Key Takeaways & Executive Findings
- •• • Hf0.85Ce0.15O2−δ films on (011)-oriented STO achieve 2Pr = 102.1 μC/cm2, the highest reported for HfO2-based ferroelectrics, enabling reduced capacitor cell area and stronger ferroelectric field modulation for high-density non-volatile memory. • • Fatigue resistance remains <10% degradation after 107 switching cycles, satisfying industrial endurance requirements for embedded memory and in-memory computing arrays. • • The (011) substrate orientation maximizes Ce3+ content and unit cell volume, inducing chemical negative strain that stabilizes the ferroelectric orthorhombic phase; this orientation-dependent control is critical for reproducible device performance. • • Piezoresponse force microscopy confirms robust polarization switching with ~180° phase reversal under ±10 V bipolar cycling, validating reliable ferroelectric switching at the local scale for capacitor and transistor integration.
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Abstract
Hafnium oxide (HfO2)-based ferroelectrics are compatible with complementary metal-oxide-semiconductor (CMOS) processing and lead-free, but their remanent polarization (2Pr) remains insufficient for advanced non-volatile memory. This work achieves an ultrahigh 2Pr of 102.1 μC/cm2 in Ce-doped HfO2 thin films via biaxial strain engineering. Hf0.85Ce0.15O2−δ films were grown on crystallographically oriented SrTiO3 (STO) substrates, and the substrate orientation was found to control Ce3+ content, unit cell volume, and ferroelectric phase stabilization. Films on (011)-oriented STO exhibit the highest Ce3+ fraction and largest unit cell volume, corresponding to chemical negative strain that stabilizes the metastable orthorhombic phase. The optimized films show 2Pr = 102.1 μC/cm2, the highest reported for HfO2-based ferroelectrics, and fatigue resistance with <10% degradation after 107 switching cycles. Piezoresponse force microscopy confirms robust polarization switching with ~180° phase reversal under ±10 V bipolar cycling. The results establish substrate-induced strain as a viable route to overcome the polarization bottleneck of HfO2-based materials, providing a foundation for high-density, low-power non-volatile memory and in-memory computing.
1. Introduction
HfO2-based ferroelectrics have emerged as leading candidates for non-volatile memory because of their CMOS compatibility and lead-free composition, yet their practical deployment is constrained by weak remanent polarization. Pure HfO2 and conventional single-element doping strategies—such as Si, Zr, or Al—struggle to simultaneously stabilize the metastable orthorhombic phase and suppress oxygen-vacancy-related leakage and fatigue. Si doping beyond 5 at.% promotes amorphization, Zr doping offers limited control over oxygen vacancies and suffers high-field leakage, and Al doping provides insufficient lattice distortion due to its small ionic radius (~0.540 Å). These trade-offs cap achievable polarization and reliability, impeding the scaling of ferroelectric random-access memory and ferroelectric field-effect transistors.
This study addresses the polarization bottleneck through biaxial strain engineering using crystallographically oriented SrTiO3 substrates. By growing Hf0.85Ce0.15O2−δ films on (001), (011), and (111) STO, the authors systematically correlate substrate orientation with Ce3+ content, unit cell volume, and ferroelectric phase stabilization. The (011) orientation induces the largest chemical negative strain, yielding an ultrahigh remanent polarization of 102.1 μC/cm2 and fatigue resistance below 10% degradation after 107 cycles. These metrics directly confront the endurance and polarization limitations that have stalled HfO2-based ferroelectrics in advanced memory architectures.
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LI Hangren, TU Jie, DING Jiaqi, XIA Jing, SHI Longyuan, DU Siyuan, LIU Xiuqiao, LIU Xudong, LI Menglin, TIAN Jianjun, ZHANG Linxing (2025). Ultrahigh remanent polarization of Ce-doped HfO2 ferroelectric thin films through strain engineering. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3456-6
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Frequently Asked Questions
What is the exact remanent polarization and fatigue endurance of the Ce-doped HfO2 films, and how do they compare to prior HfO2-based ferroelectrics?
The Hf0.85Ce0.15O2−δ films on (011)-oriented STO exhibit 2Pr = 102.1 μC/cm2, the highest value reported for HfO2-based ferroelectrics, with <10% degradation after 107 switching cycles. This exceeds typical reported values for Si-, Zr-, or Al-doped HfO2, which often fall below 40 μC/cm2 and show greater fatigue.
What is the mechanism by which substrate orientation enhances ferroelectric polarization?
The (011)-oriented STO substrate imposes biaxial strain that maximizes Ce3+ content and unit cell volume, creating chemical negative strain. This strain stabilizes the metastable orthorhombic phase, as evidenced by the correlation between increased unit cell volume and enhanced remanent polarization.
How does the film composition and substrate choice affect scalability and integration with CMOS?
The films are Hf0.85Ce0.15O2−δ grown on SrTiO3 substrates, which are not standard CMOS substrates. While the HfO2-based material is CMOS-compatible, the use of STO requires transfer or buffer layers for silicon integration, posing a scalability challenge that must be addressed for industrial adoption.
What is the switching behavior and local ferroelectric uniformity of the films?
Piezoresponse force microscopy over an 8 μm × 8 μm area shows robust polarization switching with ~180° phase reversal under bipolar cycling from −10 V to +10 V. The low surface roughness supports uniform surface ferroelectric switching, indicating good local uniformity.
What are the remaining barriers to commercial deployment of these Ce-doped HfO2 films?
Key barriers include the need for epitaxial growth on oriented STO substrates, which is costly and not directly compatible with silicon manufacturing. Additionally, long-term reliability beyond 107 cycles, high-temperature stability, and oxygen vacancy control under operating fields require further validation.
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