Superconducting critical temperatures of co-evaporated Nb-Ge films

Abstract

Nb-Ge films were prepared by a co-evaporation technique. The dependences of critical temperature T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> , lattice parameter a <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf> 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 <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> and a <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf> , but also between T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> and Δρ, where Δρ is the difference between ρ(300 K) and ρ(25). The T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> 's dependence on the film thickness was also observed. The T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> '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 Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ge grains of about 500 Å in diameter.