Superconducting T_c's and phase relationships of sputter-deposited Nb₃Ge

Nb₃Ge which is fully superconducting above 21 K has been prepared by sputter deposition in a very clean vacuum system at substrate temperatures in the range 500-550°C, or 200°C less than the temperatures needed in other sputtering systems. For a series of compound targets, superconducting T_c's and metallurgical phases, as established from x-ray diffraction studies, were determined as a function of substrate temperature and argon pressure. Maximum T_c's were found in films prepared over a narrow range of compositions and substrate temperatures. These conditions coincide with the region of phase stability for the A15. A small amount of oxygen (≳1.0%) as determined by Auger analysis, was found to be present in high-T_c films, while lower T_c films all had less oxygen.