Preparation and analysis of superconducting Nb-Ge films

Abstract

The dependences of ${T}_{c}$, resistivity, resistance ratio, and structure on chemical composition and sputtering conditions for Nb-Ge films have been studied. The chemical composition, impurity content, and x-ray structure were obtained using Rutherford backscattering, nuclear techniques, and x-ray diffraction. Although ${T}_{c}$ varies with composition, it is not found to be critically dependent upon exact stoichiometry; the Nb/Ge ratios vary by \ensuremath{\sim} 13% (2.6 to 3) for films with \ensuremath{\sim} 23-K onsets and by \ensuremath{\sim} 40% (2.2 to 3.3) for films with \ensuremath{\sim} 20-K onsets. For compositions similar to the bulk, the films have comparitively much higher ${T}_{c}$'s and smaller lattice parameters. X-ray results show the films to contain predominately $A\ensuremath{-}15$ phase (except for $\frac{\mathrm{Nb}}{\mathrm{Ge}}\ensuremath{\lesssim}2.5$) with lattice parameters varying from 5.15 \AA{} for Nb-rich low-${T}_{c}$ films to 5.12 \AA{} for Ge-rich films. Several percent of oxygen and carbon occur in low-${T}_{c}$ amorphous films deposited at 650 \ifmmode^\circ\else\textdegree\fi{}C but this is considerably reduced in high-${T}_{c}$ films made simultaneously at \ensuremath{\sim} 750 \ifmmode^\circ\else\textdegree\fi{}C. No argon was found and the nitrogen content was generally less than 1%. No correlation of high ${T}_{c}$'s and impurities was found. The optimum deposition temperature and resistivity are lowest, and the resistance ratio highest for Nb/Ge ratios somewhat below 3/1. A simple correlation of ${T}_{c}$ and resistance ratio is reported which is largely independent of all sputtering conditions and composition and which suggests that slightly higher ${T}_{c}$'s may be possible. Negative bias was found to be detrimental to ${T}_{c}$ while positive bias had relatively little effect. Magnetic-field-assisted sputtering led to significant increases in the sputtering rate and the optimum deposition temperature.