- Physical Review Journals
All JournalsPhysics Magazine

Physical Review Materials

Reuse & Permissions

It is not necessary to obtain permission to reuse this article or its components as it is available under the terms of the Creative Commons Attribution 4.0 International license. This license permits unrestricted use, distribution, and reproduction in any medium, provided attribution to the author(s) and the published article's title, journal citation, and DOI are maintained. Please note that some figures may have been included with permission from other third parties. It is your responsibility to obtain the proper permission from the rights holder directly for these figures.

Export citation

Export citation

Choose format for download:

Download Citation
  • Open Access

Influence of an Al2O3 capping layer on the thermal reduction of the native niobium oxide: An in situ x-ray reflectivity study

Artem Zaidman1,2,*, Vedran Vonk2, Getnet Kacha Deyu1, Robert Zierold3, Robert Blick3,4, Wolfgang Hillert1, Marc Wenskat1,4, and Andreas Stierle2,5

  • *Contact author: artem.zaidman@desy.de
PDF

Phys. Rev. Materials 9, 094806 – Published 24 September, 2025

DOI: https://doi.org/10.1103/48mp-wypj

Abstract

Superconducting radio-frequency cavities, critical components of modern particle accelerators and quantum computing hardware, fundamentally rely on the surface properties of niobium. However, native oxide formation and impurity uptake in the near-surface region can degrade superconducting performance and increase radio-frequency losses during operation. In this study, we present a systematic in situ x-ray reflectivity investigation of (110) niobium single-crystal surfaces with and without an atomic layer deposited Al2O3 capping layer under ultrahigh-vacuum conditions up to 650∘C. Our results reveal a temperature-dependent reduction of the native niobium oxide layers in both capped and uncapped samples, with similarities in the overall behavior but clear differences in the reduction pathways. The Al2O3 capping layer modifies the reduction process, prevents oxide regrowth upon air exposure, and protects the niobium surface against impurity uptake during thermal treatment. These findings demonstrate that Al2O3 capping is an effective strategy to suppress native oxide formation during thermal cycling, offering clear benefits for the performance and operational lifetime of superconducting radio-frequency cavities and related quantum computing technologies.

View figure in article

Physics Subject Headings (PhySH)

Article Text

Supplemental Material

References (67)

