Hybrid Nb/MoGe Josephson Junctions for the Development of Flux-Flow-Type Josephson Oscillators

Abstract

Josephson junctions (JJs) with layered Nb/MoGe electrodes were fabricated and characterized at 4.2 K. Mo _0.75 Ge _0.25 is known to be a superconductor with the transition temperature of up to 7.4 K. It is essentially an amorphous material that has very uniform and smooth surface in thin-film form. This allowed us to fabricate high-quality all-MoGe Josephson junctions with very thin Al overlayer to form AlO _x barrier [Supercond. Sci. Technol., vol. 35, Article no. 035008, 2022]. Here we report JJs with composite Nb/MoGe electrodes aiming at development of flux-flow-type Josephson oscillators operating at low frequencies suitable for qubit control. Using MoGe allows one to preserve a large effective magnetic penetration depth and a high kinetic inductance of the electrodes, which results in ( i ) a standing wave (Fiske) resonance to occur at lower frequencies, and ( ii ) in a higher impedance of the oscillator; the latter is important for matching it with other elements of the superconducting circuit. At the same time, using Nb as a part of the composite electrode allows one to maintain the critical parameters (critical current and voltage) of the JJ close to those of the Nb-based JJs. In addition, damping in hybrid Nb/MoGe JJs is lower than that in all-MoGe JJs, which results in a higher current of flux-flow steps, and potentially, in a higher microwave emission power.