Revealing Buried Layers: Exploring the Metal-Substrate Interface Layer in Superconducting Films

Researchers used a multimodal approach to determine the metal-substrate interface in a superconducting qubit material.

December 11, 2025

A schematic illustration of the material stack.

Image courtesy of Physical and Computational Sciences Directorate, Pacific Northwest National Lab.

The Science

Researchers have uncovered an interface layer that may affect the performance of certain superconducting qubits. Qubits are the fundamental units of quantum computers. The scientists used advanced characterization techniques and theoretical modeling to study thin films of the metal tantalum (Ta) on a substrate made of sapphire. They identified a thin layer between the substrate and the metal layer for a specific orientation of the Ta film. This layer was both previously unknown and unexpected by scientists. This study provides new insights into atomic-scale interactions at this interface. It is likely that these interactions govern the orientation of the Ta film. From this observation, scientists think that the metal-substrate layer could play a role in the length of qubits’ coherence time. Coherence time is how long a qubit can hold its quantum state. This property influences how well a quantum computer can maintain and process information.

The Impact

This research offers a pathway to identifying microscopic factors that affect superconducting qubits’ performance. With this information, researchers can uncover how the structure of the metal-substrate interface relates to coherence time. It could help them understand how the oxygen concentration on the surface of the substrate and the arrangement of surface atoms on the sapphire substrate influence Ta deposition. This knowledge could help researchers refine fabrication techniques to improve qubits’ stability. Unlike current techniques, they would not need to destroy the device to study the material stack. These findings have significant implications for quantum computing.

Summary

This study from the Department of Energy’s Brookhaven National Laboratory and Pacific Northwest National Laboratory investigated the role of the metal-substrate interface in superconducting qubit materials. In this work, researchers focused on tantalum thin films deposited on sapphire substrates. Researchers discovered an unexpected interfacial layer. In this layer, tantalum atoms intermixed with aluminum and oxygen from the substrate, potentially impacting qubit coherence. Through a combination of X-ray reflectivity, X-ray spectroscopy, and electron microscopy, scientists analyzed the structural and chemical properties of this buried layer. Complementary computational modeling revealed that oxygen concentration at the sapphire surface dictates the crystallographic orientation of tantalum films for two idealized cases. These insights provide a framework for optimizing qubit fabrication, paving the way for better coherence times and quantum device performance.

Contact

Andi M. Barbour
Brookhaven National Laboratory
abarbour@bnl.gov

Andrew L. Walter
Brookhaven National Laboratory
awalter@bnl.gov

Peter V. Sushko
Pacific Northwest National Laboratory
peter.sushko@pnnl.gov

Funding

This material is based on work supported by the Department of Energy Office of Science, National Quantum Information Science Research Centers, Co-design Center for Quantum Advantage (C²QA).The research used the resources at the National Synchrotron Light Source-II, the Center for Functional Nanomaterials, and the National Energy Research Scientific Computing Center, all DOE Office of Science User Facilities.

Publications

A.k. Anbalagan, R. Cummings, C. Zhou, J. Mun et al. Revealing the origin and nature of the buried metal-substrate interface layer in Ta/sapphire superconducting films. Advanced Science (12) 2413058 (2025). [DOI:10.1002/advs.202413058].