High Tc Josephson Nanojunctions Made by Ion Irradiation: Characteristics and Reproducibility

Reproducible High T_c Josephson junctions have been made in a rather simple two-step process using ion irradiation. A microbridge 1 to 5 mum 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 T_c 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 sine Fraunhofer patterns for rectangular junctions. The I_CR_n 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 spread 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 complex Josephson circuits.