Evolution of surface oxides and impurities in high vacuum heat treated Nb: A TEM and TOF-SIMS in-situ study, mechanism and repercussions on SRF cavity applications

Bose, A.; Mondal, Puspen; Bhalerao, G. M.; Kokil, S.V.; Raghavendra, S.; Joshi, S.C.; Srivastava, Arvind K.; Tewari, Raghunath

doi:10.1016/j.apsusc.2020.145464

Cited by 9 publications

(9 citation statements)

References 39 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In Figure b, we observe that the signal associated with Nb 2 O 5 exhibits a peak closer to the surface whereas the NbO 2 and NbO signals persist further into the film. This is in agreement with the literature indicating a change in valence state for the Nb throughout the oxide. , As the presence of Nb 2 O 5 has been specifically linked to decoherence in Nb superconducting qubit systems, we focus our analysis to this region throughout the text.…”

Section: Resultssupporting

confidence: 87%

“…Although direct quantification of these oxides can be challenging due to the possibility for molecular fragmentation prior to detection by the analyzer, monitoring these signals provides qualitative information regarding their spatial location within the oxide layer. 21 In Figure 1b, we observe that the signal associated with Nb 2 O 5 exhibits a peak closer to the surface whereas the NbO 2 and NbO signals persist further into the film. This is in agreement with the literature indicating a change in valence state for the Nb throughout the oxide.…”

Section: Resultsmentioning

confidence: 87%

“…Further, TOF-SIMS enables the identification of different oxide species based on their mass to charge ( m / z ) ratios. Although direct quantification of these oxides can be challenging due to the possibility for molecular fragmentation prior to detection by the analyzer, monitoring these signals provides qualitative information regarding their spatial location within the oxide layer . In Figure b, we observe that the signal associated with Nb 2 O 5 exhibits a peak closer to the surface whereas the NbO 2 and NbO signals persist further into the film.…”

Section: Resultsmentioning

confidence: 91%

See 2 more Smart Citations

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

et al. 2022

View full text Add to dashboard Cite

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.

show abstract

Section: Resultssupporting

confidence: 87%

Section: Resultsmentioning

confidence: 87%

Section: Resultsmentioning

confidence: 91%

See 1 more Smart Citation

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

et al. 2022

View full text Add to dashboard Cite

show abstract

“…The relative in-plane uniformity present in these profiles confirms that depth profiles are taken from representative regions free of surface features and the arti-facts that they can introduce. [41,42] Furthermore, it is again apparent that the lithography process introduces increased oxygen and hydrocarbon species throughout the film.…”

Section: Methodsmentioning

confidence: 99%

TOF-SIMS Analysis of Decoherence Sources in Nb Superconducting Resonators

Murthy,

Lee,

Kopas

et al. 2021

Preprint

View full text Add to dashboard Cite

Superconducting qubits have emerged as a potentially foundational platform technology for addressing complex computational problems deemed intractable with classical computing. Despite recent advances enabling multiqubit designs that exhibit coherence lifetimes on the order of hundreds of µs, material quality and interfacial structures continue to curb device performance. When niobium is deployed as the superconducting material, two-level system defects in the thin film and adjacent dielectric regions introduce stochastic noise and dissipate electromagnetic energy at the cryogenic operating temperatures. In this study, we utilize time-of-flight secondary ion mass spectrometry (TOF-SIMS) to understand the role specific fabrication procedures play in introducing such dissipation mechanisms in these complex systems. We interrogated Nb thin films and transmon qubit structures fabricated by Rigetti Computing and at the National Institute of Standards and Technology through slight variations in the processing and vacuum conditions. We find that when Nb film is sputtered onto the Si substrate, oxide and silicide regions are generated at various interfaces. We also observe that impurity species such as niobium hydrides and carbides are incorporated within the niobium layer during the subsequent lithographic patterning steps. The formation of these resistive compounds likely impact the superconducting properties of the Nb thin film. Additionally, we observe the presence of halogen species distributed throughout the patterned thin films. We conclude by hypothesizing the source of such impurities in these structures in an effort to intelligently fabricate superconducting qubits and extend coherence times moving forward.

show abstract

“…Even though diffusion of the nitrogen into the bulk of the SRF cavity is limited in depth at these low temperatures (120-200 °C), the introduction of nitrogen is sufficient to modify the cavity surface within the rf penetration depth as seen from cavities' test results, which are similar to those previously reported for high-temperature nitrogen doped cavities. The nitridation and diffusion of nitrogen in to the bulk is expected when the Nb surface is free of Nb2O5 which occurs >300 C [84,85], but the infusion was done when the cavity was fully annealed at 800 C/3hrs and cooldown to desired temperature under UHV conditions before injecting nitrogen in to the furnace. While post-doping electropolishing is required to remove coarse nitrides from the surfaces of high-temperature nitrogen doped cavities, no further processing is required for the low-temperature "infusion" recipe showing a clear benefit in reducing processing steps as well as keeping higher gradient with high Q0 values.…”

Section: B Recipe Development At Low Temperaturementioning

confidence: 99%

Nitrogen doping and infusion in SRF cavities: A review

Dhakal

2020

Physics Open

View full text Add to dashboard Cite

Evolution of surface oxides and impurities in high vacuum heat treated Nb: A TEM and TOF-SIMS in-situ study, mechanism and repercussions on SRF cavity applications

Cited by 9 publications

References 39 publications

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

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

TOF-SIMS Analysis of Decoherence Sources in Nb Superconducting Resonators

Nitrogen doping and infusion in SRF cavities: A review

Contact Info

Product

Resources

About