Superconducting critical temperatures of co-evaporated Nb-Ge films

Nb-Ge films were prepared by a co-evaporation technique. The dependences of critical temperature T c , lattice parameter a o and electrical resistivity ρ on deposition condition, i.e., substrate temperature, deposition rate and film thickness, were investigated. Close relations were observed not only between T c and a o , but also between T c and Δρ, where Δρ is the difference between ρ(300 K) and ρ(25). The T c 's dependence on the film thickness was also observed. The T c 's value of Nb-Ge films was about 22 K for 2000 A in thickness and below 4.2 K for 50 A in thickness. From a transmission electron micrograph the Nb-Ge film was found to be a polycrystal with Nb 3 Ge grains of about 500 A in diameter.