Dual Ferroelectric Stack of HfZrO₂/Al:HfO₂ With Tunable Coercive Voltage for High-Density Memory Applications

To achieve a multilevel threshold voltage in the configuration of a ferroelectric field-effect transistor for high-density memories, it is crucial to achieve a finely tunable coercive voltage ( $\textit{V}_{\text{C}}\text{)}$ of the ferroelectric layer (FL). In this study, we demonstrate that the use of dual ferroelectric stacks with Zr-doped HfO $_{\text{2}}$ (HZO) and Al-doped HfO $_{\text{2}}$ (HAO) allows the adjustment of $\textit{V}_{\text{C}}$ and remnant polarization ( $\textit{P}_{\textit{r}}\text{)}$ over a broad range relative to the ramping voltage. When a single HZO FL is used, a rapid increase in $\textit{P}_{\textit{r}}$ is observed, reaching approximately 30 $\mu $ C/cm $^{\text{2}}$ at a specific $\textit{V}_{\text{C}}$ value. In contrast, the HAO FL progresses at a slower rate, achieving a smaller $\textit{P}_{\textit{r}}$ value approximately equal to 7 $\mu$ C/cm $^{\text{2}}$ with increasing voltage. In the dual-stack configuration, polarization occurs sequentially through each FL, enabling the incremental adjustment of $\textit{V}_{\text{C}}$ . The stress primarily arises after the deposition of the top electrode during annealing, thus emphasizing the crucial role of the upper FL in the ferroelectric properties of the dual stack. Furthermore, our findings reveal that substituting a dielectric layer (DL)for the HAO FL, rather than the HZO FL, exacerbates polarization, revealing that the synergistic effect originates from two different FLs.