Estimation of surface resistance for epitaxial NbN films in the frequency range of 0.1-1.1 THz

To improve the performance of superconducting analog devices at frequencies exceeding the Nb gap frequency, we developed a fabrication process to grow epitaxial NbN/MgO/NbN trilayers. To evaluate the RF performance of the trilayers, we fabricated Josephson junctions with an epitaxial NbN/MgO/NbN microstrip resonator. A Josephson junction was placed in the center of the resonator and it was used as both an oscillator and a detector. The width and length of the microstrip resonator were 10 and 1000 μm, respectively. Current steps of up to 1.9 mV resulting from coupling with the resonator were observed in the I-V characteristics. We found that the height of the current steps depended on the loss of the resonator. By assuming that the sum of the dielectric loss and the radiation loss is much smaller than the conductor loss, we estimated the surface resistance of the epitaxial NbN thin films. The surface resistance of the epitaxial NbN films was estimated to be approximately 2.6-88 mΩ at 0.1-1.1 THz.