Scanning Tunneling Microscopy and Spectroscopy Studies of Au-Capped Nb Films for Superconducting Qubit Development

In superconducting transmon qubits thin film niobium is used for capacitor pads to provide shunting capacitance. However, the niobium native oxide contributes to RF losses, primarily through two-level system (TLS) losses and local magnetic moments from oxygen vacancies in the niobium pentoxide, which are sources of decoherence in qubit devices. The losses from these mechanisms was thought to be the leading cause of decoherence in the qubit device. Recent advances have reduced these losses by capping niobium with a thin layer of a metal, preventing oxide formation. Au has been one of the primary metals investigated for this purpose as it does not form an oxide and can have induced superconductivity due to the proximity effect. In this study, low-temperature scanning tunneling microscopy and spectroscopy is used to investigate the superconducting properties of Nb capped with Au thin films, focusing on the spatial variation of the quasiparticle density of states, with special interest given to states within the superconducting gap near the Fermi Energy. Two different samples are studied. One with DC sputtered Nb capped ex-situ with vapor-deposited Au, where the native oxide is partially removed with Ar sputtering. The second one with molecular beam epitaxy (MBE) deposited Nb and Au, done in-situ to avoid oxide formation. Both samples show a fully proximitized Au capping layer. Differences in the conductance spectra, due to the different interface, are analyzed using the proximity model in the diffusive limit. Local defects, likely from magnetic impurities, are also investigated; however, their density in these samples is significantly lower compared to bare Nb. These findings confirm that metallic capping layers may help mitigating source of decoherence in superconducting qubits.