Critical temperature of niobium and tantalum films

A method for depositing thin, superconductive films of Nb and Ta by electron beam evaporation is presented, and the critical temperature of these films is discussed as a function of deposition conditions. Gettering, increasing the ratio of impinging vapor atoms to residual gas atoms, and raising the substrate temperature during deposition gives films with a lower impurity content and higher critical temperature. The resistance ratio of a film and its x-ray lattice parameter are taken as measures of its average impurity concentration. Critical temperatures above 9°K are measured for Nb films of resistance ratio of 3 or higher, and critical temperatures above 4.2°K are measured for Ta films of resistance ratio of 5 or higher.