Observation of nonreciprocal Josephson effect in atomic layer deposition prepared TiN/Al₂O₃/ Hf_1-xZr_xO₂ /Nb junctions

Nonreciprocal Josephson effect, known also as the Josephson diode effect, has been predicted in various Josephson junctions, including those featuring a ferroelectric tunnel barrier. However, a ferroelectric Josephson junction has not been observed experimentally. Josephson junction featuring a tunnel barrier of Hf_1-xZr_xO₂ prepared by atomic layer deposition (ALD), for which ferroelectricity was reported in films as thin as 1 nm, possesses additional advantages for being compatible with the complementary metal-oxide semiconductor (CMOS) technology. We report here the preparation of Josephson junctions using ALD to grow a TiN/Al₂O₃/Hf_1-xZr_xO₂ trilayer stack without breaking the vacuum and RF sputtering to prepare the top electrode of Nb. Both reciprocal and nonreciprocal Josephson effects were observed in our junctions along with gap features in the single-particle tunnel spectra. We will discuss the physical origin of the observed nonreciprocal effect.