Niobium near-surface composition during nitrogen infusion relevant for superconducting radio-frequency cavities

A detailed study of the near-surface structure and composition of Nb, the material of choice for superconducting radio-frequency accelerator (SRF) cavities, is of great importance in order to understand the effects of different treatments applied during cavity production. By means of surface-sensitive techniques such as grazing incidence diffuse x-ray scattering, x-ray reflectivity, and x-ray photoelectron spectroscopy, single-crystalline Nb(100) samples were investigated in and ex situ during annealing in an ultrahigh vacuum as well as in nitrogen atmospheres with temperatures and pressures similar to the ones employed in real Nb cavity treatments. Annealing of Nb specimens up to $800°\mathrm{C}$ in a vacuum promotes a partial reduction of the natural surface oxides (${\mathrm{Nb}}_2{\mathrm{O}}_5$, ${\mathrm{NbO}}_2$, and NbO) into NbO. Upon cooling to $120°\mathrm{C}$, no evidence of nitrogen-rich layers was detected after nitrogen exposure times of up to 48 h. An oxygen enrichment below the Nb-oxide interface and posterior diffusion of oxygen species towards the Nb matrix, along with a partial reduction of the natural surface oxides, was observed upon a stepwise annealing up to $250°\mathrm{C}$. Nitrogen introduction to the system at $250°\mathrm{C}$ promotes neither N diffusion into the Nb matrix nor the formation of new surface layers. Upon further heating to $500°\mathrm{C}$ in a nitrogen atmosphere, the growth of a new subsurface ${\mathrm{Nb}}_x{\mathrm{N}}_y$ layer was detected. These results shed light on the composition of the near-surface region of Nb after low-temperature nitrogen treatments, which are reported to lead to a performance enhancement of SRF cavities.

Figure 1

Schematic of the GIXRD setup. The high-energy x-ray beam falls on the sample surface at grazing incident angle ${\alpha }_i$, and the diffraction patterns are obtained by a 2D detector while the sample undergoes different annealing treatments under UHV and ${\mathrm{N}}_2$. The blue circle represents the position of the interstitial diffuse scattering along the [111] direction (red line), while Nb reflections, indexed by their $hkl$ values, are blocked with lead pieces to locally protect the detector. In order to sample 3D reciprocal space, the sample is typically rotated around its surface normal (dashed line), and at each angular position a diffraction pattern is recorded.

Figure 2

(a) X-ray reflectivity data measured with Mo ${\mathrm{K}}_{\alpha }$ radiation (open circles) with respective fits (solid lines) for a Nb(100) single crystal as received and after $800°\mathrm{C}$ annealing in UHV for 1 h. The dashed line represents the simulated curve for a Nb surface without any surface oxide and with a constant background. The scans are offset in the $y$ direction for better visibility. (b) Electron-density profiles for both sets of data obtained from the fits.

Figure 3

(a) X-ray reflectivity data measured with ${\mathrm{CuK}}_{\alpha }$ radiation (open circles) with respective fits (solid lines) for various steps of the infusion procedure. The sample curvature compromises the fit in the first few data points. The scans are offset in the $y$ direction for better visibility. (b) Electron density profiles for each step. The extracted densities are lower than the expected bulk values for the niobium oxides and metallic niobium due to the pronounced sample curvature.

Figure 4

Thickness and substrate roughness (inset) obtained by XRR in each step of the sample treatment.

Figure 5

(a) X-ray reflectivity data (open circles) measured with photon energies of 70 keV with the respective fits (solid lines). The scans are offset in the $y$ direction. (b) Electron-density profiles obtained from the fits for the data collected from RT up to the addition of ${\mathrm{N}}_2$ at $3.33\times {10}^{-2}\mathrm{mbar}$ at $250°\mathrm{C}$. (c) Comparison between the electron density profiles obtained at $250°\mathrm{C}$ and $500°\mathrm{C}$ in a ${\mathrm{N}}_2$ atmosphere.

Figure 6

Diffraction patterns obtained at different temperatures with an incident angle of 0.04°, corresponding to a scattering depth ${\mathrm{\Lambda }}_{\text{eff}}$ of $\sim 100\mathrm{nm}$. At $120°\mathrm{C}$, a clear increase in diffuse intensity is observed due to the presence of subsurface oxygen. Lead (Pb) pieces block the Nb Bragg peaks. Powder rings originating from the beryllium (Be) windows of the chamber are also observed. The region of interest employed in the data analysis is schematically shown in blue (see the text for details).

Figure 7

(a) Diffuse scattering intensities obtained for each temperature step (squares) and the respective fit (solid line). The curves are offset in the $y$ direction. (b) Interstitial concentration obtained from the fit plotted in the linear scale and the concentration in the log scale (inset). The profiles are represented by connected squares (room temperature), solid ($120°\mathrm{C}$), dashed ($200\mathrm{°C}$), dot-dashed ($250°\mathrm{C}$), and dotted lines ($250°\mathrm{C}$ under a ${\mathrm{N}}_2$ atmosphere).

Figure 8

Deconvolution of Nb 3d and N 1s lines in normal exit mode (a),(c) and with a 70° exit angle (b),(d). Phases corresponding to niobium, niobium oxides, and niobium carbides are represented in gray. Contributions from niobium nitrides are shown in black.

Figure 9

Schematic representation of the stepwise annealing of Nb(100). The natural oxide layers gradually dissolve, and the liberated oxygen atoms diffuse into the Nb matrix. At $500\mathrm{°C}$ under a ${\mathrm{N}}_2$ atmosphere, a ${\mathrm{Nb}}_x{\mathrm{N}}_y$ layer is formed underneath the remaining niobium oxides.