Ferroelectricity in ultrathin Hf_1-xZr_xO₂and its use as a tunnel barrier in Josephson junctions

Nb/Hf-HfO­_x/Nb Josephson junctions were shown to potentially be a better suited platform for various superconducting electronics applications including rapid single-flux-quantum logic than the Nb/Al-AlO­_x/Nb counterpart because of the improved thermal stability needed for possible integration with Si-based CMOS technologies. Ultrathin HfO₂ has also demonstrated to be a promising tunnel barrier for ferroelectric tunnel junction applications because it can be made ferroelectric through rapid thermal annealing (RTA), a thickness at which direct quantum mechanical tunneling including Josephson tunneling of Cooper pairs is possible. We explored Nb/Hf_1-xZr_xO₂/Nb structures with the ultrathin Hf_1-xZr_xO₂ (HZO) layer prepared by plasma enhanced atomic layer deposition (PEALD), which was found previously to be ferroelectric down to 1 nm. We found that RTA at 500 °C required to stabilize the ferroelectric phase in HZO poses challenges to achieving Josephson tunneling due to interface degradations. We report the fabrication of Nb/HZO/Nb junctions featuring a PEALD prepared HZO and the Josephson effect observed in them. We will also report our work in stabilizing the ferroelectric phase in ultrathin HZO.