  1. U. Amaldi, S. Braccini, and P. Puggioni, High frequency linacs for hadrontherapy, Rev. Accel. Sci. Tech. 02, 111 (2009).
  2. W. Decking et al., A MHz-repetition-rate hard x-ray free-electron laser driven by a superconducting linear accelerator, Nat. Photonics 14, 391 (2020).
  3. T. K. Kroc, Superconducting accelerators for high-power x-ray production, arXiv:2202.07702 (2022).
  4. W. DeSorbo, Effect of dissolved gases on some superconducting properties of niobium, Phys. Rev. 132, 107 (1963).
  5. W. Singer, Fabrication of elliptical SRF cavities, Supercond. Sci. Technol. 30, 033001 (2017).
  6. J. T. Maniscalco, D. Gonnella, and M. Liepe, The importance of the electron mean free path for superconducting radio-frequency cavities, J. Appl. Phys. 121, 043910 (2017).
  7. M. Martinello, A. Grassellino, M. Checchin, A. Romanenko, O. Melnychuk, D. A. Sergatskov, S. Posen, and J. F. Zasadzinski, Effect of interstitial impurities on the field dependent microwave surface resistance of niobium, Appl. Phys. Lett. 109, 062601 (2016).
  8. C. Z. Antoine, Influence of crystalline structure on rf dissipation in superconducting niobium, Phys. Rev. Accel. Beams 22, 034801 (2019).
  9. P. Dhakal, G. Ciovati, and A. Gurevich, Flux expulsion in niobium superconducting radio-frequency cavities of different purity and essential contributions to the flux sensitivity, Phys. Rev. Accel. Beams 23, 023102 (2020).
  10. P. Garg, C. Muhich, L. D. Cooley, T. R. Bieler, and K. N. Solanki, Possible role of grain-boundary and dislocation structure for the magnetic-flux trapping behavior of niobium: A first-principles study, Phys. Rev. B 101, 184102 (2020).
  11. F. Barkov, A. Romanenko, and A. Grassellino, Direct observation of hydrides formation in cavity-grade niobium, Phys. Rev. Spec. Top.–Accel. Beams 15, 122001 (2012).
  12. Y. Trenikhina, A. Romanenko, J. Kwon, J.-M. Zuo, and J. F. Zasadzinski, Nanostructural features degrading the performance of superconducting radio frequency niobium cavities revealed by transmission electron microscopy and electron energy loss spectroscopy, J. Appl. Phys. 117, 154507 (2015).
  13. G. D. L. Semione, V. Vonk, A. D. Pandey, E. Grånäs, B. Arndt, M. Wenskat, W. Hillert, H. Noei, and A. Stierle, Temperature-dependent near-surface interstitial segregation in niobium, J. Phys.: Condens. Matter 33, 265001 (2021).
  14. J. P. Turneaure, J. Halbritter, and H. A. Schwettman, The surface impedance of superconductors and normal conductors: The Mattis-Bardeen theory, J. Supercond. 4, 341 (1991).
  15. T. Kubo, Effects of nonmagnetic impurities and subgap states on the kinetic inductance, complex conductivity, quality factor, and depairing current density, Phys. Rev. Appl. 17, 014018 (2022).
  16. V. Ngampruetikorn and J. A. Sauls, Effect of inhomogeneous surface disorder on the superheating field of superconducting rf cavities, Phys. Rev. Res. 1, 012015 (2019).
  17. T. Kubo, Multilayer coating for higher accelerating fields in superconducting radio-frequency cavities: A review of theoretical aspects, Supercond. Sci. Technol. 30, 023001 (2017).
  18. T. Kubo, Superheating fields of semi-infinite superconductors and layered superconductors in the diffusive limit: Structural optimization based on the microscopic theory, Supercond. Sci. Technol. 34, 045006 (2021).
  19. A. Gurevich and T. Kubo, Surface impedance and optimum surface resistance of a superconductor with an imperfect surface, Phys. Rev. B 96, 184515 (2017).
  20. T. Kubo and A. Gurevich, Field-dependent nonlinear surface resistance and its optimization by surface nanostructuring in superconductors, Phys. Rev. B 100, 064522 (2019).
