Internally shunted Josephson junctions with barriers tuned near the metal-insulator transition for RSFQ logic applications

The fabrication of self-shunted SNS (superconductor/normal conductor/superconductor) Josephson junctions for rapid single flux quantum (RSFQ) logic could potentially facilitate increased circuit density, as well as reduced parasitic capacitance and inductance over the currently used externally shunted SIS (superconductor/insulator/superconductor) trilayer junction process. We report the deposition, fabrication, and device characterization of Josephson junctions prepared with Nb_1-yTi_yN electrodes and Ta_xN barriers tuned near the metal-insulator transition, deposited on practical large-area oxide-buffered silicon wafers. When scaled to practical device dimensions, this type of junction is found to have an I _cR_n product of over 0.5 mV and a critical current (I _c) and normal resistance (R_n) of magnitudes suitable for single flux quantum digital circuits. A longer than expected normal-metal coherence length (ξ_n) of 5.8 nm is inferred from the thickness dependence of J_c at 4.2 K for junctions fabricated using a barrier resistivity of 13 mΩ cm. Although not well understood and not quantitatively predicted by conventional theories, this results in a sufficiently high I_c and I_cR_n to make the junctions suitable for practical applications. Similar observations of unexpectedly large Josephson coupling currents in SNS junctions have been documented in other systems, particularly in cases when the barrier is near the M-I transition, and have become known as the giant proximity effect. The temperature dependence of ξ_n, I_cR_n, and J_c are also reported. For this technology to be used in practical applications, significant improvements in our fabrication process are needed as we observe large variations in I_c and R_n values across a 100 mm wafer, presumably as a result of variations in the Ta:N stoichiometry and the resulting changes in the Ta_xN barrier resistivity.