Atomic-Scale Characterization and Electronic Properties of Gold-Capped Niobium Films for Superconducting Qubits

Niobium thin films are central to superconducting qubits, but their complex native oxides contribute significantly to microwave losses. A promising mitigation strategy is to suppress oxide formation using engineered thin-film encapsulation layers. Recent experiments have shown that ∼10 nm metallic overlayers on Nb capacitor films can improve the energy relaxation time <i>T</i>₁ of transmon qubits. Here, we present a comparative study of Au-capped Nb films fabricated <i>in situ</i> by molecular beam epitaxy and <i>ex situ</i> by sequential deposition benchmarked against bare Nb films. Using complementary structural, chemical, and spectroscopic techniques, we correlate the interface quality with superconducting electronic properties of the Au surface. Low-temperature scanning tunneling spectroscopy (STS) provides spatially resolved quasiparticle density-of-states maps. While both Au-capped Nb samples exhibit large areas with uniform, fully gapped density of states, clear differences emerge between the two interfaces. Compared to <i>in situ</i> Nb-Au, <i>ex situ</i> Nb-Au exhibits a reduced induced superconducting gap, broadened coherence peak, and localized in-gap states, consistent with a residual Nb_<i>x</i>O_<i>y</i> layer at the interface. Supported by complementary structural and chemical analyses, these findings demonstrate that STS directly links nanoscale superconducting properties to interface preparation, highlighting the importance of controlled <i>in situ</i> encapsulation for minimizing dissipation and improving quantum coherence.