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

Abstract

Superconducting thin films of niobium have been extensively employed in transmon qubit architectures. Although these architectures have demonstrated 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/oxide interface 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 1–3 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 the highly disordered 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.

Probing Oxidation-Driven Amorphized Surfaces in a Ta(110) Film for Superconducting Qubit

“…We must note that the Ta oxidation mode is clearly distinct from that of Nb: (i) Nb forms a thicker native oxide layer (5− 6 nm), while the Ta native oxide is only 3−4 nm thick, 19,22 and (ii) the Nb native oxide layer contains Nb suboxides over a thickness of a few nm, 18 while Ta suboxide forms just a monolayer. Furthermore, beyond confirming the presence of Ta suboxide via synchrotron-based variable-energy XPS 30 after the long-held belief that only Ta 2 O 5 exists in the naturally oxidized Ta surface, 22 we provide an atomistic model of Ta suboxide and its chemical identity, structure, and spatial distribution from the current study.…”

Topical Review of Quantum Materials and Heterostructures Studied by Polarized Neutron Reflectometry

“…Adapted with permission. [ 34 ] Copyright 2022, American Chemical Society. e) An example of PNR measured in a high‐temperature superconductor/ferromagnet YBCO/STO/LCMO heterostructure.…”

“…An exciting development since 2019 has been the rise in popularity of quantum computing using “simple” elemental superconductors such as Nb and Al. In particular, Nb/NbO x and Al/AlO x are widely used in the transmon qubit architecture, [ 34 ] as depicted in Figure 3d. These architectures were the basis of the first claims (by the Google team in 2019) of a 54‐qubit system that was presented as the first example of “quantum supremacy,” i.e., the ability to do a specific type of calculation faster than a classical computer.…”

“…d) An illustration of the modern application of Nb superconductors in the transmon qubit geometry, where the black box illustrates the components comprised of Nb and NbO x layers, similar to those depicted by the cross-sectional TEM image on the right. Adapted with permission [34]. Copyright 2022, American Chemical Society.…”

Tuning Epitaxial Growth of Nb on MgO(100)

“…Niobium is a superconductor with the highest superconducting transition temperature among elemental metals (T C =9.3 K in bulk). In this respect two main applications are the superconducting radio-frequency cavities for particle accelerators, [1] and Josephson junctions, [2,3] which are the basis of superconducting qubit operation. [4] In addition, niobium is able to form highquality interfaces with various non-superconducting materials, which makes Nb-based heterostructures promising in many applications of superconducting electronics.…”