Impurity doping of chemical-vapor-deposited Nb3Ge and its effect on critical-current density

In an earlier work, we demonstrated that high self-field and low-field critical-current densitites J/sub c/ of the order of 106 A cm-2 can be attributed to flux pinning on a dispersed Nb5Ge3 tetragonal phase present in Nb3Ge3 layers grown by chemical-vapor deposition (CVD). In this study, we examined the effect of impurity gas additions on J/sub c/ and the critical temperature T/sub c/ of the A15 superconducting phase. The gas impurities were N2, C2H6, and CO2. The impurity concentration in the gas phase was varied over three to four orders of magnitude to establish tradeoffs between T/sub c/ deterioration and J/sub c/ enhancement. The x-ray phase analysis of samples containing the highest impurity concentrations indicated by the presence of niobium nitrides and carbides, respectively. The T/sub c/ was affected least by N2 and most by CO2 additions. Doping by N2 or C2H6 resulted in A15 deposits free of the tetragonal phase and having J/sub c/'s of the order of 106 A cm-2, comparable to the best Nb5Ge3-containing samples. The grain size of deposits was estimated from thinned specimens using transmission electron microscopy. Correlation of J/sub c/ with the microstructure of the sample suggested that predominant flux pinning occurs on impurities incorporated in Nb-Ge layers and on the accompanying defects