Adrian Acosta1,John Mark P. Martirez1,Norleakvisoth Lim1,Jane Chang1,Emily Carter1,2
University of California, Los Angeles1,Princeton University2
Adrian Acosta1,John Mark P. Martirez1,Norleakvisoth Lim1,Jane Chang1,Emily Carter1,2
University of California, Los Angeles1,Princeton University2
HfO<sub>2</sub> and HfO<sub>2</sub>-based ferroelectric materials have received increasing attention for their potential to be components of next-generation memory and transistors in electronics. Although researchers have explored the influence of various variables (e.g. dopants, strain, and applied electric fields) on the stability of HfO<sub>2</sub>’s polar orthorhombic phase compared to its nonpolar tetragonal and monoclinic phases [1-3], the effect of the surface composition has not been investigated fully. We therefore used density functional theory to first assess the effect of ferroelectric polarization on the stabilities of an array of reconstructed surfaces of polar orthorhombic HfO<sub>2</sub>(001). We determined that while a stoichiometric slab with symmetrically terminated oxygen surfaces is unstable due to its inability to screen surface polarization charges, a nonstoichiometric slab with a relatively oxygen-rich, positively polarized surface can adequately screen such charges. We then studied the reciprocal interaction and determined the influence of the surface composition on the stability of the ferroelectric polarization for free-standing orthorhombic slabs. At thicknesses of five to 11 HfO<sub>2</sub> layers, the symmetric slabs depolarize to a monoclinic-like phase. In contrast, asymmetric slabs can maintain a stable orthorhombic phase with polarizations (all above the bulk value) increasing with decreasing thickness. However, for the ultrathin case of three-HfO<sub>2</sub>-layer thickness, the symmetric slab can sustain a strong polarization without an explicit dipole screening mechanism whereas the asymmetric slab undergoes a transition to a polar rhombohedral (R3)-like phase. Our findings can account for the recently observed absence of a ferroelectric critical thickness in HfO<sub>2</sub>-based materials and are consistent with polarization enhancement at reduced thickness even at the ultrathin limit [4].<br/><br/>[1] Phys. Rev. B 102, 214108 (2020)<br/>[2] Phys. Rev. Lett. 125, 257603 (2020)<br/>[3] Phys. Rev. Lett. 127, 087602 (2021)<br/>[4] Nature 580, 478 (2020)