EPITAXIAL-GROWTH AND CRYSTAL-STRUCTURES OF NBN ALN/NBN TRILAYERS FABRICATED AT AMBIENT SUBSTRATE-TEMPERATURE/

We report on the deposition and crystal structures of NbN/AlN/NbN tril ayers for the fabrication of Josephson SIS tunnel junctions. The cryst al structures of the NbN/AlN/NbN trilayers were studied by electron mi crograph diffraction patterns and cross-sectional transmission electro n micrograph (TEM) observations. Even though the substrates were not h eated, the base NbN films demonstrated excellent superconducting prope rties and had single-phase orientation without columnar or granular st ructures. We found that the crystal structures of the counter NbN film s depended on the thickness of the AlN barriers. The counter NbN films deposited on 2-nm-thick AlN barriers showed polycrystalline structure s while they had a single crystal structure when the AlN barriers were thinner than 1 nm.