Suppression of superconuctivity in Nb and NbN thinfilm nano-structures

Abstract

We have performed a systematic study of the superconducting properties of Nb and NbN thin-film Nano-bridges, which are readily usable as structural units of single-photon detectors. We present dependencies of the superconduction transition temperature of Nb and NbN thin-film bridges on their thickness and width, with particular emphasis on the nanometer wide structures made from ultra-thin filmes. Typically, for bridges thicker than 50 nm and wider than 1 µm, the transition temperature does not depend on the bridge dimensions. For smaller bridges, a significant reduction of the critical temperature has been found. The reduction is attribtued to the proximity effect in normal-metal-superconducting-normal-metal (NSN) structures with perfect N-S interfaces (Cooper's limit). Both the effective thickness and width of normal areas in the bridgte have been evaluated from comparison of the experimental results and theoretical simulations. We discuss arising of the normal areas in the manufacture of nano-bridges.