UC Santa Barbara
In-Situ Characterization of the Surfaces & Interfaces in Superconducting Coplanar Waveguides, RNiO3 heterostructures, and Self Assembled ErSb Nanostructures Grown by Molecular Beam Epitaxy
- Author(s): Wilson, Nathaniel Spencer
- Advisor(s): Palmstrøm, Chris J
- et al.
Surfaces and interfaces are responsible for many of the properties observed in electronic devices. However, characterizing the properties and effects of these interfaces remains one of the most challenging problems in materials science. This is due to both the limited number of characterization techniques that are sensitive to small interfacial regions, as well as the problems associated with surface contamination when exposing a clean surface to atmosphere. In this thesis these problems are overcome through the use of in-situ growth and characterization. Taking advantage of the layer by layer interfacial control achievable with molecular beam epitaxy (MBE) highly precise surfaces and interfaces are created. In-situ surface characterization via x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) is then used to characterize the grown structures. Three different materials are examined, Al superconducting coplanar waveguides (SCW), metal to insulator transitions in rare earth nickelates, and self-assembled ErSb nanostructures. Collectively, these material systems demonstrate the importance of contaminants at interfaces, interfacial control of material properties, and the effect of surfaces on material growth.
Two level states are thought to be one of the primary loss mechanisms for qubits and resonators fabricated from SCW. In-situ XPS is used to analyze the physical changes that occur at the surface during cleaning procedures used to improve qubit coherence times. Possible sources of TLS are identified, and include the presence of hydrogen and carbon on the initial sapphire substrate. Requirements for creating a similar TLS free surface when transitioning to SCW fabrication using silicon substrates is also discussed.
Materials in the RNiO3 (R = a trivalent rare earth ion) family exhibit a metal to insulator transition(MIT) as a function of temperature and film thickness. Strain is shown to clearly suppress or enhance the MIT observed in PrNiO3 thin films. A separate MIT transition is observed as a function of LaNiO3 film thickness and characterized by in-situ STM and XPS. A clear gap is observed in the density of states for thin LaNiO3 films providing insight into the nature of the MIT and suggesting a source other than disorder. LaAlO3/LaNiO3 superlattices are characterized and show that the MIT and transport properties of LaNiO3 may be tunable via superlattice structure.
ErxGa1-xSb is a model system for examining self-assembled nanostructures producing nanoparticles, out-of-plane nanowires, in-plane nanowires and nanosheets. The morphology of the growth surface is characterized via in-situ STM. The growth mechanism responsible for the transition from out-of-plane to in-plane nanowire growth is identified as the formation of large macrosteps with step heights of ~7nm flowing across the surface and a surface energy model is developed to predict this behavior. A novel nanoparticle consisting of extremely thin in-plane nanowires was identified for low temperature, high Sb pressure growths and may prove beneficial to a number of technological applications.