Thermodynamic and experimental studies of the CVD of A-15 superconductors I. Nb3Ga

This paper deals with the experimental and thermodynamic study of the CVD synthesis of Nb₃Ga layers on various metallic and insulting substrates using the coreduction of mixed halides by hydrogen. Thermodynamic equilibrium in the seven-components system Nb-Ga-H-Cl-Si-O-Ar has been calculated using the method of minimization of the system Gibbs free energy as a function of the variables directly available in the CVD system. The chosen variables were the chloride ratio, the reduction and dilution parameters and the temperature of the deposition zone. The equilibrium compositions were calculated for the two composition limits of the A-15 phase: NbGa_0.15 and Nb₃Ga. They are presented in the form of CVD phase diagrams. A CVD reactor has been set up and more than one hundred measurements have been made in order to check the validity of the equilibrium calculations. The comparisons between equilibrium and experimental results show a good agreement and lead to a better understanding of the chemistry and thermodynamics of the system.