Effect of metal encapsulation on bulk superconducting properties of niobium thin films used in qubits

TL;DR

This study demonstrates that in-situ metal encapsulation of Nb thin films, intended to passivate surfaces, also significantly modifies bulk superconducting properties across 110–163 nm films. Using MO imaging, magnetization, resistivity, and London and Campbell penetration-depth techniques, Au-encapsulated Nb shows the highest $T_c$ and ${RRR}$ but the lowest $H_{c2}(0)$, $ ext{λ}(0)$, and $j_c$, indicating reduced bulk disorder and weaker pinning; Au capping also suppresses thermo-magnetic flux avalanches, suggesting improved thermal management. The results are rationalized by a bulk disorder mechanism that extends beyond surface oxides, with TOF-SIMS indicating lower oxygen and NbN-related impurities in Au-capped films, correlating with improved superconducting properties. Collectively, the findings imply that bulk disorder contributes to pair-breaking and decoherence in transmon qubits, and that in-situ Au capping can simultaneously passivate surfaces and tune bulk properties to enhance qubit coherence.

Abstract

Niobium metal occupies nearly 100\% of the volume of a typical 2D transmon device. While the aluminum Josephson junction is of utmost importance, maintaining quantum coherence across the entire device means that pair-breaking in Nb leads, capacitive pads, and readout resonators can be a major source of decoherence. The established contributors are surface oxides and hydroxides, as well as absorbed hydrogen and oxygen. Metal encapsulation of freshly grown surfaces with non-oxidizing metals, preferably without breaking the vacuum, is a successful strategy to mitigate these issues. While the positive effects of encapsulation are undeniable, it is important to understand its impact on the macroscopic behavior of niobium films. We present a comprehensive study of the bulk superconducting properties of Nb thin films encapsulated with gold and palladium/gold, and compare them to those of bare Nb films. Magneto-optical imaging, magnetization, resistivity, and London and Campbell penetration depth measurements reveal significant differences in encapsulated samples. Both sputtered, and epitaxial Au-capped films exhibit the highest residual resistivity ratio and superconducting transition temperature, as well as the lowest upper critical field, London penetration depth, and critical current. These results are in good agreement with the microscopic theory of anisotropic normal and superconducting states of Nb. We conclude that pair-breaking in the bulk of niobium films, driven by disorder throughout the film rather than just at the surface, is a significant source of quantum decoherence in transmons. We also conclude that gold capping not only passivates the surface but also affects the properties of the entire film, significantly reducing the scattering rate due to defects likely induced by surface diffusion if the film is not protected immediately after fabrication.

Figures (12)

• Figure 1: Electrical resistivity. (a) Full temperature scale resistivity of all studied films. Inset in (a): residual resistivity ratio, $RRR(T)=\rho(300\;\text{K})/\rho(T)$ showing distinctly higher values in Au-capped films. (b) Zoom in on the superconducting transition, $T_c$. Au-capped films show the highest $T_{c}$. (c) Resistivity at $T_{c}$ as a function of $T_{c}$ showing monotonic behavior except for the un-optimized Au-capped film, Nb/Au-2.
• Figure 2: Superfluid density. Normalized superfluid density, $\rho_{s}=(\lambda(0)/\lambda(T))^{2}$ (symbols), calculated from the measured London penetration depth, $\lambda(T)$. The solid red curves show the isotropic weak-coupling BCS theory. The only free parameter to adjust the experimental data to match the theory was $\lambda(0)$, and this is how it was obtained for Fig.\ref{['fig:Hc2_Tc']}.
• Figure 3: Parallel vs. perpendicular orientation. Temperature dependence of magnetic susceptibility, $\chi(T)$, of PdAu-capped Nb film. The green symbols correspond to a magnetic field applied perpendicular to the film’s plane. Blue symbols show the case when a magnetic field was applied parallel to the film’s plane. Note a significant difference in noise level due to extreme demagnetization.
• Figure 4: Thermo-magnetic flux avalanches. Magnetization loops (raw data in memu) measured at indicated temperatures in a parallel orientation in PdAu-capped Nb film. The vertical jumps appearing above a certain threshold (shown by light shaded areas) are well-known in niobium films, occurring due to thermomagnetic instabilities (a.k.a. vortex avalanches) Duran1995Welling2004.
• Figure 5: DC magnetization. Comparison of normalized magnetic hysteresis loops, $4\pi m(H)$ (in gauss), at 2 K in the studied films. The magnetic field was applied parallel to the sample plane. The hysteresis is the largest in a bare Nb film and the lowest in an Au-coated film, implying stronger pinning in the former and weaker pinning in the latter.