  21. W. Singer, X. Singer, A. Brinkmann, J. Iversen, A. Matheisen, A. Navitski, Y. Tamashevich, P. Michelato, and L. Monaco, Superconducting cavity material for the european XFEL, Supercond. Sci. Technol. 28, 085014 (2015).
  22. D. Reschke, V. Gubarev, J. Schaffran, L. Steder, N. Walker, M. Wenskat, and L. Monaco, Performance in the vertical test of the 832 nine-cell 1.3 GHz cavities for the European X-ray Free Electron Laser, Phys. Rev. Accel. Beams 20, 042004 (2017).
  23. J. Knobloch, The “q disease” in superconducting niobium rf cavities, AIP Conf. Proc. 671, 133 (2003).
  24. P. Dhakal, G. Ciovati, U. Pudasaini, S. Chetri, S. Balachandran, and P. J. Lee, Surface characterization of nitrogen-doped high purity niobium coupons compared with superconducting rf cavity performance, Phys. Rev. Accel. Beams 22, 122002 (2019).
  25. R. Darren Veit, R. G. Farber, N. S. Sitaraman, T. A. Arias, and S. J. Sibener, Suppression of nano-hydride growth on Nb(100) due to nitrogen doping, J. Chem. Phys. 152, 214703 (2020).
  26. A. Grassellino, A. Romanenko, D. Sergatskov, O. Melnychuk, Y. Trenikhina, A. Crawford, A. Rowe, M. Wong, T. Khabiboulline, and F. Barkov, Nitrogen and argon doping of niobium for superconducting radio frequency cavities: A pathway to highly efficient accelerating structures, Supercond. Sci. Technol. 26, 102001 (2013).
  27. A. Grassellino, A. Romanenko, Y. Trenikhina, M. Checchin, M. Martinello, O. S. Melnychuk, S. Chandrasekaran, D. A. Sergatskov, S. Posen, A. C. Crawford, S. Aderhold, and D. Bice, Unprecedented quality factors at accelerating gradients up to 45 MVm−1 in niobium superconducting resonators via low temperature nitrogen infusion, Supercond. Sci. Technol. 30, 094004 (2017).
  28. D. Gonnella, S. Aderhold, A. Burrill, E. Daly, K. Davis, A. Grassellino, C. Grimm, T. Khabiboulline, F. Marhauser, O. Melnychuk, A. Palczewski, S. Posen, M. Ross, D. Sergatskov, and K. Wilson, Industrial cavity production: Lessons learned to push the boundaries of nitrogen-doping, in Proceedings of the 19th International Conference on RF Superconductivity (SRF'19), Dresden, Germany (JACoW Publishing, Geneva, Switzerland, 2019), pp. 1199–1205.
  29. M. Wenskat, C. Bate, A. Dangwal Pandey, A. Jeromin, T. F. Keller, J. Knobloch, J. Köszegi, F. Kramer, O. Kugeler, S. Kulkarni, D. Reschke, J. Schaffran, G. D. L. Semione, S. Sievers, L. Steder, A. Stierle, and N. Walker, Nitrogen infusion R&D at DESY a case study on cavity cut-outs, Supercond. Sci. Technol. 33, 115017 (2020).
  30. K. Umemori, E. Kako, T. Konomi, S. Michizono, T. Okada, H. Sakai, and J. Tamura, Study on nitrogen infusion using KEK new furnace, in Proceedings of the 19th International Conference on RF Superconductivity (SRF'19), Dresden, Germany (JACoW Publishing, Geneva, Switzerland, 2019), pp. 95–98.
  31. M. Wenskat, A. L. Prudnikava, D. Reschke, and J. Schaffran, Nitrogen infusion R&D on single cell cavities at DESY, in Proceedings of the 18th International Conference on RF Superconductivity (SRF'17), Lanzhou, China (JACoW, Geneva, Switzerland, 2018), pp. 759–761.
  32. P. Dhakal, G. Ciovati, G. R. Myneni, K. E. Gray, N. Groll, P. Maheshwari, D. M. McRae, R. Pike, T. Proslier, F. Stevie, R. P. Walsh, Q. Yang, and J. Zasadzinzki, Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity, Phys. Rev. Spec. Top.–Accel. Beams 16, 042001 (2013).
  33. S. Posen, A. Romanenko, A. Grassellino, O. S. Melnychuk, and D. A. Sergatskov, Ultralow surface resistance via vacuum heat treatment of superconducting radio-frequency cavities, Phys. Rev. Appl. 13, 014024 (2020).
  34. H. Ito, H. Araki, K. Takahashi, and K. Umemori, Influence of furnace baking on q-e behavior of superconducting accelerating cavities, Prog. Theor. Exp. Phys. 2021, 071G01 (2021).
