Composition versus a0 and T c relations in superconducting ’’Nb3Ge’’ thin films

A large series of well-crystallized ’’Nb3Ge’’ thin films have been prepared by carefully controlled rf sputtering. The compositions expressed as Nb/Ge ratios for these essentially single-phase ’’Nb3Ge’’ thin films were determined by electron microprobe (EMP) analysis, ion scattering spectrometry, and wet chemistry (WC). Based on lattice-parameter considerations, this series of films spanned the entire homogeneity range of ’’Nb3Ge’’ phases, from the metastable Ge-rich end (a0<5.167 Å) to the metastable Nb-rich end (a0≳5.176 Å). The data from the different analytical methods and annealing experiments suggest that typical films are Ge enriched at the substrate interface and germanium and/or oxygen enriched for almost 600 Å from the exposed surface. Intercorrelations of increased reliability have now been established between Tc, the Nb/Ge ratio, and a0. An extrapolation of the EMP Nb/Ge ratio versus a0 and Tc data predict an a0 of 5.127 Å and a Tc of 23.4 °K for a phase of Nb3Ge stoichiometry. The correlation between the EMP Nb/Ge ratio and a0 obtained from a single-phase bulk ’’Nb3Ge’’ sample was found to be in excellent agreement with the data obtained from the present study’s rf-sputtered ’’Nb3Ge’’ thin films.