From localization to superconductivity in granular niobium nitride thin films

Abstract

The variations with temperature T of the resistivities of reactively sputtered NbN films have been studied from 1 to 300 K. The nitrogen concentration in the films has been increased by using higher and higher nitrogen pressures during sputtering. The porosity of the films is also increased and this favours their post oxidation at ambient atmosphere. The oxygen concentration in the films revealed to play a prime part on the resistivities p. Series of samples were obtained with room temperature resistivities from 14 Ω.cm to 1 mΩ.cm. The resistivity variations with T regularly evolve from a disordered semiconductor like behaviour to the bad metal behaviour of the classical superconducting NbN. The samples are highly inhomogeneous with a wide distribution of the energy barriers between conducting grains. In the high resistivity samples, thermally activated hopping is present and Ln p ∝ T-n with 1/2≤ n ≤1/4. In the less resistive samples, a percolating path is established through the lowest barriers and beyond some critical temperature T* the conductivity becomes linear in T. A superconducting transition may then appear which could be observed even on samples whose behaviour is markedly of the semiconductor type in the normal state. The highest resistivity before transition was 60 mΩ.cm. This corresponds to a resistivity ratio of 60 with respect to the value at room temperature. Application of a magnetic field up to 4 T decreases the critical temperature in the same way as for a homogeneous dirty type II superconductor.