Developing a Chemical and Structural Understanding of the Surface Oxide in a Niobium Superconducting Qubit

TL;DR

This study provides a detailed chemical and structural analysis of the surface oxide in niobium superconducting qubits, revealing variations in composition and crystallinity that impact qubit coherence.

Contribution

It introduces a correlative microscopy approach to characterize the oxide's composition and structure at nanometer resolution, linking oxygen vacancies to potential decoherence.

Findings

Surface oxide varies in stoichiometry from NbO to Nb$_2$O$_5$

Nb$_2$O$_5$ is semicrystalline with nanometer-sized grains

Oxygen vacancies are associated with amorphous regions and weaker Nb–O bonds

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 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 NbO$_2$ found closer to the niobium film and Nb$_2$O$_5$ found closer to the surface. In terms of structural analysis, we find that the Nb$_2$O$_5$ region is semicrystalline in nature and exhibits randomly oriented grains on the order of 1-2 nm corresponding to monoclinic N-Nb$_2$O$_5$ that are dispersed throughout an amorphous matrix. Using fluctuation electron microscopy, we are able to map the relative crystallinity in the Nb$_2$O$_5$ 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.