Effects of deposition parameters on the properties of superconducting rf reactively sputtered NbN films

Abstract

Thin films of NbN were reactively rf sputtered using a range of substrate temperatures and nitrogen partial pressures. The effects of these deposition parameters was to change the morphology rather than the stoichiometry of the sputtered films. Auger composition analysis, x-ray diffractometry, resistivity, resistivity ratio, superconducting critical temperature (Tc), and critical magnetic field [Hc2(T)] data were all used to determine the microstructure of the films and how it related to the measured properties. The best metallic films had Tc’s?16 K, Hc2(T)’s of 18 T and were prepared at 600°C using 1 Pa argon, 0.03 Pa N2, and a target power density of 9.8 W/cm2. All the films had carbon impurities at concentrations estimated to be between 1 and 15%. X-ray analysis indicated the presence of a face- centered-cubic structure, assumed to be the Bl phase, in addition to a second phase in one of the samples. The best superconducting films had several percent carbon. Those films prepared with an excess nitrogen partial pressure had negative temperature derivatives of resistance indicating a granular column void type microstructure. Films prepared with a deficient nitrogen partial pressure had very low Tc0’s (<4.2 K). A film prepared at ambient temperature had the least carbon, was nearly stoichiometric NbN, but had degraded Tc and enhanced Hc2(0).