Coherent epitaxial growth of superconducting NbN ultrathin films on AlN by sputtering

We investigated the structural and electrical properties of superconducting NbN films epitaxially grown on AlN single-crystalline films using a sputtering technique. The NbN(111) films grown on AlN under optimized temperatures exhibited clear peaks with Pendelloesung fringes attributed to the growth of the atomically flat surfaces in 2θ/ω X-ray diffraction patterns. Scanning transmission electron microscopy also confirmed the formation of sharp NbN/AlN interfaces. Reciprocal space mapping revealed that the NbN films were coherently grown on the AlN templates, which indicates that the NbN films have the same in-plane lattice constants as AlN. It was also determined that the shape of the unit cell of NbN depends strongly on the epitaxial growth temperature. The NbN films coherently grown on AlN exhibited superconducting transition temperatures (T_c) ranging from 12 to 16 K, which also depends on the epitaxial growth temperature. These results indicate that the epitaxial strain (or change in crystal structure) in NbN modifies T_c. (author)