Josephson Tunneling Behaviors in NbN/AlN/NbN Junctions with an Ultrathin NbN Film

We fabricated epitaxial NbN/AlN/NbN tunnel junctions with an ultrathin NbN film as the base layer on single-crystal MgO (100) substrates. The measured thickness of the base layer was 4.7 nm. By measuring the electrical characteristics of the junctions, we showed that the junction had excellent Josephson tunneling properties, with a large gap voltage, a small gap voltage width, and a quality factor
R
sg/
R
_N
of 42 at 4.2 K. The temperature dependence of the critical current for the junctions agreed with a theoretical calculation using the Ambegaokar–Baratoff relation over the temperature range of 3–12 K. An ideal Fraunhofer diffraction pattern appeared for the Josephson critical current as a function of magnetic field. Fiske steps were observed in current-voltage characteristics of the Josephson junctions at different applied magnetic field.