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

Aswin kumar Anbalagan^1*, Rebecca Cummings², Chenyu Zhou³, Junsik Mun^2,3, Vesna Stanic⁴, Jean Jordan-Sweet⁴, Juntao Yao^2,5, Kim Kisslinger³, Conan Weiland⁶, Dmytro Nykypanchuk³, Steven L. Hulbert¹, Qiang Li^2,7, Yimei Zhu², Mingzhao Liu³, Peter V. Sushko⁸, Andrew L. Walter¹, Andi M. Barbour¹

¹National Synchrotron Light Source II, Brookhaven National Laboratory, Upton, New York 11973, USA
²The Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA
³Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
⁴IBM, T. J. Watson Research Center, 1101 Kitchawan Road, Yorktown Heights, New York 10598, USA
⁵Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York 11794, USA
⁶Material Measurement Laboratory, National Institute of Standard and Technology, Gaithersburg, Maryland 20899, USA
⁷7Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11794, USA
⁸Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, USA

* Presenter:Aswin kumar Anbalagan, email:aanbalaga1@bnl.gov

Despite constituting a smaller fraction of the qubit’s 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 nm ± 0.05 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.

Keywords: Superconducting films, Tantalum, HAADF-STEM, Synchrotron X-ray reflectivity, Density functional theory modeling