Superconducting Transition Temperature of Nb–Ge Thin Films Prepared by Co-Evaporation

Nb–Ge thin films are prepared by co-evaporating Nb and Ge. Dependences of the superconducting critical temperature T c the lattice parameter a o and the resistivity on preparation conditions are studied. As the substrate temperature lowers from 1000 to 800°C, a o remains constant but T c is lowered. Lowering the deposition rate results in a decrease in a o and an increase in T c . T c stays constant for the film thickness d larger than 2000 Å and is lowered for d smaller than 2000 Å. Especially, T c is less than 4.2 K when d is smaller than 50 Å. The lattice parameter of the stoichiometric Nb 3 Ge is estimated to be 5.12 7 Å by extrapolating the a o 's of single phase Nb–Ge thin films. A morphological study of Nb–Ge thin films is made by transmission electron microscopy. They consist of polycrystalline grains of about 500 Å in diameter.