Structural, electronic and superconducting properties of room temperature deposited VN[formula omitted] thin films

Abstract

In recent years, vanadium mononitride (VN) thin films have emerged for their multifaceted functionalities e.g. as electrodes in battery and supercapacitors, as catalysts for hydrogen and oxygen evolution reactions, as superconductors for hot-electron bolometer detectors. In this study, we investigate the structural, electronic and superconducting properties of polycrystalline VNx thin films synthesized at room temperature using a reactive dc magnetron sputtering. The structural evolution of VNx samples with increase in partial N2 gas flow (RN2) during the reactive sputtering and corresponding change in the oxidation states were systematically studied. At a first glance, no visible changes appear in the lattice parameters within RN2= 5–50% samples. Despite this, only RN2 = 5 and 10% samples exhibit superconductivity. Probing the electronic structure around the V L-edges and O, N K-edge again did not reflect any minimal changes in RN2 = 5–100% samples. Across the V K-edge, the oxidation state was also found to be similar. However, the ambiguity was resolved from the Fourier transformed EXAFS spectra immediately revealing presence of a higher number of cationic vacancies. These vacancies drive the VNx system towards poor metals exhibiting dρ/dT = negative in the temperature dependent resistivity. This is in sharp contrast to the previous reports so far exhibiting positive slopes evident for metals. The present work insights on several ambiguities observed for the first time in the V-N system. The work further emphasizes the need to perform a similar study on epitaxially grown VNx thin film samples with reduced defects.