Scanning Tunneling Spectroscopy Studies of Metal Capping layers on Nb for Superconducting Qubit Development*

Recent studies have shown significant improvement in the T₁ decay time of transmon qubits by using a thin (~10 nm) capping layer of Al, Ta, or TiN on top of the Nb capacitor film, thereby preventing the formation of the RF lossy Nb oxide layer. Here we demonstrate that scanning tunneling spectroscopy (STS) can play an important role in identifying optimal capping layers, including metals that have little or no surface oxide layer such as Re. STS directly reveals the proximity induced gap in normal metal capping layers and probes any sub-gap quasiparticle states that might cause non-TLS losses, down to atomic scale. Inhomogeneities in superconducting gap values can also lead to potential TLS losses. A focus is on a comparison of sub-gap states capped Nb films as compared to bare Nb films where the native oxide has been removed by ion milling. Preliminary data indicate that capping layers can lead to a reduction in sub-gap quasiparticle states.