Electrically controllable nonreciprocal Josephson junctions with an atomic layer deposition prepared tunnel barrier

We report the observation of nonreciprocal Josephson or Josephson diode effect in Josephson tunnel junctions (JTJs) featuring a composite tunnel barrier of Al₂O₃/Hf_0.8Zr_0.2O₂ (AO/HZO) prepared by atomic layer deposition, which is compatible with Complementary Metal-Oxide-Semiconductor (CMOS) technologies. These junctions, featuring TiN and Nb as the two superconducting electrodes, were found to exhibit Josephson coupling whose magnitude depends on the direction of the supercurrent in the absence of an external magnetic field. The substantial nonreciprocity could be tuned by a simple scheme of applying an electrical training current several times the critical current of the junction. We attribute these observations to the coexistence of positive and negative Josephson couplings with the latter facilitated by the indirect tunneling through localized states in the tunnel barrier. While ferroelectricity of the ultrathin layer of HZO is not demonstrated in the current devices, a ferroelectric tunnel barrier will provide additional tunability by exploiting its reversible remnant polarization, a direction that is currently being explored. Our work suggests a potential pathway for integrating nonreciprocal JTJs into CMOS technology for quantum technologies based on superconducting electronics.