Electronmicroscopic evidence for a columnar-void-type structure in sputtered NbN films

Abstract
Transmission electron microscopy of sputtered NbN films shows that the films contain columnar grains with an average axial diameter of 100 Å or less. A large number of narrow (≈ 20 Å) voids separate most of the grains. The voids extend through the film and are probably responsible for the strong pinning observed in these films. The high resistivity and relatedly the high upper critical field values of the films are also believed to be due to this columnar‐void structure.