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An evaluation of A15 Nb3Al superconducting thin films for application in quantum circuits

Joseph Falvo, Brooke Henry, Bernardo Langa, Rohit Pant, Ashish Alexander, Jason Dong, Kasra Sardashti

TL;DR

The study demonstrates that Nb$_3$Al, an A15 intermetallic, can be grown as high-quality thin films via DC/RF co-sputtering followed by rapid thermal processing to achieve $T_c$ up to 16.64 K and strong high-field performance ($B_c(0) \approx 33$ T). Microwire devices reveal a coherence length of $\xi_0 \approx 3.1$ nm and a sheet kinetic inductance enabling substantial kinetic inductance while maintaining a bulk-like superfluid density $n_s \approx (1.1-2.7)\times 10^{26}$ m$^{-3}$; CPW resonators reach a single-photon internal quality factor $Q_i \approx 2.26\times 10^5$ at 4.5–5.25 GHz. The data indicate quasi-2D superconducting electrodynamics in films thicker than $\xi_0$ and highlight TLS- and quasiparticle-related losses linked to surface Al oxides and RTP contaminants, pointing to surface-cleaning strategies to improve microwave performance. Overall, Nb$_3$Al is a promising material platform for high-temperature superconducting quantum circuits, with clear pathways to further enhance inductive properties and device quality through controlled interfacial chemistry and surface morphology.

Abstract

A15 superconductors are distinguished by their high critical temperatures, magnetic fields, and current-carrying capabilities. Among them, Nb$_3$Al is of particular interest for superconducting quantum circuits as a means to extend device operating temperatures, provided that its electrodynamic properties are well understood. Here, we report on the synthesis of Nb$_3$Al thin films by magnetron co-sputtering followed by rapid thermal processing, yielding superconducting transition temperatures above 16~K. Microwire devices patterned from these films exhibit a coherence length of $3.2\,\mathrm{nm}$ and superfluid densities as low as $1.1\times 10^{26}\,\mathrm{m}^{-3}$, suggesting that Nb$_3$Al may enable high kinetic inductance in thinner films. Coplanar waveguide resonators fabricated on Nb$_3$Al demonstrate single-photon internal quality factors up to $2.26\times 10^{5}$. These results establish Nb$_3$Al as a promising material platform for the development of superconducting quantum circuits operating at elevated temperatures, contingent on appropriate control of interfacial chemistry and surface morphology.

An evaluation of A15 Nb3Al superconducting thin films for application in quantum circuits

TL;DR

The study demonstrates that NbAl, an A15 intermetallic, can be grown as high-quality thin films via DC/RF co-sputtering followed by rapid thermal processing to achieve up to 16.64 K and strong high-field performance ( T). Microwire devices reveal a coherence length of nm and a sheet kinetic inductance enabling substantial kinetic inductance while maintaining a bulk-like superfluid density m; CPW resonators reach a single-photon internal quality factor at 4.5–5.25 GHz. The data indicate quasi-2D superconducting electrodynamics in films thicker than and highlight TLS- and quasiparticle-related losses linked to surface Al oxides and RTP contaminants, pointing to surface-cleaning strategies to improve microwave performance. Overall, NbAl is a promising material platform for high-temperature superconducting quantum circuits, with clear pathways to further enhance inductive properties and device quality through controlled interfacial chemistry and surface morphology.

Abstract

A15 superconductors are distinguished by their high critical temperatures, magnetic fields, and current-carrying capabilities. Among them, NbAl is of particular interest for superconducting quantum circuits as a means to extend device operating temperatures, provided that its electrodynamic properties are well understood. Here, we report on the synthesis of NbAl thin films by magnetron co-sputtering followed by rapid thermal processing, yielding superconducting transition temperatures above 16~K. Microwire devices patterned from these films exhibit a coherence length of and superfluid densities as low as , suggesting that NbAl may enable high kinetic inductance in thinner films. Coplanar waveguide resonators fabricated on NbAl demonstrate single-photon internal quality factors up to . These results establish NbAl as a promising material platform for the development of superconducting quantum circuits operating at elevated temperatures, contingent on appropriate control of interfacial chemistry and surface morphology.

Paper Structure

This paper contains 7 sections, 6 equations, 9 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Experimental
  3. Thin film growth
  4. Microfabrication
  5. Characterization
  6. Results and Discussion
  7. Conclusion

Figures (9)

  • Figure 1: Chemical and structural characteristics of co-sputtered Nb-Al thin films: (a) Aluminum content in the Nb-Al thin films, expressed in atomic percent (at. %), as a function of substrate angle during co-deposition. The gray shading highlights a region within 3 at. % of stoichiometric 3:1 Nb:Al ratio. (b) XRD patterns for select samples where green dashed lines indicate expected peak positions from Nb3Al in the A15 structure, and * indicates peaks resulting from the sapphire (0001) substrate. (c) and (d) Surface topography maps for sample L1 before and after RTP under argon, respectively. RMS roughness shown in bottom right corner.
  • Figure 2: Superconducting-normal transition in L1, H1, and H2 sample groups: Sheet resistance vs temperature for samples derived from depositions (a) L1, (b) H1, and (c) H2 measured using a van der Pauw geometry. In each panel, the as-deposited sample (blue lines) is compared with samples treated by RTP under vacuum (-VAC, orange lines), and in Ar flow (-AR, green lines).
  • Figure 3: Superconducting characteristics of H2-AR microwires: (a) Sheet resistance vs temperature curves for a set of microwires fabricated from H2-AR (inset: optical micrograph of the microwire test device). Vertical dashed line indicates $T_c$ of H2-AR measured in VdP configuration. (b) Critical magnetic field ($B_c$) vs temperature normalized to $T_c$ for a typical microwire, which is fit to the GL relation for 2D superconductors.
  • Figure 4: Measuring microwave loss in H2-AR superconducting resonators: (a) $Q_i$ vs microwave power expressed as the average photon number for 4 hanging CPW resonators. (b) Boxplots of $Q_i$ vs average photon number for all resonators in a set across two measurements (total of 8 measurements). The green arrow denotes the mean $Q_i$ within the set. (c) $Q_i$ vs temperature for multiple applied microwave powers.
  • Figure 5: XPS composition analysis for samples H2, H2-AR, and H2-VAC surface and bulk after 1 min and 30 min of Ar etching, respectively: (a) Nb3d spectra showing Nb oxide (Nb3d5/2 at 203 eV) at the surface (blue lines) of the film with metallic Nb (Nb3d5/2 at 202.3 eV) dominating in the bulk (red lines). (b) Al2p spectra showing high oxidation at the surface (Al2p at 75.5 eV) of H2 and H2-AR giving way to metallic Al in the bulk of all films (Al2p at 72 eV). (c) Si2p spectra indicating SiOx (Si2p at 103 eV) and elemental Si (Si2p doublet at 99 eV) at the surface of films only after RTP. (d) Elemental composition of the three films after 1 minute (light shade) and 30 minutes (dark shade) of Ar sputtering using peak areas normalized by RSF.
  • ...and 4 more figures