Scanning Tunneling Microscopic and Spectroscopic Studies of Au-Capped Nb Films for Superconducting Qubit Development*

Niobium thin films are important components in superconducting qubit devices; however, the complex native oxide that forms naturally on Nb surfaces introduces microwave losses that can degrade qubit performance and affect the coherence time. Recent research has shown that removing the oxide layer and capping Nb with a metallic layer can significantly enhance qubit performance by preventing oxide formation. In this study, we utilized low-temperature scanning tunneling microscopy and spectroscopy (STM/S) to investigate the superconducting properties of Au-capped Nb films. Our focus was on the spatial inhomogeneity and the density of states (DOS) near the Fermi energy—both critical factors for optimizing qubit performance. Our results demonstrate that Au-capped Nb exhibits a uniform quasiparticle DOS without sub-gap states and a low density of defects, positioning it as an ideal material for qubit applications. Furthermore, our STM/S measurements reveal key distinctions between ex-situ and in-situ Au capping processes. Specifically, ex-situ capped samples display magnetic impurity-like defects at the Au-Nb interface, likely resulting from residual niobium suboxide at the interface between gold and niobium. In contrast, in-situ capped samples exhibit surface defects that may arise from insufficient coverage of the thin capping layer or contamination such as hydrogen. These findings offer valuable insights into optimizing surface preparation and tuning the density of states, to improve the coherence time of transmon qubits.