Correlating the quality factor with the deposition temperature of Nb thin films used in superconducting qubits

Niobium thin films are widely used in transmon qubits for readout resonators, capacitor pads, and coupling lines. Consequently, there is a major effort to correlate the performance with material properties. One of the important parameters of epitaxial growth is the deposition temperature. While there is no correlation with the residual resistivity ratio, RRR, it is determined that lower deposition temperature results in higher transition temperature, T_c, and improved intrinsic mean quality factor, Q_i [1]. By using magneto-optical (MO) imaging of the superconducting state, we show a direct correlation between Qi and uniformity of magnetic flux penetration as well as critical current [2, 3]. The quasiparticle spectroscopy using a tunnel-diode resonator (TDR) shows a higher population of quasiparticles in a low Qi film (deposited at a higher temperature). Not only does superfluid density deviate from the exponential BCS behavior at low temperatures, but it also exhibits non-monotonic features suggesting localized in-gap states, perhaps overlapping two-level systems [2, 4]. We conclude that the combined MO and TDR measurements represent a powerful novel tool for rapid characterization of superconducting qubits.

References:

[1] McFadden, Anthony P., et al. "Interface-sensitive microwave loss in superconducting tantalum films sputtered on c-plane sapphire." Physical Review Materials 9.9 (2025): 096201.

[2] Joshi, Kamal R., et al. "Quasiparticle spectroscopy, transport, and magnetic properties of Nb films used in superconducting qubits." Physical Review Applied 20.2 (2023): 024031.

[3] Oh, Jin-Su, et al. "Exploring the relationship between deposition method, microstructure, and performance of Nb/Si-based superconducting coplanar waveguide resonators." Acta Materialia 276 (2024): 120153.

[4] Ghimire, Sunil, et al. "Quasiparticle spectroscopy in technologically relevant niobium using London penetration depth measurements: experiment and theory." Materials for Quantum Technology 4.4 (2024): 045201.