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Zinc Oxide Heterostructures for Electron Confinement

Sparks, Matthew; (2021) Zinc Oxide Heterostructures for Electron Confinement. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Two-dimensional electron gases (2DEGs) at the ZnO/ZnMgO interface are promising for spintronics and quantum computing applications due to the combination of low spin-orbit coupling and high electron mobility. This thesis proposes that these 2DEGs could be used to achieve voltage-tuneable Josephson junctions (JJs). To achieve this, the 2DEG needs a high superconducting coherence length, which requires both a high carrier concentration and electron mobility. This thesis summarises methods to enhance ZnO 2DEG quality using pre-growth processing on ZnO substrates, and testing ZnO/ZnMgO heterostructure fabrication on alternative substrate materials. This is achieved through rapid thermal treatment of commercially available ZnO and by exploring growth on a- and c-plane sapphire. Rapid thermal annealing of ZnO substrates is shown to greatly improve substrate quality. Devices produced on thermally treated substrates demonstrate 2DEG behaviour with mobilities and carrier concentrations of 4.8 × 10⁴ cm²/Vs and 5.05 × 10¹² cm⁻² respectively. This translates to a long mean free path (1800 nm) and a long clean-limit coherence length (120 nm) at 2K, making the sample well suited for voltage-tuneable Josephson junctions. The high 2DEG mobility is attributed to a reduction in substrate defect density due to the thermal treatment. Later chapters show how thin MgO buffer layers can be used to achieve Zn-polar ZnO layers on c-plane sapphire. The Zn-polar layers are used to produce ZnMgO/ZnO heterostructures which could provide the basis of 2DEG formation on non-ZnO substrates. Practical means of controlling Zn-polarity on a-plane sapphire could not be found, so an alternative heterostructure is produced which utilises polarity discontinuity at the ZnO/ZnMgO interface. Finally a road-map for future research is presented. This includes suggestions for retrofitting working ZnO 2DEG devices to measure proximity superconductivity through the 2DEG. It is explained how this proximity superconductivity can be tuned through electrical gates, and how this component could be used to achieve tuneable superconducting qubits.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Zinc Oxide Heterostructures for Electron Confinement
Event: University College London
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Copyright © The Author 2021. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request.
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Electronic and Electrical Eng
URI: https://discovery.ucl.ac.uk/id/eprint/10127476
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