Thin Films of Niobium Tin by Codeposition

Abstract

Thin films of niobium tin were deposited by the simultaneous condensation of niobium and tin vapor from two separate evaporation sources in a vacuum system. The composition and structure of the deposit was examined as a function of substrate temperature and the relative impingement rate of niobium to tin. If this rate ratio is near 3:1, the cubic phase Nb₃Sn is the predominant product, regardless of substrate temperature, even if it is much below the temperature where Nb₃Sn forms in the bulk. For a ratio of 3:2, and at substrate temperatures less than 800°C, formation of Nb₃Sn is almost entirely suppressed, and phases higher in tin predominate. Annealing at 900°C converts the phases high in tin to Nb₃Sn. The superconducting critical temperature of codeposited niobium tin films is below 14°K before annealing and above 14°K after annealing at 900°C. The superconducting energy gap is the order of 1 meV at 4.2°K, considerably below the value predicted by the BCS theory. This suggests that the material on the surface of the films has a lower critical temperature than in the interior.