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

Josephson junctions (JJs) with layered Nb/MoGe electrodes were fabricated and characterized at 4.2 K. Mo<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub>Ge<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> 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<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">_x</i> 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 (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</i>) a standing wave (Fiske) resonance to occur at lower frequencies, and (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ii</i>) 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.