Dimensional effects on current and field properties in NbN films

We have investigated dimensional effects, i.e., variations in thickness, width, grain size, and separation between grains, on the current and field properties of NbN films. The films, all of which had T
c
's of ∼ 16K were prepared by reactive sputtering, Self-field current densities measured at 4.2K ranged from
\sim 5 \times 10^{5}
to > 10
⁷
Amps/cm
²
. Measured upper critical fields at 4.2K varied from < 100 kG to > 220 kG. Extrapolated H
c2
's of over 500 kG were calculated from data taken near T
c
. All of these results are correlated with transmission electron microscopy studies. The very highest upper critical fields are attributed to an H
c3
arising from a column-void microstructure. In general, we conclude that dimensional effects are a dominant factor in achieving the very high current and field values observed in these films.