Diffusion mediated growth of superconducting Nb-Ti composite films by high temperature annealing

Abstract

The fabrication of superconducting Nb-Ti alloy by high temperature annealing of Nb/Ti bilayer thin films is reported here. During the annealing process, Nb and Ti diffuse into each other and Nb-Ti composite film formation occurs at the interface of the bilayer. Two types of substrates, namely, SiO2/Si and Si3N4/Si are used to grow the bilayers of Nb/Ti by using dc magnetron sputtering. Annealing at temperature about 820 °C leads the substrates to take part into the diffusion process. The alloying of Nb-Ti and the effect of substrates on the structural properties are studied by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Ti-rich Nb-Ti phases are present in the XRD while interface studies through XPS confirms the interdiffusion of the two elements Nb and Ti along with the presence of the decomposed elements from the substrates. Appearance of nitride phases in case of Si3N4/Si substrate confirms the substrate’s involvement in the diffusion process. Further low temperature transport measurements are carried out to study the superconducting properties of the Nb-Ti composite films grown on both oxide (SiO2/Si) and nitride (Si3N4/Si) substrates. Nb-Ti composite films offer higher transition temperature (T C ) compared to that of pure Nb with similar thickness used in Nb/Ti bilayer films. Higher normal state resistance (R N ) with wider transition width for Nb-Ti on nitride substrate in comparison with the oxide substrate indicates a possible role of N atoms in tuning the disorder and hence controlling the transport properties. Finally, the presented method can be used to fabricate superconducting stoichiometric NbTi and NbTiN thin films for future phase slip and superconducting single photon detector-based applications.