Surface morphology and superconductivity of Nb thin films by biased target ion beam deposition

TL;DR

This study uses biased target ion beam deposition to produce Nb thin films with improved surface smoothness and superconducting properties, highlighting the influence of deposition parameters on film quality.

Contribution

It introduces a novel deposition technique for Nb films and identifies critical parameters affecting their surface morphology and superconductivity.

Findings

High target current degrades crystallinity and reduces Tc.

Optimized conditions produce ultra-smooth films with Tc close to bulk Nb.

Films exhibit a very thin proximity layer indicating high quality.

Abstract

One of many challenges for niobium (Nb) based superconducting devices is the improvement over the surface morphology and superconducting properties as well as the reduction of defects. We employed a novel deposition technique, i.e. biased target ion beam deposition technique (BTIBD) to prepare Nb thin films with controlled crystallinity and surface morphology. We found that the target current (ITarget) and the target bias (VTarget) were critical to the crystallinity and surface morphology of Nb films. The high target current (ITarget >500 mA and VTarget = 400 V bias) during the deposition degraded the Nb crystallinity, and subsequently reduced the critical temperature for superconductivity (Tc). VTarget was critical to the surface morphology, i.e. grain size and shape and the surface roughness. The optimized growth condition yielded very smooth film with RMS roughness of 0.4 nm that was an order of magnitude smoother than that of Nb films by sputtering process. The critical temperature for superconductivity was also close to the value of the bulk Nb. The quality of Nb film was evident in the presence of a very thin proximity layer (~ 0.8 nm). The experimental results demonstrated that the preparation of smooth Nb films with adequate superconductivity by BTIBD could serve as a base electrode for the in-situ magnetic layer or insulating layer for superconducting electronic devices.