  35. M. Delheusy, A. Stierle, N. Kasper, R. P. Kurta, A. Vlad, H. Dosch, C. Antoine, A. Resta, E. Lundgren, and J. Andersen, X-ray investigation of subsurface interstitial oxygen at Nb/oxide interfaces, Appl. Phys. Lett. 92, 101911 (2008).
  36. G. D. L. Semione, A. D. Pandey, S. Tober, J. Pfrommer, A. Poulain, J. Drnec, G. Schütz, T. F. Keller, H. Noei, V. Vonk, B. Foster, and A. Stierle, Niobium near-surface composition during nitrogen infusion relevant for superconducting radio-frequency cavities, Phys. Rev. Accel. Beams 22, 103102 (2019).
  37. A. Gurevich, Enhancement of rf breakdown field of superconductors by multilayer coating, Appl. Phys. Lett. 88, 012511 (2006).
  38. T. Proslier, J. Zasadzinski, J. Moore, M. Pellin, J. Elam, L. Cooley, C. Antoine, J. Norem, and K. E. Gray, Improvement and protection of niobium surface superconductivity by atomic layer deposition and heat treatment, Appl. Phys. Lett. 93, 192504 (2008).
  39. M. Wenskat, G. K. Deyu, I. González Díaz-Palacio, R. H. Blick, R. Zierold, and W. Hillert, Successful Al2O3 coating of superconducting niobium cavities with thermal ALD, Supercond. Sci. Technol. 36, 015010 (2023).
  40. Y. Kalboussi, B. Delatte, S. Bira, K. Dembele, X. Li, F. Miserque, N. Brun, M. Walls, J. L. Maurice, D. Dragoe, J. Leroy, D. Longuevergne, A. Gentils, S. Jublot-Leclerc, G. Jullien, F. Eozenou, M. Baudrier, L. Maurice, and T. Proslier, Reducing two-level systems dissipations in 3D superconducting niobium resonators by atomic layer deposition and high temperature heat treatment, Appl. Phys. Lett. 124, 134001 (2024).
  41. A. Romanenko and D. I. Schuster, Understanding quality factor degradation in superconducting niobium cavities at low microwave field amplitudes, Phys. Rev. Lett. 119, 264801 (2017).
  42. Y. Kalboussi, I. Curci, F. Miserque, D. Troadec, N. Brun, M. Walls, G. Jullien, F. Eozenou, M. Baudrier, L. Maurice, Q. Bertrand, P. Sahuquet, and T. Proslier, Crystallinity in niobium oxides: A pathway to mitigate two-level-system defects in niobium three-dimensional resonators for quantum applications, Phys. Rev. Appl. 23, 044023 (2025).
  43. D. Bafia, A. Murthy, A. Grassellino, and A. Romanenko, Oxygen vacancies in niobium pentoxide as a source of two-level system losses in superconducting niobium, Phys. Rev. Appl. 22, 024035 (2024).
  44. M. Wenskat, J. Čižek, M. O. Liedke, M. Butterling, M. Stiehl, G. D. L. Semione, C. Backes, C. Bate, O. Melikhova, E. Hirschmann, A. Wagner, H. Weise, A. Stierle, M. Aeschlimann, and W. Hillert, Vacancy dynamics in niobium and its native oxides and their potential implications for quantum computing and superconducting accelerators, Phys. Rev. B 106, 094516 (2022).
  45. A. A. Murthy, P. Masih Das, S. M. Ribet, C. Kopas, J. Lee, M. J. Reagor, L. Zhou, M. J. Kramer, M. C. Hersam, M. Checchin, A. Grassellino, R. D. Reis, V. P. Dravid, and A. Romanenko, Developing a chemical and structural understanding of the surface oxide in a niobium superconducting qubit, ACS Nano 16, 17257 (2022).
  46. G. Wendin, Quantum information processing with superconducting circuits: A review, Rep. Prog. Phys. 80, 106001 (2017).
  47. D. A. Bennett, L. Longobardi, V. Patel, W. Chen, D. V. Averin, and J. E. Lukens, Decoherence in rf squid qubits, Quantum Inf. Process. 8, 217 (2009).
  48. J. Koch, T. M. Yu, J. Gambetta, A. A. Houck, D. I. Schuster, J. Majer, A. Blais, M. H. Devoret, S. M. Girvin, and R. J. Schoelkopf, Charge-insensitive qubit design derived from the cooper pair box, Phys. Rev. A 76, 042319 (2007).
  49. A. Romanenko, R. Pilipenko, S. Zorzetti, D. Frolov, M. Awida, S. Belomestnykh, S. Posen, and A. Grassellino, Three-dimensional superconducting resonators at t<20 mk with photon lifetimes up to t=2s, Phys. Rev. Appl. 13, 034032 (2020).
