Investigation into surface composition of nitrogen-doped niobium for superconducting RF cavities

Abstract

Systematic analysis of the surface morphology, crystalline phase, chemical composition and elemental distribution along depth for nitrogen-doped niobium was carried out using different methods of characterization, including Scanning Electron Microscopy (SEM), Atomic-Force Microscopy (AFM), Grazing Incidence X-ray Diffraction (GIXRD), Rutherford Backscattering Spectrometry (RBS) and layer-by-layer X-ray Photoelectron Spectroscopy (XPS) analysis. The results showed that, after nitrogen doping, the surface was covered by densely distributed trigonal precipitates with an average crystallite size of 32 ± 8 nm, in line with the calculation result (29.9 nm) of nitrogen-enriched β-Nb2N from GIXRD, demonstrating the phase composition of trigonal precipitates. The depth analysis through RBS and XPS indicated that β-Nb2N was dominant in the topmost 9.7 nm and extended to a depth of 575 nm, with gradually decreased content. In addition, the successive change along depth in the naturally oxidized states of niobium after nitrogen doping, was revealed. It was interesting to find that the oxygen diffusion depth could be moderately enhanced by the nitridation process. These results established the near-surface phase composition of nitrided niobium, which is of great significance in evaluating the effect of nitrogen doping and further understanding the Q improvement of the superconducting radio frequency cavities.

Characterization of dissipative regions of a N-doped superconducting radio-frequency cavity

“…Here XPS provides information about the chemical composition of the surface oxide layer while STM/STS probes the quasiparticle density of states, the superconducting gap, subgap states, as well as their spatial inhomogeneity (Lechner et al, 2020). The samples' surface was characterized with a scanning electron microscope (SEM) for any evidence of defects such as etch pits (Zhao et al, 2010) or scarring due to hydrides (Barkov et al, 2013) or nitrides (Trenikhina et al, 2015;Dangwal Pandey et al, 2018;Dhakal et al, 2019;Spradlin et al, 2019;Yang et al, 2021).…”

Cleaning of nitrogen-containing carbon contamination by atmospheric pressure plasma jet

“…To have better understanding of chemical bonds in the deposited C-N films, high-resolution XPS (HR-XPS) with a scanning step size of 0.1 eV was carried out for W_CN and the results are shown in figure 6, which gives both the original test data and the peak fitting results. The rules of peak fitting using Multipak software can be found in our previously reported paper [27]. An area constraint of 4:3 was applied to the spinorbit doublets of the W 4f 7/2 and W 4f 5/2 energy levels and their binding energies (BEs) were set apart for 2.18 eV.…”

“…The measured sputtering rate of SiO 2 is about 4.76 nm min −1 and the derived sputtering rate of Si is 5.04 nm min −1 . Details of the measurement process can be found in the supplemental file of [27].…”

Significant improvement of the lower critical field in Y doped Nb: potential replacement of basic material for the radio-frequency superconducting cavity

“…However, in practical terms, the pure Nb cavity is approaching its fundamental limit in terms of the magnetic flux entry field B sh [19][20][21], and it is highly desirable to have materials with better performance than Nb to make the SRF cavity [2,9]. In order to improve the peak magnetic field or quality factor of a Nb cavity, some treatments have been carried out, such as the so-called International Linear Collider (ILC)recipe with the combination of the electro-polishing and baking at 120 • C for 48 h [22][23][24][25], nitrogen-doping [26][27][28][29], nitrogen-infusion [21,30], Nb 3 Sn coating [31][32][33], rare earth elements doping [34], etc. Some ideas for achieving higher energy barrier of flux entry were also proposed [2].…”