Insights into Improving Performance of Niobium Superconducting Transmon Qubit Devices through Advanced Microscopy

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Abstract
Superconducting thin films of niobium have been extensively employed in transmon qubit architectures. Although these architectures have demonstrated remarkable improvements in recent years, further improvements in performance through materials engineering will aid in large-scale deployment. Here, we use information retrieved from electron microscopy to conduct a detailed assessment of defects and inhomogeneities on the atomic and nanoscale that may potentially induce quantum decoherence in transmon qubit test devices. In the niobium thin film, we observe the presence of localized strain at the metal/substrate and grain boundaries, which may amplify interactions between two-level systems and impose limits on T$_1$ and T$_2$ relaxation times. Additionally, we observe the presence of a surface oxide with varying stoichiometry and bond distances, which can generate a broad two-level system noise spectrum. Finally, a similarly disordered and rough interface is observed between Nb and the Si substrate. We propose that this interface can also degrade the overall superconducting properties. Based on these findings, we propose solutions to eliminate these potentially problematic imperfections in future quantum devices.

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