Anisotropy of critical currents and fields in sputtered and evaporated superconducting Nb3Ge films

Nb3Ge films with thicknesses between 0.06 and 1.5 μm were deposited onto heated sapphire substrates by dc sputtering in a pure argon atmosphere as well as by thermal coevaporation of niobium and germanium in ultrahigh vacuum. Both preparation methods result in Nb3Ge films with high transition onset temperatures up to 22.7 K which crystallize in a single phase A 15-structure with a lattice parameter of 5.14 Å. The sputtered films exhibit columnar growth of the crystallites normal to the substrate surface in contrast to random crystallite arrangement in the evaporated films. Critical current densities and upper critical magnetic fields were measured as a function of magnetic field orientation, temperature and film thickness. In sputtered films critical currents and fields show maxima for the field orientation normal to the sample surface, whereas in evaporated films these maxima occur for parallel field orientation.