Ion scattering and Auger electron spectrometry of superconducting ’’Nb3Ge’’ sputtered films

Low-energy ion scattering spectrometry (ISS) in conjunction with Auger electron spectrometry (AES) has been applied to the analysis of a series of superconducting ’’Nb3Ge’’ thin films deposited on alumina substrates by rf sputtering. ISS was used to determine the Nb/Ge ratio as a function of depth, while AES in combination with ISS was used to determine the impurities. No gradients in composition with depth were observed except in a thin (∼13% of the film thickness) niobium-deficient region near the film-alumina interface. The applicability of ISS to quantitative measurements of the Nb/Ge ratio was shown by comparing the variation of this ratio with lattice parameter a0 with a similar analysis performed on the same samples using electron microprobe (EMP) measurements on the Nb and Ge composition. Comparison of the Nb/Ge ratio determined by ISS with EMP data showed that no preferential sputtering of Ge from the films occurred during neon ion bombardment. The variation of Tc with the Nb/Ge ratios determined by ISS and EMP supports the idea that for ’’Nb3Ge’’, Tc increases as the composition approaches the stoichiometric ratio of 3:1.