Electron microscopy study of sputtered NbN films

The microstructures of sputtered NbN thin films (103 μm thick) has been analyzed by transmission electron microscopy utilizing transverse sections. The micrographs reveal that the films consist of columnar grains of predominantly FCC NbN, oriented with a <111> direction normal to the substrate. These grains are surrounded by intergranular regions of unknown composition which are 10-30 A in width, and the average grain diameter increases linearly as a function of distance from the film/sapphire substrate interface. The new and surprising result is that films grown under different sputtering conditions, which exhibit vastly different superconducting properties, have essentially identical grain size and shape. However, defects have been observed within the grains and their density correlates well with the transition temperature Tc and the slope of the upper critical field at Tc