Superconducting properties and structure of ion bombarded Nb layers

The influence of ion bombardment on the superconducting transition temperature T_c and the structure of thin evaporated niobium layers has been investigated as a function of ion species and layer purity. Irradiation through pure layers with neon ions and fluences of typically 1016 ions/cm² leads to relatively small T_c decreases (δT_c × 0.5 K), while in oxygen contaminated layers larger effects depending on oxygen concentration are observed. Homogeneous implantation of chemically active impurities (nitrogen, oxygen) also drastically depresses T_c reaching the detection limit of 1.2K at a concentration of 15 at. %N. The T_c depressions correlate with a lattice parameter expansion of the Nb bcc structure at a rate of about 0.1 %per 1 at. % impurity.