Self-Shunted NbN Junctions With ${\rm NbN}_{x}/{\rm AlN}$ Bilayered Barriers for 4 K Operation

We report the fabrication and electrical characteristics of self-shunted Josephson junctions consisting of NbN electrodes, an AlN barrier, and an NbN_x normal layer for an SFQ circuit application. To fabricate such junctions with a high characteristic voltage V _c, we have studied their preparation conditions including the surface treatments of MgO substrates. Junction characteristics at 4 K were dependent on the electrical properties and film thickness of the NbN_x layer; the underdamped junctions changed into overdamped ones with an increase in the film thickness. The overdamped junctions showed a characteristic voltage V_c of 0.8 mV and a critical current density J_c of 22 A/cm² at 4.2 K.