A Study of Superconducting Behavior in Ruthenium Thin Films

TL;DR

The paper demonstrates superconductivity in ultrahigh-vacuum–grown Ru thin films down to 11.9 nm, with Tc spanning 557–658 mK and a thickness-dependent trend well described by $T_c(d) = 470 + A/d$ where $A = 11.840$ K·nm. Structural characterizations show fine-grained, polycrystalline Ru, and transport measurements indicate a 1/d scaling of Tc and parallel critical fields, placing these films in the dirty, Type-II regime with coherence lengths between $13.5$ and $27$ nm. The Ru films exhibit robust air stability with minimal RuOx growth over seven weeks, supporting their integration into superconducting devices such as Josephson junctions and qubits, where Ru can serve as a low-diffusivity, oxidatively stable electrode. Overall, the work provides a detailed link between thickness, microstructure, and superconducting properties in Ru thin films, offering practical pathways for Ru-based superconducting electronics.

Abstract

Ruthenium (Ru) is a promising candidate for the next-generation of electronic interconnects due to its low resistivity, small mean free path, and superior electromigration reliability at nanometer scales. Additionally, Ru exhibits superconductivity below 1 K, with resistance to oxidation, low diffusivity, and a small superconducting gap, making it a potential material for superconducting qubits and Josephson Junctions. Here, we investigate the superconducting behavior of Ru thin films (11.9 - 108.5 nm thick), observing transition temperatures from 657.9 mK to 557 mK. A weak thickness dependence appears in the thinnest films, followed by a conventional inverse thickness dependence in thicker films. Magnetotransport studies reveal type-II superconductivity in the dirty limit (ξ >> l), with coherence lengths ranging from 13.5 nm to 27 nm. Finally, oxidation resistance studies confirm minimal RuOx growth after seven weeks of air exposure. These findings provide key insights for integrating Ru into superconducting electronic devices.

Figures (5)

• Figure 1: Structural characteristics of the UHV-grown Ru thin films on c-plane sapphire substrates: (a) X-ray diffraction spectra of thin films with varying thicknesses and growth temperatures (Samples C, E, H, F, and G). (b) AFM topographical maps of Ru thin films of different thicknesses and growth temperatures (Samples C, E, F, and H).
• Figure 2: Thickness Dependence of Critical Temperature and Normal Resistance: (a) Sheet resistance as a function of temperature for Samples C (d = 30.4 nm), E (d = 58.9 nm), F (d = 82.2 nm), and G (d = 108.5 nm). (b) Critical temperature as a function of sample thickness fitted to the equation Tc(d) = 470 + A/d, where A = 11.840 K.nm is a fit parameter and d is the film thickness. (c) Normal sheet resistance ($\mathbf{R_s}$) vs sample thickness at 300 K and 1 K. The fit uses the equation $R_s=\rho/d$, where $\mathbf{\rho}$ is the resistivity of the film and d is the film thickness. The resistivities of the films at 300 K and 1 K were $\mathbf{\rho_{300K}} = 2.57 \times 10^{-5}$$\Omega$.cm and $\mathbf{\rho_{1K}} = 1.95 \times 10-5$$\Omega$.cm.
• Figure 3: Superconducting-normal transition in parallel magnetic field: (a) Sheet resistance as a function of the in-plane magnetic field at varying temperatures for Sample C. (b) Critical magnetic field as a function of temperature. The data is fitted using the model: $B_c^\parallel(T)=B_0^\parallel\sqrt{1-t}$. (c) $B_0^\parallel$ values for each Ru thin film fitted to $B_0^\parallel=\sqrt{24}B_{cb}^\parallel(0) \lambda_b(0)/d$.
• Figure 4: Thickness and temperature dependence of parallel critical magnetic field:$(B_c^\parallel(T))^2$ as a function of the temperature function $(1-t^2)/(1+t^2)$ for six Ru thin film samples of different thicknesses (Samples B, C, D, E, F, and G). The proportionality factor estimated from the linear fit is $\sqrt{24}\lambda^2(0)(B_c^\parallel(0))^2/d^2$, where $\lambda$ is the London penetration depth and d is the thickness of the film.
• Figure 5: Ru thin film surface composition as a function of air exposure: (a) Ru3p spectra with metallic Ru and RuOx highlighted in light and dark blue, respectively. (b) O1s spectra as a function of exposure time. (c) Surface composition as a function of air exposure calculated from the Ru3p and O1s intensities (normalized to their respective relative sensitivity factor values).