N concentration effects on structure and superconductivity of NbN thin films

Abstract

In this work, we present a study on niobium nitride (NbN) thin films deposited using a reactive magnetron sputtering of Nb at different partial pressures of N2 (RN2). The effect of N concentration on the structural and superconducting properties of niobium mononitride (NbN) thin films was investigated using x-ray diffraction and resistivity measurements. During the initial nitridation process (RN2 = 0.5 to 2%) N atoms occupy interstitial sites within Nb, leading to an expansion and distortion in Nb lattice. At an intermediate RN2 range (RN2 = 8 to 16%), nearly stoichiometric NbN phase emerges and further nitridation (RN2 = 20 to 100%) results in expansion and distortion of NbN, similar to the effect observed in Nb. Low temperature and high magnetic field resistivity measurements reveal that the superconducting transition temperature (TC) and the upper critical field (HC2(0)) maximizes to 12.8 K and 30 T at RN2 = 16%. Both deficiency or excess of N from equiatomic NbN composition leads to a reduction in TC and HC2(0). The effect of an applied radio frequency (rf) bias during the growth of NbN thin films was also studied and it was found that under optimized rf biasing conditions, the adatom mobility can be enhanced leading to superior long-range crystalline ordering with reduced disorder/defects. As a result, higher value of TC = 13.2 K was obtained. In essence through this work, we demonstrate possibilities to attain a stoichiometric NbN phase without the need of high growth temperatures during the deposition process.