Ultrathin ZrO₂ Seed Layers for Low-Temperature Orthorhombic Ferroelectric Hf_1-xZr_xO₂

Ferroelectric Hf_1-xZr_xO₂ has emerged as a promising material for next-generation nonvolatile memory and logic applications due to its complementary metal-oxide-semiconductor (CMOS) compatibility and scalability. However, stabilizing the ferroelectric orthorhombic-phase at back-end-of-line (BEOL)-compatible temperatures (<400 °C) remains a critical challenge, particularly in achieving both high remnant polarization and device reliability. In this work, we demonstrate that the incorporation of an ultrathin ZrO₂ seed layer enables low-temperature crystallization and robust ferroelectricity in Hf_1-xZr_xO₂ thin films. The ultrathin ZrO₂ seed layer facilitates the tetragonal-phase to orthorhombic-phase transformation during low-temperature post-metallization annealing, thereby promoting robust orthorhombic-phase formation under BEOL-compatible conditions. By optimizing ZrO₂ seed layer thickness, we achieved a significant enhancement in ferroelectric performance: a remnant polarization of 30.3 μC/cm² at 350 °C for films with a 5-cycle ZrO₂ seed layer, outperforming both seedless films (12.1 μC/cm²) and those with a thicker 7-cycle ZrO₂ seed layer (13.0 μC/cm²). Furthermore, the optimized structure exhibited a low leakage current and endurance exceeding 10⁹ cycles. We confirm an increased o-phase fraction and suppressed interfacial layer formation, attributed to the (111)-textured ZrO₂ seed layer. These findings underscore the critical role of atomic-level seed layer engineering in enabling high-performance, BEOL-compatible ferroelectric Hf_1-xZr_xO₂ devices for future memory architectures.