Composition versus a0 and Tc relations in superconducting ’’Nb3Ge’’ thin films

摘要

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.