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

• Main Authors: Anbalagan, Aswin kumar, Cummings, Rebecca, Zhou, Chenyu, Mun, Junsik, Stanic, Vesna, Jordan-Sweet, Jean, Yao, Juntao, Kisslinger, Kim, Weiland, Conan, Nykypanchuk, Dmytro, Hulbert, Steven L, Li, Qiang, Zhu, Yimei, Liu, Mingzhao, Sushko, Peter V, Walter, Andrew L, Barbour, Andi M
• Format: Journal Article
• Language: English
• Published: 16.09.2024
• Subjects: Physics - Materials Science; Physics - Superconductivity
• DOI: 10.48550/arxiv.2409.10780

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.