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We present recent results in developing SNS self-shunted NbN-TaxN-NbN Josephson junctions operating above 9 K; good control over the barrier resistivity allows us to fabricate high RnIc product junctions. We believe that our all-nitride technology is suitable for next generation RSFQ (Rapid Single Flux Quantum) circuits. Films are sputtered on 3 inch thermally oxidized silicon wafers. An MgO buffer is used to induce a cubic NbN texture, assuring low resistivity and high transition temperature Tc up to 16 K. The barrier has been characterized by using several thin TaxN films sputtered at ambient temperature and 300 °C, showing a resistivity behavior spanning from metallic to the insulator range. Junctions are achieved by using respectively a SNOP (Selective Niobium Overlap Process) and a SNEP (Selectively Niobium Etching Process) method. In-situ DC-Magnetron sputtered NbN/TaxN/NbN trilayers show very narrowed transitions up to 15.5 K. The junctions fabricated with a 7 nm thin high-resistive TaxN barrier show current densities in the 10kA/cm2 range and a large RnIc product up to 0.3mV at 10K. Shapiro steps and Fraunhofer diffraction pattern have been observed, revealing Josephson behavior up to 14K. We also developed a complete all-NbN 10 photo-masks technology, including a common ground plane and bias resistors, devoted to RSFQ applications.

Study of NbN Josephson junctions with a tantalum nitride barrier tuned to the metal-insulator transition