Peter V. Sushko

Fabrication and Structural Analysis of Trilayers for Tantalum Josephson Junctions with Ta2O5 Barriers

  1. Raahul Potluri,
  2. Rohin Tangirala,
  3. Sage Bauers,
  4. Alejandro Barrios,
  5. Praveen Kumar,
  6. Peter V. Sushko,
  7. David P. Pappas,
  8. and Serena Eley
Tantalum (Ta) has recently emerged as a promising low-loss material, enabling record coherence times in superconducting qubits. This enhanced performance is largely attributed to its
stable native oxide, which is believed to host fewer two-level system (TLS) defects key − contributors to decoherence in superconducting circuits. Nevertheless, aluminum oxide (AlOx) remains the predominant choice for Josephson junction barriers in most qubit architectures. In this study, we systematically investigate various techniques for forming high-quality oxide layers on α-phase tantalum (α-Ta) thin films, aiming to develop effective Josephson junction barriers. We explore thermal oxidation in a tube furnace, rapid thermal annealing, as well as plasma oxidation of both room-temperature and heated Ta films, and propose a mechanistic picture of the underlying oxidation mechanisms. All methods yield Ta2O5, the same compound as tantalum’s native oxide. Among these, plasma oxidation produces the smoothest and highest-quality oxide layers, making it particularly well-suited for Josephson junction fabrication. Furthermore, we demonstrate the successful epitaxial growth of α-Ta atop oxidized α-Ta films, paving the way for the realization of trilayer Ta/Ta-O/Ta Josephson junctions with clean, low-loss interfaces.
arxiv pdf

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.
arxiv pdf