Single-charge devices with ultrasmall Nb/AlOx/Nb trilayer Josephson junctions

TL;DR

This paper reports the fabrication and characterization of ultrasmall Nb/AlOx/Nb trilayer Josephson junctions and their arrays, demonstrating high-quality devices with notable single-electron effects at millikelvin temperatures.

Contribution

It introduces a fabrication process for sub-100 nm Josephson junctions and arrays with detailed electrical characterization and observation of single-electron phenomena.

Findings

High-quality junctions with low leakage currents and high energy gap.

Observation of single-electron behavior in devices below 100 nm.

Capacitance scales with junction size, affecting device performance.

Abstract

Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200 nm down to 70 nm were fabricated from standard Nb/AlOx/Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25 mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of 1.35 meV. The characteristics of the transistors and arrays exhibit some features in the subgap area, associated with tunneling of Cooper pairs, quasiparticles and their combinations due to the redistribution of the bias voltage between the junctions. Total island capacitances of the transistor samples ranged from 1.5 fF to 4 fF, depending on the junction sizes. Devices made of junctions with linear dimensions below 100 nm by 100 nm demonstrate a remarkable single-electron behavior in both superconducting and normal state. We also investigated the area dependence of the junction capacitances for transistor and array samples.