Influence Mechanism of Vertical Mirror Symmetry on Out-of-Plane Dielectric Properties in 2D Materials

In vertically integrated transistor structures, understanding the out-of-plane dielectric properties of 2D materials is crucial. We performed first-principles calculations to investigate the out-of-plane dielectric properties, including electronic and ionic polarization, of three types of 2D materials: transition metal dichalcogenides MX₂ (M = Mo, W; X = S, Se, Te), transition metal halide nitrides MNX (M = Ti, Zr, Hf; X = Cl, Br), and MF₄ (M = Sn, Pb). We identified a suppression mechanism for ionic polarization caused by vertical mirror symmetry, which becomes more pronounced with higher structural symmetry and atomic coordination. This mechanism ultimately reduces the out-of-plane ionic contribution of 2D MX₂ to nearly zero. Using the classical spring oscillator model and second-order perturbation theory, we identified relevant physical quantities to specifically analyze the variation of the dielectric constant. This study provides a framework for analyzing 2D materials' dielectric properties and guides the design of novel dielectric materials.