Structure and Formation Mechanisms in Tantalum and Niobium Oxides in Superconducting Quantum Circuits

Improving the qubit's lifetime (T₁) is crucial for fault-tolerant quantum computing. Recent advancements have shown that replacing niobium (Nb) with tantalum (Ta) as the base metal significantly increases T₁, likely due to a less lossy native surface oxide. However, understanding the formation mechanism and nature of both surface oxides is still limited. Using aberration-corrected transmission electron microscopy and electron energy loss spectroscopy, we found that Ta surface oxide has fewer suboxides than Nb oxide. We observed an abrupt oxidation state transition from Ta₂O₅ to Ta, as opposed to the gradual shift from Nb₂O₅, NbO₂, and NbO to Nb, consistent with thermodynamic modeling. Additionally, amorphous Ta₂O₅ exhibits a closer-to-crystalline bonding nature than Nb₂O₅, potentially hindering H atomic diffusion toward the oxide/metal interface. Finally, we propose a loss mechanism arising from the transition between two states within the distorted octahedron in an amorphous structure, potentially causing two-level system loss. Our findings offer a deeper understanding of the differences between native amorphous Ta and Nb oxides, providing valuable insights for advancing superconducting qubits through surface oxide engineering.