Internally shunted Josephson junctions with barriers tuned near the metal–insulatortransition for RSFQ logic applications

Abstract

The fabrication of self-shunted SNS (superconductor/normal conductor/superconductor)Josephson junctions for rapid single flux quantum (RSFQ) logic couldpotentially facilitate increased circuit density, as well as reduced parasiticcapacitance and inductance over the currently used externally shunted SIS(superconductor/insulator/superconductor) trilayer junction process. We report thedeposition, fabrication, and device characterization of Josephson junctions prepared withNb1−yTiyN electrodesand TaxN barriers tuned near the metal–insulator transition, deposited on practical large-area oxide-bufferedsilicon wafers. When scaled to practical device dimensions, this type of junction is found to have anIcRn product of over 0.5 mV and a critical current(Ic) and normalresistance (Rn) of magnitudes suitable for single flux quantum digital circuits. A longer than expected normal-metal coherencelength (ξn) of 5.8 nm is inferred from the thickness dependence ofJc at 4.2 K for junctions fabricated using a barrier resistivity of13 mΩ cm. Althoughnot well understood and not quantitatively predicted by conventional theories, this results in a sufficientlyhigh Ic and IcRn to make the junctions suitable for practical applications. Similar observationsof unexpectedly large Josephson coupling currents in SNS junctions have beendocumented in other systems, particularly in cases when the barrier is near theM–I transition, and have become known as the giant proximity effect. The temperature dependence ofξn,IcRn,and Jc are also reported. For this technology to be used in practical applications, significantimprovements in our fabrication process are needed as we observe large variations inIc and Rn values across a 100 mm wafer, presumably as a result of variationsin the Ta:N stoichiometry and the resulting changes in theTaxN barrier resistivity.