Epitaxial growth of Nb 3Ge on Nb 3Ir and Nb 3Rh

Abstract

This is a report on a cooperative research carried out in Stanford University to investigate the possibility of using epitaxy to prepare the high T c superconductor Nb 3Ge in an A15 crystalline structure at the 3:1 stoichiometry. Nb 3Ir polycrystalline films with the A15 structure deposited on sapphire were used as substrates for the epitaxial growth of Nb 3Ge because of the favorable lattice parameter match. The experimental results clearly show that epitaxial growth indeed occurs and helps to extend the range of homogeneity of the A15 phase up to 26.3 at.% Ge as compared with the thermodynamic equilibrium boundary at 19 at.% Ge. We also used Nb 3Rh films as substrates and found them inferior to Nb 3Ir because of the multiphase nature of the films. In addition to extending the A15 phase boundary epitaxy results in a considerable rise in the superconducting transition temperature for Ge-rich samples together with a reduction in the transition width. The work suggests that polycrystalline epitaxy can be an important tool in the synthesis of thin-film intermetallic compounds.