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 secondary ion mass spectrometry and electron microscopy to conduct a detailed assessment of the surface oxide that forms in ambient conditions for transmon test qubit devices patterned from a niobium film. We observe that this oxide exhibits a varying stoichiometry with NbO and NbO2 found closer to the niobium film and Nb2O5 found closer to the surface. In terms of structural analysis, we find that the Nb2O5 region is semicrystalline in nature and exhibits randomly oriented grains on the order of 1-2 nm corresponding to monoclinic N-Nb2O5 that are dispersed throughout an amorphous matrix. Using fluctuation electron microscopy, we are able to map the relative crystallinity in the Nb2O5 region with nanometer spatial resolution. Through this correlative method, we observe that amorphous regions are more likely to contain oxygen vacancies and exhibit weaker bonds between the niobium and oxygen atoms. Based on these findings, we expect that oxygen vacancies likely serve as a decoherence mechanism in quantum systems.
@article{arxiv.2203.08710,
title = {Developing a Chemical and Structural Understanding of the Surface Oxide in a Niobium Superconducting Qubit},
author = {Akshay A. Murthy and Paul Masih Das and Stephanie M. Ribet and Cameron Kopas and Jaeyel Lee and Matthew J. Reagor and Lin Zhou and Matthew J. Kramer and Mark C. Hersam and Mattia Checchin and Anna Grassellino and Roberto dos Reis and Vinayak P. Dravid and Alexander Romanenko},
journal= {arXiv preprint arXiv:2203.08710},
year = {2022}
}