, William; Manjeshwar, Anusha K; Nair, Sreejith; Jalan, Bharat; bjalan@umn.edu; Jalan, Bharat; University of Minnesota Jalan MBE Lab
    X-ray Diffraction data of the RuO2 samples were obtained to assess the crystallinity and epitaxial nature of the thin films grown on different substrates. Atomic Force Microscopy was used to determine the surface morphology changes as a function of thickness and epitaxial strain and correlate it with the observed oxygen evolution activity.