Superconductivity transition temperatures of rf reactively sputtered NbN Films

Niobium nitride films of a thickness ranging from 500 to 3000 Å have been deposited by reactive rf sputtering of niobium in a nitrogen/argon atmosphere. A water-cooled substrate holder was used. The partial pressure of nitrogen was varied systematically between 0 and 10^-3 Torr at two different argon pressures. Generally, the substrate was set on anode potential but in some cases a bias voltage was applied. The experiments showed that the transition temperature was strongly dependent upon the partial pressures of nitrogen and argon. In the sputtering experiments without bias very steep transition curves were obtained. The highest transition temperature was T _c = 14.5 K. The structures of films were examined by x-ray and TEM transmission methods. The investigations made evident that rf sputtering is a method by which high transition temperatures can be obtained, even at high residual gas pressures and unheated substrates.