Fabrication and Characterization of Epitaxial TiN-Based Josephson Junctions for Superconducting Circuit Applications

We investigated full-epitaxial TiN/AlN/TiN Josephson junctions on MgO substrates for superconducting qubit applications. The critical temperature (T_C) of TiN film is relatively low at around 5 K, but its lattice constant is about 0.424 nm, which is close to the lattice constant of MgO of 0.421 nm. TiN and AIN films were prepared by DC magnetron sputtering in a load-lock sputtering system with an ultra-high vacuum chamber. The deposition temperature was varied from ambient temperature to 1073 K. In XRD analysis, (200) peaks were observed in both the TiN single layer film and the TiN/AIN/TiN trilayer film. No other XRD peaks were observed in the single layer or trilayer films. The lattice constant of TiN was determined to be 0.4242 nm from XRD analysis, close to the value of 0.4212 nm for MgO. The 150 nm-thick single-layer TiN film on the MgO substrate showed a T_C of 5.3 K and a resistivity of 3.5 μΩcm at 10 K. Based on these epitaxial TiN films, we fabricated TiN/AIN/TiN Josephson junctions and measured their current-voltage characteristics. At 1.9 K, the electrical parameters of junctions with J_C = 50 A/cm² showed that the gap voltage and the ratio of R_sg/R_N were about 2.5 mV and 2.1, respectively.