Low-loss epitaxial NbN/MgO/NbN trilayers for THz applications

To improve the performance of Josephson oscillators and SIS mixers over 700 GHz, we have developed a fabrication process to grow epitaxial NbN/MgO/NbN trilayers by reactive de-sputtering. Trilayers were fabricated on single-c rystal MgO substrate at ambient temperature. NbN and MgO films were deposit ed by reactive de-sputtering with an Nb and Mg target, respectively. MgO in ter-layer thickness was changed up to 480 nm, but, T-C and 20-K resistivity of the NbN upper-layer showed no remarkable dependency and they were 15.7 K and about 60 mu Omega cm, respectively. To evaluate the RF performance of the epitaxial NbN films, we fabricated Josephson junctions with a microstr ip resonator constructed from an epitaxial NbN/MgO/NbN trilayer. The I-V ch aracteristics of the junction exhibited resonance steps up to 2.5 mV, which suggests that the epitaxial NbN films have low loss up to 1.2 THz.