Optimization of the superconducting properties of NbTiN thin films by variation of the N2 partial pressure during sputter deposition

Abstract

We report the effect of nitrogen partial pressure on the growth of niobium-titanium nitride (NbTiN) thin films through reactive sputtering of the NbTi target in the presence of Ar and N2 atmosphere. The N2 partial pressure was varied from 5.8% to 15.15% with respect to the Ar flow available in the chamber. We measured a critical temperature (T C) as high as 15.77 K for an N2 partial pressure of 6.8% for a 50 nm film deposited on an MgO substrate. The epitaxial growth of the NbTiN films is evident from the observed (200) and (400) XRD peaks with respect to the substrate. The superconducting properties were analyzed with respect to the observed values of T C, ΔT C, temperature dependence of the upper critical field (B C2), coherence length (ξ), diffusion coefficient (D), and the thermally activated flux flow behavior following the Arrhenius equation. The T C variation observed for the entire spectrum of N2 partial pressure variation is ∼11%, with T C getting saturated towards the higher end of the N2 content. While the T C variation with respect to N2 content is non-linear, the variation of B C2(0), diffusion coefficient, and zero temperature coherence length ξ(0) show a linear dependence. While the B C2(0) values show an increasing trend, D and ξ(0) values show a decreasing trend with respect to the increase of N2 partial pressure. It is concluded that the optimum window of N2 partial pressure for the reactive sputtering of NbTiN using a target of Nb0.7Ti0.3 for the present experimental conditions, in terms of epitaxial growth and obtaining the highest T C is within the range of 5.8% to 8.51%.