Operation of single-flux-quantum logic cells based on all-NbN integrated circuit technology

We developed NbN-based SFQ circuits consisting of high-quality epitaxial NbN/AlN/NbN trilayer grown on MgO (100) substrate. The SFQ logic cells such as JTL, DC/SFQ converter, voltage driver, pulse splitter, TFF, and CB were designed by setting the critical current density J_C at 2.5 kA/cm². We succeeded in demonstrating correct operations of all the designed SFQ cells, but the J_C range allowing the operation was quite narrow as around 70% for the designed value. The circuit simulation revealed that the TFF cell requires the J_C range from 65% to 115% for the correct operation, while the DC/SFQ converter was not operational for the J_C of more than 70%.