Transmission electron microscope studies of Nb-Ge superconducting films

Abstract

Superconducting Nb—Ge films have been prepared by getter sputtering over a range of conditions which yielded superconducting onset temperatures of up to 22 K. The structure of the films has been studied by transmission electron microscopy and diffraction. Films with the highest transition temperatures are composed substantially of A15 Nb₃Ge grains but, when a second phase occurs, it is apparently D8_mNb₅Ge₃ often in the form of lamellar structures. The latter may form by a martens tic transformation, and possible effects of the second phase upon film electrical properties are examined. Films deposited at below the optimum growth temperature contain a proportion of non-crystalline material and, for a sufficiently low growth temperature, they can be completely non-crystalline with a transition temperature of > 4·2 K. The annealing behaviour of radiation-damaged Nb-Ge has also been studied. The way in which the basic characteristics of the A15 lattice and its point defect content may control the electrical properties of the material are discussed.