Tail-to-Tail and Head-to-Head Domain Walls in Ferroelectric Hafnium Zirconium Oxide—A First-Principle Analysis

Hafnium Zirconium Oxide (HZO) is a promising ferroelectric (FE) material for future electronic devices owing to its CMOS compatibility, strong ferroelectricity in ultrathin films and rich multidomain characteristics. Due to its unique atomic arrangement, the domain walls (DW) in different spatial directions can exhibit different properties. Most of the studies so far have focused on lateral DWs in HZO based FEs [1][2]. On the other hand, Tail-to-Tail/ Head-to-Head (T-T/H-H) DWs have not been well explored. To understand domain growth mechanism in HZO more comprehensively, there is a strong need to analyze the nature and characteristics of T-T/H-H DWs formed along the polarization direction.<br/>In this work, we use First-Principle Density Functional Theory (DFT) calculations to predict domain formation along the polarization direction in HZO. We find that, although stable T-T DWs can form in HZO, H-H DW formation is energetically unfavorable. This phenomenon originates due to the unique segmentation of FE HZO unit cells in polar and non-polar layers with different ionic charge density as well as the energy minimization through oxygen termination at the surface/interface. We substantiate this claim further by studying the electrostatic potential profiles obtained from DFT calculations. Moreover, due to the depolarization energy, the formation of T-T DW is only possible above a critical thickness. The important consequence of the instability of H-H DW is twofold. H-H/T-T DW of periodic nature cannot form. Also, the nucleation must occur from interfacial sites to avoid H-H DW formation. Previous theoretical works [3] have shown different microscopic configurations of both upward and downward polarized domains in HZO. Considering all different combinations of the domains, we obtain the most energetically favorable T-T DW as the configuration where <i>pbcn</i> phase forms between the two neighboring FE domains. We analyze both freestanding HZO and metal-HZO-metal samples to establish the conditions for the stability of T-T DW.<br/>[1] W. Ding et al., <i>Acta Mat.,</i>196, 556-564 (2020)<br/>[2] Xu, X. et al. <i>Nat. Mater.,</i> 20, 826–832 (2021)<br/>[3] D. -H. Choe et al., <i>Mat. Today,</i> 50, 8-15, (2021)