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

Abstract

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 (a₀a₀≳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 T_c, the Nb/Ge ratio, and a₀. An extrapolation of the EMP Nb/Ge ratio versus a₀ and T_c data predict an a₀ of 5.127 Å and a T_c 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.