Ultrathin Hf_0.5Zr_0.5O₂ Films with ZrO₂ Seed Layer for Ferroelectric Tunnel Junctions in Crossbar Array

Ferroelectric tunnel junctions (FTJs) are promising candidates for next-generation nonvolatile memories, offering fast switching, low-power operation, and scalability. However, achieving robust ferroelectricity and high tunneling electroresistance (TER) at nanometer-scale thickness remains challenging due to phase instability and interfacial degradation. Here, we report ultrathin FTJs based on a 2.5 nm-thick Hf_0.5Zr_0.5O₂ (HZO) ferroelectric barrier stabilized by a 0.5 nm ZrO₂ seed layer. The seed layer promotes the orthorhombic phase and enhances ferroelectricity, as confirmed by structural, electrical, and piezoresponse analyses. The resulting Pt/HZO/ZrO₂/TiN devices exhibit a TER of 8 under ±1.5 V pulses, remnant polarization of 8 μC cm^-2, and stable ferroelectric switching. Integration into a 3 × 3 crossbar array demonstrates pattern programmability and stable ON/OFF readout. This work presents a practical pathway toward highly scalable and complementary metal-oxide-semiconductor (CMOS)-compatible ferroelectric nonvolatile memories with competitive performance at ultrathin thickness.