XPS studies of nitrogen doping niobium used for accelerator applications

TL;DR

This study uses XPS to analyze nitrogen-doped niobium surfaces for superconducting RF cavities, revealing chemical composition changes and the absence of nitrides at the surface and depth after specific treatments.

Contribution

It provides detailed surface chemical analysis of nitrogen-doped niobium, highlighting the absence of nitrides and depth-dependent nitrogen content after various surface treatments.

Findings

No nitrides NbNx detected on surface before GCIB etching.

Nitrogen content below detection limit beyond 30nm depth.

Surface treatments influence nitrogen distribution and chemical composition.

Abstract

Nitrogen doping study on niobium (Nb) samples used for the fabrication of superconducting radio frequency (SRF) cavities was carried out. The samples' surface treatment was attempted to replicate that of the Nb SRF cavities, which includes heavy electropolishing (EP), nitrogen doping and the subsequent EP with different amounts of material removal. The surface chemical composition of Nb samples with different post treatments has been studied by XPS. The chemical composition of Nb, O, C and N was presented before and after Gas Cluster Ion Beam (GCIB) etching. No signals of poorly superconducting nitrides NbNx was found on the surface of any doped Nb sample with the 2/6 recipe before GCIB etching. However, in the depth range greater than 30nm, the content of N element is below the XPS detection precision scope even for the Nb sample directly after nitrogen doping treatment with the 2/6 recipe.