Microstructure, resistivity and the anisotropy of the upper critical field in NbN thin films.

The upper critical field of most polycrystalline NbN thin films is anisotropic, with the perpendicular critical field H(c2)(perpendicular) larger than the parallel critical field H(c2)(parallel). We have measured the angular dependence H (c2)(theta) for samples with both a columner and non-columnar microstructure. In both cases we find a rounded maximum near H(c2) (perpendicular). The shape of H(c2)(e), combined with the known microstructure of the films and the linear temperature dependence of H(c2) for both field orientations leads us to conclude that the mechanism responsible for this anisotropy is an anisotropic conductivity in the film, probably due to differences in the grain boundary resistance in the plane of the film and normal to the film. In contrast, a single crystal film shows only surface superconductivity.