Surfaces, Interfaces, and Impurity Elements in Niobium-based Materials for Superconducting Quantum Computing Analyzed by Atom-Probe Tomography

Superconductivity with T_c=9.3 K makes niobium the material of choice for thin film resonator structures in transmon qubits, one of the leading candidates in quantum computing. While the superconducting properties of the Nb thin film itself are robust, the qubit coherency times are thought to be limited by dielectric losses originating from defects at surfaces and interfaces. We apply atom-probe tomography (APT) to compositionally and structurally characterize the surfaces, interfaces, and grain boundaries in Nb thin-films fabricated by high-power impulse magnetron sputtering (HIPIMS) on silicon substrate. The results indicate the formation of a 5-8 nm thick surface oxide film with a NbO or Nb₂O₅ stoichiometry on top of the Nb thin film, and the formation of a 6-8 nm thick silicide interreaction layer between the Nb thin-film and the Si substrate. The single-atom time-of-flight mass spectrometric sensitivity of APT is particularly suitable to track presence, location, and concentrations of interstitial impurity elements, H, C, N, and O.