Key Takeaways & Executive Findings
- •• • Improper ferroelectrics, where polarization is a secondary order parameter, exhibit intrinsic immunity to depolarization fields, enabling polarization stability in ultrathin films (e.g., below 5 nm) without the critical thickness limit observed in proper ferroelectrics like BaTiO3 and PbTiO3. • • Hyper-ferroelectric materials, such as LiZnSb, retain polarization under strong depolarization fields due to their unique electronic structure, with theoretical predictions indicating polarization stability even at 2D limits, as supported by first-principles calculations. • • HfO2-based quasi-2D ferroelectrics demonstrate thickness-independent polarization and depolarization-field immunity, making them promising for high-density memory integration with endurance exceeding 10^10 cycles and switching speeds below 10 ns. • • 2D van der Waals ferroelectrics, including exfoliated Aurivillius oxide flakes of sub-unit cell thickness, exhibit robust in-plane ferroelectricity at room temperature, with polarization values up to 20 μC/cm², enabling scalable device fabrication without lattice mismatch constraints.
Abstract
Ferroelectric memory, with its promise of low power consumption, high writing speed and exceptional endurance, requires the scaling of ferroelectric films to ultrathin dimensions—often just a few atomic layers thick. However, such extreme thinning risks destabilizing or even erasing electric polarization, mainly due to the detrimental depolarization field. Remarkably, certain ferroelectrics exhibit an intrinsic immunity to this effect, as predicted theoretically and confirmed experimentally. Examples include improper ferroelectrics, hyper ferroelectrics, engineered heterostructures, and low-dimensional van der Waals ferroelectrics. This review systematically examines these unique materials, unravelling the fundamental physics behind their polarization robustness and the mechanisms enabling them to resist the depolarization field. By bridging theory with experimental advances, we aim to inspire the design of next-generation ferroelectrics capable of overcoming critical challenges encountered in practical ferroelectric memory devices.
1. Introduction
Ferroelectric memory technologies promise low power consumption, high write speeds, and exceptional endurance, positioning them as leading candidates for next-generation nonvolatile memory. However, scaling ferroelectric films to ultrathin dimensions—essential for high-density integration—destabilizes polarization due to the intrinsic depolarization field, which opposes spontaneous polarization and can suppress it entirely below a critical thickness. This bottleneck has hindered commercial FRAM scalability beyond a few megabits, as conventional proper ferroelectrics like BaTiO3 and PbTiO3 suffer from severe polarization degradation at nanoscale thicknesses.
This review systematically addresses the depolarization-field challenge by examining four classes of materials that exhibit intrinsic immunity: improper ferroelectrics, hyper-ferroelectrics, engineered heterostructures, and low-dimensional van der Waals ferroelectrics. By analyzing the fundamental physics and experimental evidence, we identify mechanisms that enable polarization robustness, offering a pathway to overcome the scaling limits of ferroelectric memory and achieve high-density, low-power storage solutions.
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Xudong Wang, Guichen Teng, Xiangjian Meng, Shujun Zhang, Tie Lin, Hao Shen, Jianlu Wang, Junhao Chu (2026). Why Are Some Special Ferroelectrics Immune to the Depolarization Field?. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-4173-x
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Frequently Asked Questions
What is the critical thickness for polarization stability in improper ferroelectrics compared to proper ferroelectrics?
Improper ferroelectrics, where polarization is a secondary order parameter, exhibit polarization stability down to a few atomic layers (e.g., below 5 nm), whereas proper ferroelectrics like BaTiO3 typically lose polarization below a critical thickness of ~2-3 nm due to depolarization field effects.
How do hyper-ferroelectric materials like LiZnSb achieve polarization retention under strong depolarization fields?
Hyper-ferroelectrics possess a unique electronic structure where the Born effective charges are anomalously large, leading to a negative depolarization field contribution that stabilizes polarization. First-principles calculations predict that LiZnSb retains polarization even in the 2D limit, with a spontaneous polarization of ~50 μC/cm².
What are the endurance and switching speed metrics for HfO2-based ferroelectric devices?
HfO2-based ferroelectrics exhibit thickness-independent polarization and depolarization-field immunity, with endurance exceeding 10^10 cycles and switching speeds below 10 ns, making them suitable for high-density FRAM applications.
Can 2D van der Waals ferroelectrics be integrated into existing semiconductor manufacturing processes?
2D vdW ferroelectrics, such as exfoliated Aurivillius oxide flakes, show robust in-plane ferroelectricity at sub-unit cell thicknesses, with polarization values up to 20 μC/cm². Their van der Waals nature allows transfer onto arbitrary substrates without lattice matching, potentially simplifying integration with CMOS technology.
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