Granular-composite-like electrical transport properties of polycrystalline cubic TaNx thin films prepared by rf sputtering method

Abstract

We have systematically investigated the electrical transport properties of polycrystalline TaNx (0.83 ≲ x ≲1.32) films with rocksalt structure from 300 down to 2 K. It is found that the conductivity varies linearly with lnT from ∼6 to ∼30 K, which does not originate from the conventional two dimensional weak-localization or electron-electron interaction effects, but can be well explained by the intergrain Coulomb effect which was theoretically proposed in the granular metals. While the fluctuation-induced tunneling conduction process dominates the temperature behaviors of the conductivities (resistivities) above ∼60 K. Normal state to superconductive state transition is observed in the x ≳1.04 films in low temperature regime. The superconductivity can still be retained at a field of 9 T. The upper critical field increases linearly with decreasing temperature in the vicinity of the superconductive transition temperature, which is the typical feature of granular superconductors or dirty type-II superconductors. The granular-composite-like electrical transport properties of the polycrystalline TaNx films are related to their microstructure, in which the TaNx grains with high conductivity are separated by the poorly conductive amorphous transition layers (grain boundaries).