EPITAXIAL GROWTH OF PALLADATES AND RELATED OXIDES FOR OXIDE ELECTRONICS BY MOLECULAR-BEAM EPITAXY

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Abstract
Complex oxides are an important class of materials with properties that are of great interest to a wide range of audiences from condensed matter physics to the semiconductor industry. Owing to the ultra-high vacuum environment, high purity and excellent sub-monolayer deposition control, molecular beam epitaxy (MBE) is the ‘gold standard’ for the epitaxial growth of complex oxides; the film properties are among the best achieved by thin film deposition techniques and even bulk single crystals. MBE grown film qualities are often plagued, however, by off-stoichiometry due to the needs to individually control source fluxes. In efforts to tackle this problem, in the first part of this thesis I describe an alternate method of calibration that relies on the epitaxial growth of individual binary oxides and metals rather than in situ quartz crystal microbalance measurements and reflection high-energy electron diffraction (RHEED) oscillations, which lack precision and general applicability, respectively. After describing the calibration mechanisms and presenting growth conditions of 39 elements, I demonstrate the merits of this approach through the growth of high quality, quaternary La0.5Sr0.5CoO3-? thin films and show that their transport properties rival the best techniques providing ‘stoichiometric transfer’. In the latter parts of this thesis I elaborate on 3 signature projects that either presented solutions to novel problems or significantly improved/optimized existing processes in condensed matter physics and oxide electronic devices. Delafossites with the general chemical formula ABO2 are an emerging class of compounds in condensed matter physics. The metallic Pd and Pt based oxides, namely PdCoO2 and PtCoO2 are the most conductive oxides known with mean free paths at low temperature exceeding 4 µm. Moreover, these materials exhibit large surface Rashba spin splitting where the magnitude increases with B-site spin orbit strength. Until recently there only existed very small single crystal (with dimensions smaller than 3 mm) and no reports of in situ growth by MBE. I describe the first in situ, direct epitaxial growth of PdCoO2 on c-plane sapphires using ozone-assisted MBE. I show that MBE-grown PdCoO2 films have excellent transport characteristics that are among the best reported in literature. Through scanning transmission electron microscopy studies, I show that the nucleation of PdCoO2 requires a strict deposition sequence and various structural defects such as twinning and out-of-phase boundaries that arise due to lattice and symmetry mismatch with c-plane sapphire. I then showcase my work on growth of an all a-axis YBa2CuO7-?/PrBa2Cu3O7-?/YBa2Cu3O7-? superconductor/normal metal/superconductor (SNS) vertical stack. Due to strong crystal anisotropies, the Cooper pair coherence length along the a-axis is a few times longer than along the c-axis even though c-axis is the thermodynamically favored growth direction. Although smooth c-axis SNS stack has been demonstrated, the a-axis counterpart remains elusive. Through novel calibration strategies based on binary oxide growth and developments of sophisticated temperature ramping recipes, I demonstrate the successful growth of a-axis YBa2CuO7-?/PrBa2Cu3O7-?/YBa2Cu3O7-? tri-layers with atomically sharp interfaces with less than 3% c-axis nucleation and excellent superconducting properties. I discuss in detail how key materials properties, namely superconducting Tc and transition width as well as surface roughness scale with YBa2CuO7-? layer thickness. As traditional approaches of transistor scaling gradually reach a bottleneck, innovative device architectures such as 3D logic and memory are becoming increasingly sought after. One such architecture, monolithic 3D integration, requires the back-end of line (BEOL) growth and fabrication of devices using oxides as channel materials. While n-type oxides with high mobility such as indium gallium zinc oxide (IGZO) are abundant, high mobility p-type oxides remain elusive as flat O 2p orbitals usually dominate valence band edge. Through the design principles of introducing highly correlated orbitals to the electronic structure, we have identified PdO as a back-end-of-line (BEOL) compatible p-type oxide semiconductor for monolithic 3D integrated circuits applications. Using molecular-beam epitaxy, we have successfully synthesized high quality epitaxial thin films of PdO on TiO2 (110) and MgO (100) substrates at BEOL conditions and measured its band structure using angle resolved photoemission spectroscopy for the first time. Our work unambiguously establishes PdO as a direct gap p-type semiconductor with 2.05 eV band gap and room-temperature hole mobility as high as 28.3 cm2/Vs.
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199 pages
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2021-12
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Electronics; Epitaxy; Oxide; Physics; Semiconductor
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Schlom, Darrell
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Fennie, Craig J.
Shen, Kyle M.
Degree Discipline
Materials Science and Engineering
Degree Name
Ph. D., Materials Science and Engineering
Degree Level
Doctor of Philosophy
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dissertation or thesis
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