High Tc Josephson nanoJunctions made by ion irradiation : characteristics and reproducibility

TL;DR

This paper presents a simple, reproducible ion irradiation method to fabricate high-temperature superconducting Josephson nanojunctions with consistent properties, suitable for quantum interference devices operating at liquid nitrogen temperatures.

Contribution

The authors introduce a novel two-step ion irradiation process for creating high Tc Josephson nanojunctions with high reproducibility and tunable characteristics.

Findings

Reproducible Josephson junctions with low dispersion (5-10%)

Critical current densities up to 30 kA/cm²

Successful fabrication of microSQUIDs with high sensitivity

Abstract

Reproducible High Tc Josephson junctions have been made in a rather simple two-step process using ion irradiation. A microbridge 1 to 5 micrometers wide is firstly designed by ion irradiating a c-axis-oriented YBa2Cu3O7 film through a gold mask such as the unprotected part becomes insulating. A lower Tc part is then defined within the bridge by irradiating with a much lower dose through a 20 nm wide narrow slit opened in a standard electronic photoresist. These planar junctions, whose settings can be finely tuned, exhibit reproducible and nearly ideal Josephson characteristics. Non hysteretic Resistively Shunted Junction (RSJ) like behavior is observed, together with sinc Fraunhofer patterns for rectangular junctions. The IcRn product varies with temperature ; it can reach a few mV. The typical resistance ranges from 0.1 to a few ohms, and the critical current density can be as high as 30 kA/cm2. The dispersion in characteristics is very low, in the 5% to 10% range. Such nanojunctions have been used to make microSQUIDs (Superconducting Quantum Interference Device) operating at Liquid Nitrogen (LN2) temperature. They exhibit a very small asymmetry, a good sensitivity and a rather low noise. The process is easily scalable to make rather complex Josephson circuits.