Refractory Niobium Nitride NbN Josephson Junctions and Applications (original) (raw)

The Refractory Nitride Josephson Tunnel Junctions with NbN superconducting thin-film base-electrode (Tc~16.5K) were studied since 1982 using thermal or plasma oxide tunnel barriers, showing attracting I-V non-linear characteristics only when a superconducting soft-metal (Pb-In) was used as counter-electrode. Few years later, introduction of in-situ deposited tri-layers including MgO or AlN tunnel barrier permits a large improvement in the junction quality, yield and reproducibility, opening new application fields. Since, NbN (or one of its parent compounds such as Nb1-xTixN) was established as a possible alternative to refractory Nb Josephson devices (operating at or below 4.2K) for building Superconductive Electronics circuits achieving higher frequencies, up to the THz range for SIS mixers or oscillators as well as operating temperature up to 10K. “Self-shunted” nitride SNS and SS’S junctions, such as NbN-TaNx-NbN JJs, are taking advantage of large RNIC products induced by efficient quasi-particles diffusion and good nitride barriers matching at interfaces with nitride electrodes which solve frequency limitation problems induced by the capacitance of SIS junctions. RF frontends, MUX, ADC and digital RSFQ nitrides circuits have been demonstrated, some of them optimized to operate near 10K inside closed cycle refrigerators for minimizing energy dissipation in a space satellite applications. While NbN circuits are today still less mature than the established Nb-Al-AlOx-Nb JJ state-of-art technology (~20k JJs with JJ ~8kA/cm2, ~1µm2) suitable to achieve prototypes of low dissipation integrated circuit SFQ processors described in [1] and references inside. However important properties of NbN technology seems more economically relevant for the future: a conventional Si-CMOS foundry factories should be used to establish a NbN-SiO2 stack-layered process, stable up to ~350°C, not sensitive to hydride species diffusion, while present Nb trilayers are degraded above 180°C and hydrogen diffusion is detrimental of junctions yield. A recent NbN IC processing demonstration (0.5µm linewidth) has been done on 8-inch silicon wafers at the CEA-LETI CMOS facility, making possible to integrate further NbN ADC and processors chips reliably designed and fabricated with high yield at reasonable cost in any CMOS foundry. Another advantage of NbN thin and very thin (few nm thick) films and nanowires with very short electron-phonon relaxation time leads to fast, light or current sensitive self-resetting disjunctions switches with few ps jitter and fast heat dissipation in the substrate. On-chip integration of nitride front-ends circuits such as resonators, microwave filters, registers, highly sensitive and fast photo-sensors (KIDs, SNSPD, HEB,..) and superconducting-semiconducting interfaces circuits widen the field of superconductive electronics applications from mK up to 10K. Moreover recently, deep submicron size (~0.01µm2), very low capacitance NbN-MgO-NbN SIS and SQUID circuits were achieved using a self-aligned process. Such junctions, beside their attracting properties for nitride IC, are able to emit or detect in a controllable way single microwave photons and could be a new building block of quantum information circuits.