Polarization switching in Hf0.5Zr0.5O2-dielectric stack: The role of dielectric layer thickness

Abstract

Understanding the role of the dielectric (DE) layer in ferroelectric (FE) Hf0.5Zr0.5O2 (HZO) based devices (e.g., ferroelectric-field-effect-transistors, FE-FETs) is important to enable their application-driven optimizations. To that end, in this work, we systematically investigate the polarization switching mechanisms in FE–DE stacks and analyze their dependence on the dielectric layer thickness (TDE). First, we fabricate a HZO–Al2O3 (FE–DE) stack and experimentally demonstrate a decrease in remanent polarization and an increase in coercive voltage with an increase in TDE. As such dependencies are out of the scope of commonly used single domain polarization switching models, therefore, we argue that the consideration of the multi-domain model is essential for analyzing the polarization switching in HZO. Then, using phase-field simulations of the FE–DE stack, we show that an increase in TDE results in a larger number of reverse domains in the FE layer to suppress the depolarization field, which leads to a decrease in the remanent polarization and an increase in the coercive voltage. Furthermore, our analysis signifies that the polarization switching mechanism in HZO can be modulated from domain-nucleation based to domain-wall motion based by increasing the TDE and that can serve as a potential knob for application-specific optimization of FE-FETs. In addition, we show that the effective polarization–voltage characteristics of the FE layer in the FE–DE stack exhibit a negative slope region that leads to the charge enhancement effects in the FE–DE stack. While such effects are most commonly misinterpreted as either the transient effects or the stabilized single-domain negative capacitance effects, we demonstrate that the appearance of a negative slope in the hysteretic polarization–voltage characteristics is quasi-static in nature and that originates from the multi-domain polarization switching in the FE.