Detailed Structural and Chemical Analysis of Amorphous Compounds in Superconducting Qubit Systems*

Following improvements in device coherence times and gate fidelities over the past two decades, defects, impurities, and interfaces have emerged as the key barriers currently limiting the performance of superconducting quantum systems. As experimental and theoretical investigations have suggested that deviations from crystalline order can lead to quantum decoherence, we use a combination of scanning transmission electron imaging and diffraction methods to interrogate the thin metal films integral for superconducting qubit operation. Specifically, we investigate disordered compounds at the metal/air and metal/substrate interfaces. We observe that highly disordered regions in the oxide that forms at the surface of the niobium film are more likely to contain oxygen vacancies and exhibit weaker bonds between the niobium and oxygen atoms. Additionally, at the interface between niobium and the underlying silicon substrate, we observe an amorphous alloyed region that exhibits variations in stoichiometry and bond distances. We hypothesize that each of these features may contribute to quantum decoherence. Equipped with these findings, we seek to engineer the atomic ordering in these regions to intelligently fabricate superconducting qubits and extend coherence times.