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 ( <tex-math notation="LaTeX">VC)</tex-math> of the ferroelectric layer (FL). In this study, we demonstrate that the use of dual ferroelectric stacks with Zr-doped HfO <tex-math notation="LaTeX">₂</tex-math> (HZO) and Al-doped HfO <tex-math notation="LaTeX">₂</tex-math> (HAO) allows the adjustment of <tex-math notation="LaTeX">VC</tex-math> and remnant polarization ( <tex-math notation="LaTeX">Pᵣ)</tex-math> over a broad range relative to the ramping voltage. When a single HZO FL is used, a rapid increase in <tex-math notation="LaTeX">Pᵣ</tex-math> is observed, reaching approximately 30 <tex-math notation="LaTeX">μ</tex-math> C/cm <tex-math notation="LaTeX">²</tex-math> at a specific <tex-math notation="LaTeX">VC</tex-math> value. In contrast, the HAO FL progresses at a slower rate, achieving a smaller <tex-math notation="LaTeX">Pᵣ</tex-math> value approximately equal to 7 <tex-math notation="LaTeX">μ</tex-math> C/cm <tex-math notation="LaTeX">²</tex-math> with increasing voltage. In the dual-stack configuration, polarization occurs sequentially through each FL, enabling the incremental adjustment of <tex-math notation="LaTeX">VC</tex-math> . 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.