Fabrication and evaluation of hetero-epitaxial multilayer films of Nb/AlN/Nb/NbN for SIS junction

We investigated the fabrication of single-phase Nb films and hetero-epitaxial multi-layered films of Nb/AlN/Nb for SIS junctions. The Nb and multi-layered Nb/AlN/Nb films were deposited on NbN buffered MgO(100) single-crystal substrates at 753 K by using a dc magnetron sputtering system. We observed that the Nb films grow to Nb(200) orientation by measuring a standard 2θ/θ X-ray diffraction. The φ-scan diffraction peaks of Nb(200) were rotated by 45 degrees in-plane orientation for those of both NbN(220) and MgO(220). We found that NbN buffer layer with the thickness of a few nm was effective to enhance the Nb(200) orientation. The best thickness for a NbN buffer layer to grow epitaxial of Nb film was approximately 3 nm. Also, we examined dependence of degree of Nb(200) orientation and surface roughness(Ra) on multi-layered films. As a result, we found that the multi-layered films had flat surface interfaces for SIS junctions almost equivalent to single-layered films.