Properties of Unshunted and Resistively Shunted Nb/AlOx-Al/Nb Josephson Junctions With Critical Current Densities from 0.1 mA/{\mu}m^2 to 1 mA/{\mu}m^2

摘要

We investigated current-voltage characteristics of unshunted and externallyshunted Josephson junctions (JJs) with high critical current densities, Jc, inorder to extract their basic parameters and statistical characteristics for JJmodeling in superconducting integrated circuits and to assess their potentialfor future technology nodes. Nb/AlOx-Al/Nb JJs with diameters from 0.5 {\mu}mto 6 {\mu}m were fabricated using a fully planarized process with Mo or MoNxthin-film shunt resistors with sheet resistance Rsq = 2 {\Omega}/sq and Rsq = 6{\Omega}/sq, respectively. We used our standard MIT LL process SFQ5ee tofabricate JJs with Jc = 0.1 mA/{\mu}m^2 and our new process SFQ5hs to make JJswith Jc = 0.2 mA/{\mu}m^2 and higher current densities up to about 1mA/{\mu}m^2. Using LRC resonance features on the I-V characteristics of shuntedJJs, we extract the inductance associated with molybdenum shunt resistors of1.4 pH/sq. The main part this inductance, about 1.1 pH/sq, is the inductance ofthe 40-nm Mo resistor film, while the geometrical inductance of superconductingNb wiring contributes the rest. We attribute this large inductance to kineticinductance arising from the complex conductivity of a thin normal-metal film inan electromagnetic field with angular frequency {\omega},{\sigma}({\omega})={\sigma}0/(1+i{\omega}{\tau}), where {\sigma}0 is the staticconductivity and {\tau} the electron scattering time. Using a resonance in alarge-area unshunted high-Jc junction excited by a resistively coupledsmall-area shunted JJ, we extract the Josephson plasma frequency and specificcapacitance of high-Jc junctions in 0.1 to 1 mA/{\mu}m^2 Jc range. We alsopresent data on Jc targeting and JJ critical current spreads. We discuss using0.2-mA/{\mu}m^2 JJs in VLSI Single Flux Quantum circuits and 0.5-mA/{\mu}m^2JJs in high-density integrated circuits without shunt resistors.