Chemical Passivation of Yttrium Barium Copper Oxide and Nanoscale High-Temperature Superconducting Devices
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Chemical Passivation of Yttrium Barium Copper Oxide and Nanoscale High-Temperature Superconducting Devices

  • Author(s): Vinay, Miranda Lynn
  • Advisor(s): Cybart, Shane A
  • et al.
Creative Commons 'BY' version 4.0 license
Abstract

High-temperature superconductivity (HTS) was supposed to change the world. And it did, albeit less dramatically than previously predicted over 40 years ago. Thin films of HTS materials have been applied in qubits for quantum computing, analog-to-digital converters, and neuromorphic circuits; and are necessary for the most sensitive magnetometers used in geological surveys and medical technology.

This technological revolution hinges on Yttrium Barium Copper Oxide, or YBa2Cu3Ox+6 (YBCO). This ceramic oxide from the cuprate family has been the benchmark for “high temperature” superconductors, in which the superconducting state is reached above the boiling temperature of nitrogen. However, its processing and technical implementation in thin films come with challenges. It is a non-stoichiometric crystal, and its superconducting properties are highly dependent on its concentration of oxygen (and therefore the variable oxidation state of copper). Most importantly – and the focus of this dissertation – is its stability as a thin film under 40 nanometers thick: YBCO expands differently from its substrates under thermal cycling, loses oxygen at high temperatures, and in air, reacts to form non-superconducting (and oxygen-depleted) products.

The work presented here explores how YBCO thin films fail, and how to protect their surface and preserve thin-film superconductivity. The focus lies on the use of bromine – a halogen that tightly binds to the YBCO surface – as a means of chemical passivation, protecting YBCO from being reduced in the atmosphere without harming the desirable superconducting properties, and also without harming the performance of superconducting devices such as YBCO-based nanoelectronics. We found that thermal stress is not the root cause of failure in superconducting thin films, which led the study into further chemistry. Remarkably, treatment with dilute bromine in ethanol solutions was found to have little effect on the material’s performance and does not destroy the electronic properties of Josephson Junctions. The reaction, while an etching process, only attacks the surface, leaving the underlying material (and written junctions) intact. Through this work, we aim to increase the longevity of HTS nanoelectronics. This dissertation uses the toolkits of chemistry to solve problems in electrical engineering – ultimately inching closer to less expensive, more robust HTS devices.

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