Critical Current Densities of Nb Thin Films

The focus of this study is to investigate the inductive critical current of thin films of niobium as the normalized thickness of the film (d/ξ) and “dirtiness” (λ__tr) are varied. The niobium films were primarily grown via magnetron sputtering onto both silicon and sapphire substrates, with thickness ranging from ~ 10 to ~ 180 nm, and dirtiness values (λ__tr) ranging from 0.5 to 20. We measured the critical current of the niobium films using an inductive third-harmonic (3f) technique as a function of temperature (Claassen, Rev. Sci. Instrum. 62, 996 (1991)). The inductive critical current was extracted by using a linear offset method applied to the third harmonic signal versus the coil current. Critical current densities were subsequently calculated using the film thickness and the coil conversion factor. Jc was plotted versus temperature, and fit to a functional form of J_c(T) = J_c(0)(1-T/T_c)^γ. From this fit, we observe how T_c, J_c(0), and γ vary with the normalized thickness of the film (d/ξ) and the “dirtiness” (λ__tr) of the film. Our initial analysis indicates that the value of J_c(0) decreases as both the thickness and the dirtiness of the films increase.