The effects of inserted layers (HfO2, ZrO2, Y2O3, La2O3) on the ferroelectric and synaptic properties of Zr-doped HfO2 sandwich structure

HfO₂-based ferroelectrics (FEs) have gained significant attention as synaptic devices due to multistate capability enabled by multi-domain FEs and nonvolatile characteristics. However, when these FE thin films are applied to synaptic devices, the polarization switching often results in non-linear behavior during the potentiation and depression of synaptic weights. In this study, we introduced insertion layer structures to induce a depolarization field, effectively suppressing abrupt polarization switching and increasing the coercive field (E_c) to enhance synaptic device performance. The study investigated the behavior of Zr-doped HfO2 (HZO) FE properties using different insertion layers (HfO₂, ZrO₂, Y₂O₃, La₂O₃). The FE performance was further evaluated for synaptic devices, including the linearity of remnant polarization (P_r) under varying electric field, endurance characteristics, and cycle-to-cycle variation. Among the materials tested, HfO₂ as the insertion layer yielded the most promising results, showing superior endurance (2ⅹ10⁶ cycle) and minimal cycle-to-cycle variation (14.3%) due to its lower oxygen vacancy levels (6.6%). The HfO₂-inserted FeFET device demonstrated improved synaptic weight control, providing a more states and enhanced linearity in potentiation and depression compared to FeFET without insertion layer. Finally, pattern recognition simulations using the MNIST dataset revealed an accuracy of 87.76%, which represents an improvement of 3.75% over reference.