Comparison of Tc´s and phase relationships of Nb3Ge films deposited on A15 Nb3Ir and Al2O3substrates

Nb-Ge films containing approximately 25 at% Ge were simultaneously d.c. sputtered on both sapphire and A15 Nb3Ir substrates. The films were analyzed using Auger electron spectroscopy, X-ray diffraction, and low temperature resistance measurements. X-ray diffraction data showed that the films deposited on A15 Nb3Ir remained single phase up to a higher concentration of Ge than those deposited on sapphire. Superconducting transition temperatures of films deposited on A15 Nb3Ir were almost always lower than those of films deposited on sapphire under identical conditions. Annealing at 750°C decreased Tc´s for films deposited on Al2O3, while it increased Tc´s for those deposited on A15 Nb3Ir substrates. Under optimum conditions, Tconsets greater than 20 K were obtained for both as-deposited and annealed Nb-Ge films sputtered on Nb3Ir substrates.