Superconductivity of Nb-Ge thin films deposited by ion beam sputtering

The superconductivity of the Nb-Ge thin films deposited by ion beam sputtering was studied. The ions bombarding a target were extracted from a separate ion source so that the films are not affected by the plasma, and can be deposited at the environmental pressure below 0.1 Pa. The dependence of the crystal structure, the superconducting transition temperature Tc and the lattice parameter (asub(o)) of the films on their deposition condition have been investigated. The crystal structure of the films was investigated by X-ray diffractometry. The Tc was measured by the four-probe method, and the composition of the films was determined by the electron-probe micro-analysis (EPMA). The result showed that the films with A15 phase was synthesized at the Ge concentration below 23 at. percent. The films deposited at Ts of 600 degree C and the Ge concentration of 13 at. percent showed the minimum asub(o) of 5.161 x 10-8 cm and the maximum Tc of 9.1 K. The value of Tc was lower than that of the films deposited by conventional plasma sputtering method. These results imply that the high Tc metastable A15 phase was destroyed by the bombardment of high energy particles, and that the high Tc Nb3Ge films may not be obtained by ion beam sputtering technique. (Kato, T.)