Scanning Tunneling Microscopy/Spectroscopy of Nb films capped with Re for Potential Qubit Development

Transmon qubits incorporating Nb thin film capacitors experience significant RF losses (both TLS and non-TLS) due to the amorphous Nb oxide. Capping layers on the Nb have shown promise to reduce such losses and here we use STM/STS to study the surface superconductivity of Nb capped with Re, a refractory metal with minimal surface oxide. Bilayers were fabricated in the same chamber with sequential d.c. sputter deposition on sapphire of Nb followed by Re, without breaking vacuum. Uniform Re coverage is observed in TEM. For scanning tunneling the Re surface is prepared first by a mild anneal of the bilayer film and this is followed later by a brief sputter etch. Wide swaths of area on the Re surface exhibit uniform, fully gapped superconductivity with zero sub-gap quasiparticle states and gap parameters close to that of Nb. Tunneling conductances are successfully fit with a diffusive proximity effect model incorporating the Usadel equations. Small regions of area with nanoscale defects reveal spectra consistent with magnetic moments including zero bias conductance peaks due to Kondo tunneling channels inside a broadened Nb gap. Some defects exhibit well defined Shiba states at T =0.4K. The origin of these defects have not yet been established but the spectral features are consistent defective NbOx regions observed on uncapped Nb films. STM topographic images reveal a dendritic Re layer which might support the diffusion of Nb and O to the Re surface.