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

Abstract

One of the many challenges for niobium (Nb) based superconducting devices is the concurrent improvement over the surface morphology without undermining superconducting properties. In this work, the authors demonstrated the modification to the surface morphology of 10–100 nm Nb thin films deposited by bias target ion beam deposition (BTIBD). Target current (IT) and target bias (VT) were the main parameters explored and were found to be critical to the control over crystallinity and surface morphology of Nb films. The optimized growth conditions yielded a ∼50 nm thick Nb film with the root-mean-square roughness of 0.4 nm, which was an order of magnitude smoother than that of Nb films by sputtering process. The critical temperature for superconductivity was close to the value of the bulk Nb (∼9.3 K) with a very homogeneous transition (the superconducting transition width ΔTc of 0.1 K). The quality of Nb film was evident in the presence of a very thin proximity layer (∼0.75 nm). The experimental results demonstrated that the preparation of smooth Nb films with expected superconductivity by BTIBD could serve as a base electrode for the in situ magnetic layer or insulating layer for superconducting electronic devices.