  50. M. Bal et al., Systematic improvements in transmon qubit coherence enabled by niobium surface encapsulation, npj Quantum Inf. 10, 43 (2024).
  51. E. J. Mittemeijer and U. S. Welzel, Modern Diffraction Methods (Wiley-VCH, 2013).
  52. L. G. Parratt, Surface studies of solids by total reflection of x-rays, Phys. Rev. 95, 359 (1954).
  53. A. Stierle, T. F. Keller, H. Noei, V. Vonk, and R. Roehlsberger, DESY Nanolab, J. Large Scale Res. Facil. 2, A76 (2016).
  54. See Supplemental Material at ahttp://link.aps.org/supplemental/10.1103/48mp-wypj for detailed fit parameters.
  55. A. Glavic and M. Björck, GenX 3: The latest generation of an established tool, J. Appl. Crystallogr. 55, 1063 (2022).
  56. M. Grundner and J. Halbritter, On the natural Nb2O5 growth on Nb at room temperature, Surf. Sci. 136, 144 (1984).
  57. Q. Ma, P. Ryan, J. W. Freeland, and R. A. Rosenberg, Thermal effect on the oxides on Nb(100) studied by synchrotron-radiation x-ray photoelectron spectroscopy, J. Appl. Phys. 96, 7675 (2004).
  58. A. Prudnikava, Y. Tamashevich, A. Makarova, D. Smirnov, and J. Knobloch, In-situ synchrotron x-ray photoelectron spectroscopy study of medium-temperature baking of niobium for SRF application, Supercond. Sci. Technol. 37, 075007 (2024).
  59. M. Yu, S. Huang, Y. Zhao, X. Gu, L. Peng, J. Lai, T. Zhu, Y. Wang, A. Wu, T. Tan, Y. He, H. Cao, Y. Cao, and K. Zhang, Study on the interstitial oxygen diffusion to understand the reduction of cryogenic rf loss for the superconducting radio-frequency niobium cavities, Supercond. Sci. Technol. 37, 105014 (2024).
  60. G. Ciovati, Improved oxygen diffusion model to explain the effect of low-temperature baking on high field losses in niobium superconducting cavities, Appl. Phys. Lett. 89, 022507 (2006).
  61. E. M. Lechner, J. W. Angle, F. A. Stevie, M. J. Kelley, C. E. Reece, and A. D. Palczewski, RF surface resistance tuning of superconducting niobium via thermal diffusion of native oxide, Appl. Phys. Lett. 119, 082601 (2021).
  62. A. Dangwal Pandey, T. F. Keller, M. Wenskat, A. Jeromin, S. Kulkarni, H. Noei, V. Vonk, W. Hillert, D. Reschke, N. Walker, H. Weise, and A. Stierle, Grain boundary segregation and carbide precipitation in heat treated niobium superconducting radio frequency cavities, Appl. Phys. Lett. 119, 194102 (2021).
  63. A. Bose, P. Mondal, G. M. Bhalerao, S. V. Kokil, S. Raghavendra, S. C. Joshi, A. K. Srivastava, and R. Tewari, 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, Appl. Surf. Sci. 510, 145464 (2020).
  64. J. F. Smith, O. N. Carlson, and R. R. de Avillez, The niobium-carbon system, J. Nucl. Mater. 148, 1 (1987).
  65. S. Jakschik, U. Schroeder, T. Hecht, M. Gutsche, H. Seidl, and J. W. Bartha, Crystallization behavior of thin ALD-Al2O3 films, Thin Solid Films 425, 216 (2003).
  66. M. Broas, J. Lemettinen, T. Sajavaara, M. Tilli, V. Vuorinen, S. Suihkonen, and M. Paulasto-Kröckel, In-situ annealing characterization of atomic-layer-deposited Al2O3 in N2, H2 and vacuum atmospheres, Thin Solid Films 682, 147 (2019).
  67. O. M. E. Ylivaara, A. Langner, S. Ek, J. Malm, J. Julin, M. Laitinen, S. Ali, S. Sintonen, H. Lipsanen, T. Sajavaara, and R. L. Puurunen, Thermomechanical properties of aluminum oxide thin films made by atomic layer deposition, J. Vac. Sci. Technol. A 40, 062414 (2022).

Outline

Information

Phys. Rev. Materials 9, 094806– Published 24 September, 2025
  • Received 2 July 2025
  • Revised 1 August 2025
  • Accepted 12 August 2025
Crossmark - Check for updates button
Creative Commons Logo

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Sign In to Your Journals Account

Filter

Filter

Article Lookup

Enter a citation