Revealing the Origin and Nature of the Buried Metal-Substrate Interface Layer in Ta/Sapphire Superconducting Films

  1. Aswin kumar Anbalagan,
  2. Rebecca Cummings,
  3. Chenyu Zhou,
  4. Junsik Mun,
  5. Vesna Stanic,
  6. Jean Jordan-Sweet,
  7. Juntao Yao,
  8. Kim Kisslinger,
  9. Conan Weiland,
  10. Dmytro Nykypanchuk,
  11. Steven L. Hulbert,
  12. Qiang Li,
  13. Yimei Zhu,
  14. Mingzhao Liu,
  15. Peter V. Sushko,
  16. Andrew L. Walter,
  17. and Andi M. Barbour
Despite constituting a smaller fraction of the qubits electromagnetic mode, surfaces and interfaces can exert significant influence as sources of high-loss tangents, which brings forward the need to reveal properties of these extended defects and identify routes to their control. Here, we examine the structure and composition of the metal-substrate interfacial layer that exists in Ta/sapphire-based superconducting films. Synchrotron-based X-ray reflectivity measurements of Ta films, commonly used in these qubits, reveal an unexplored interface layer at the metal-substrate interface. Scanning transmission electron microscopy and core-level electron energy loss spectroscopy identified an approximately 0.65 \ \text{nm} \pm 0.05 \ \text{nm} thick intermixing layer at the metal-substrate interface containing Al, O, and Ta atoms. Density functional theory (DFT) modeling reveals that the structure and properties of the Ta/sapphire heterojunctions are determined by the oxygen content on the sapphire surface prior to Ta deposition, as discussed for the limiting cases of Ta films on the O-rich versus Al-rich Al2O3 (0001) surface. By using a multimodal approach, integrating various material characterization techniques and DFT modeling, we have gained deeper insights into the interface layer between the metal and substrate. This intermixing at the metal-substrate interface influences their thermodynamic stability and electronic behavior, which may affect qubit performance.
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