NbN/TaN/NbN Tunnel Junctions With Ultrahigh Critical Current Densities

We report the fabrication and characterization of NbN/TaN/NbN tunnel junctions with critical current densities (Jc) greater than 100 kA/cm2, realized through chemical–mechanical polishing technology. By reducing the TaN barrier thickness to 1.70 nm, we achieved aJcexceeding 600 kA/cm2, and the retrapping-to-critical current ratioIr/Icapproaches unity, indicating nearly non-hysteretic behavior without external shunting. From thickness-dependent fits, the average barrier height was estimated to be 0.048 eV, consistent with previous reports and confirming the structural stability and reproducibility of TaN barriers across the investigated thickness range. The specific capacitanceCswas further extracted using Fiske resonance acrossJcrange (0.6–62.5 kA/cm2), yielding a segmented empirical formula with a breakpointJc= 13 kA/cm2. Compared with other barrier materials, NbN/TaN/NbN SIS junctions exhibited significantly largerCsfor a givenJc, suggesting thatCsreduction will be essential for optimizing their high-frequency performance. Statistical analysis of critical current distributions in large arrays revealed excellent uniformity atJc= 4.5 kA/cm2, the standard deviation (1σ) remained about 3.0%. Even at highJc= 101 kA/cm2a 2,000-junction array maintained 1σ ≈ 3.7%, demonstrating that TaN barriers can achieve high uniformity under stringent process control.