Curator's Take
AI Commentary
This article shows that a few‑nanometer Ta buffer layer between silicon and a TaN seed dramatically raises resonator internal quality factors from ~1.5×10^5 to nearly 6×10^5, indicating a strong reduction of interface‑related two‑level system loss. By suppressing nitrogen accumulation and structural disorder at the substrate‑metal boundary, the work makes tantalum nitride a more viable material for both seed layers and barrier layers in superconducting qubit stacks, potentially expanding the toolbox beyond traditional Al or Nb films. The result is especially relevant as the community seeks higher‑coherence planar resonators and transmons that can be integrated with emerging 3D cavity architectures. Further work will be needed to confirm that these interface gains translate into full‑device qubit performance at typical operating temperatures.
— Mark Eatherly
Summary
Tantalum has been demonstrated as a promising material for superconducting qubits. However, comparatively little attention has been given to its nitrides. Tantalum nitride exhibits a range of stoichiometries, resulting in a variety of material properties, including both superconducting and non-superconducting phases. Owing to this versatility, tantalum nitrides can serve multiple purposes in superconducting qubits: as seed layers for alpha-Ta growth, as a superconducting base material and as a non-superconducting barrier in the Josephson junction. In this study, we explore the performance of superconducting TaN and Ta thin film combinations on silicon substrates in terms of internal quality factor Qi. We find that standalone TaN films exhibit Qi values of about 1.5x10^5 at 100mK in the single-photon regime. Surprisingly, a resonator made from Ta grown on a few-nanometers-thick TaN seed layer yields largely the same performance. However, adding an additional, few-nanometers-thick Ta buffer layer between the Si substrate and this TaN seed layer enhances Qi significantly up to 5.9x10^5. Supporting transmission electron microscopy measurements reveal nitrogen accumulation and structural disorder at the TaN-Si interface, while this interfacial modification is suppressed when the Ta buffer layer is introduced. The observed improvement in resonator performance is consistent with a reduction of interface-related two-level system losses and strongly supports the hypothesis that controlling the substrate-metal interface is pivotal for the performance of superconducting qubit circuitry.