铌超导量子比特中表面氧化物的化学和结构理解
Developing a Chemical and Structural Understanding of the Surface Oxide in a Niobium Superconducting Qubit
摘要：
铌的超导薄膜已广泛用于transmon量子位架构中。虽然这些架构近年来已经取得了显着的进步，但通过材料工程进一步提高性能将有助于大规模部署。在这里，我们使用从二次离子质谱和电子显微镜检索的信息进行详细的评估，在环境条件下形成的transmon测试量子位器件从铌膜图案化的表面氧化物。我们观察到，这种氧化物表现出不同的化学计量与NbO和NbO $_2$发现更接近铌膜和Nb $_2$ O $_5$发现更接近表面。在结构分析方面，我们发现，Nb $_2$ O $_5$区域是半结晶的性质，并表现出随机取向的晶粒上的顺序为1-2纳米对应于单斜N-Nb $_2$ O $_5$分散在整个非晶基体。使用波动电子显微镜，我们能够映射在Nb $_2$ O $_5$区域的相对结晶度与纳米空间分辨率。通过这种关联方法，我们观察到非晶区更可能含有氧空位，并且铌和氧原子之间的键合更弱。基于这些发现，我们预计氧空位可能作为量子系统中的退相干机制。
英文摘要：
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.
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