The upper critical field of NbN films. II

Abstract

Previous measurements of upper critical fields near Tc on NbN films with a small column (∼80 Å) void (∼10 Å) microstructure showed a strong anisotropy for applied fields perpendicular and parallel to the film surface. The perpendicular critical field was the larger one. Extrapolation of the perpendicular field data to zero temperature indicated an Hc2(⊥,0)>500 kG. Low temperature pulsed field measurements have now been made which show Hc2(⊥,0)=443 kG. In contrast, the measured upper critical fields of NbN films with equiaxed grains (≳275Å) or with larger columns (> 1000Å) and voids (∼500Å) indicate that these films behave like bulk superconductors. The Hc2(0)’s are ∼300 kG or less and little or no anisotropy is seen. A semiphenomenological theory incorporating size effects is used to interpret the data.