Nb3Ge and Nb3Sn Films Prepared by High-Rate Magnetron Sputtering
During the past several years, considerable effort has been put into the fabrication of A15 superconductors, with particular emphasis on Nb3Sn and Nb3Ge. Nb3Ge probably cannot be produced by bulk fabrication or diffusion processes, which are successful with Nb3Sn, due to the metastable nature of the high critical temperature A15 phase, and thus must be “frozen in” at high temperatures by methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), or low-energy sputtering. The latter technique has produced Nb3Ge films with the highest critical temperature, T c , and best self-field critical current density, J c . A variation on the low-energy sputtering technique is discussed, wherein a magnetic field in the vicinity of the sputtering target alters the plasma characteristics in such a way that deposition rates are enhanced (up to 1 µm/min) at very low operating voltages (≤400V dc). Then sputter deposition is competitive with CVD. This technique is capable of producing very high quality Nb3Sn films and Nb3Ge films with T c ≥21.5 K. By suitable adjustment of the sputtering parameters, it is possible to alter the microstructure and surface characteristics. The effect of controlled amounts of oxygen in the sputtering gas on the T c of Nb3Ge is described.
external magnetic field
upper critical field
self-field critical current density
partial pressure of oxygen
critical temperature, onset of superconductivity
sputtering gas temperature
transition temperature width
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