Niobium nitride/aluminium nitride superlattices

DC reactive magnetron sputtering has been used to prepare niobium nitride/aluminium nitride superconductor/insulator superlattices. Deposition conditions were optimized in order that multilayer fabrication could be achieved by simple computer control of the substrate positioning beneath each sputtering target. Transport measurements in the mixed state have been made to assess the behaviour of vortices in the multilayers for fields applied both normal and parallel to the layers. The upper critical fields and irreversibility lines have been determined, and their evolution investigated as a function of superconducting and insulating layer spacing. The results are promising for assisting in elucidation of the behaviour of the layered high temperature superconductors. Stacked superconductor-insulator-superconductor junctions have been fabricated. We have characterized the multilayers using X-ray diffraction and transmission electron microscopy techniques. Multilayers grown on silica substrates consist of untextured polycrystalline NbN, with AlN that is predominantly amorphous. The layer roughness is largely determined by the NbN grain size, which is on a similar scale to the NbN layer thickness, and does not significantly increase towards the top of the multilayer. With AlN inter-layers <4.5 nm thick, NbN grains are able to maintain a common orientation across several multilayer repeats as a result of protrusions of NbN through the AlN layers. The electronic structure of the multilayers has also been investigated using electron energy loss spectroscopy and Fresnel contrast analysis.