Ferroelectric Compensation Effect of the Hard Electrode for the HfO2-ZrO2 Superlattice Films at the Low-Annealing Temperature

Abstract

This study investigates the ferroelectric (FE) performance of [HfO₂/ZrO₂]₆ superlattice FE capacitors using different top electrodes (TE). The unidirectional rapid thermal annealing (RTA) process from 450 to 600 °C is conducted. The device's remanent polarization (P_r) improved with TE hardness, and is maintained with harder TE at lower temperature. Furthermore, the superlattice's endurance and the recovery feature improve with harder TE. The increased orthorhombic phase (o-phase) content and the decreased tetragonal phase (t-phase) content indicate that the hard TE's out-of-plane stress at the interface suppressed the phase transition from the t-phase to the monoclinic phase (m-phase) and promotes the o-phase formation. It's known that hard electrodes usually have low coefficient of thermal expansion (CTE), which can generate high in-plane tensile strain optimizing the FE properties, so the lower-CTE electrodes devices’ FE performances are expected to degrade more with temperature decreasing, which is opposite with the experimental results. Therefore, hard electrodes can generate high out-of-plane compressive stress to offset the reduced in-plane tensile stress, leading to a FE compensation effect in low temperature thermal process.