Superconducting critical temperatures of co-evaporated Nb-Ge films

Nb-Ge films were prepared by a co-evaporation technique. The dependences of critical temperature Tc, lattice parameter aoand electrical resistivity ρ on deposition condition, i.e., substrate temperature, deposition rate and film thickness, were investigated. Close relations were observed not only between Tcand ao, but also between Tcand Δρ, where Δρ is the difference between ρ(300 K) and ρ(25). The Tc´s dependence on the film thickness was also observed. The Tc´s value of Nb-Ge films was about 22 K for 2000 Å in thickness and below 4.2 K for 50 Å in thickness. From a transmission electron micrograph the Nb-Ge film was found to be a polycrystal with Nb3Ge grains of about 500 Å in diameter.