Fabrication of epitaxial NbN devices with TiN resistors

To extend the application range of epitaxial NbN devices, we developed epitaxial TiN thin-film resistors. TiN has approximately the same lattice constant and crystalline structure as NbN and MgO. However, high quality TiN thin films show superconductivity at the liquid helium temperature. We attempted to remove this superconductivity using TiO impurity. The ratio of Ar and N₂ was fixed at 5:1 and the O₂ flow ratio varied from 0 to 1.6% in relation to the total gas flow. When O₂ ranged from 0.5 to 1.6%, superconductivity vanished and the resistivity obtained was about 50-145 μΩcm at 4.2 K. X-ray analysis showed that the TiN films grew epitaxially on the MgO substrate as did the NbN films on the TiN films. Using the TiN resistor, a fully epitaxial NbN Josephson array oscillator was fabricated and tested. The oscillator was formed with 10 TiN-shunted NbN/MgO/NbN tunnel junctions and NbN microstrip resonators. At around 1.0THz, Shapiro steps induced by the Josephson oscillation were observed in the detector I-V characteristics.