Cheema et al. (Nature 2020) demonstrated robust, switchable ferroelectricity in Hf0.5Zr0.5O2 (HZO) films down to 1 nm thickness grown by atomic layer deposition directly on silicon, with polar distortions that are anomalously enhanced, rather than suppressed, as thickness is reduced. The paper left a connected set of issues unresolved: the remanent polarization of ~1 nm films cannot be quantified by conventional polarization-voltage measurements because of prohibitive tunnelling currents; the microscopic origin of the enhanced polar distortion (interplay of confinement strain, surface energy, texturing, and phase competition) below 2 nm is unknown; the initial ~1 nm ferroelectric tunnel junctions show polarization-driven electroresistance but no established performance limits; and the proposed confinement-strain-driven orbital-level engineering lacks a mechanism. The problem is to build a quantitative, mechanism-level, device-validated account of ~1 nm fluorite ferroelectricity on silicon: leakage-insensitive polarization quantification, evolution of phase competition below 2 nm including orbital-level electronic structure, and the resulting limits of atomic-scale ferroelectric tunnel junctions. This connected problem remains open.

Background

A ferroelectric is a material with spontaneously ordered electric dipoles whose polarization can be reversibly switched by an applied electric field; the residual switchable moment after the field is removed is the remanent polarization (PrP_r). In the archetypal perovskite oxide ferroelectrics, ferroelectricity is suppressed below a critical thickness of a few unit cells: incomplete screening of the depolarization field (the internal field generated by the polarization's own bound surface charges) and surface energies favoring high-symmetry phases destroy switchable polarization, so perovskite ultrathin films have generally failed both thickness scaling and monolithic integration with silicon.

Doped HfO2, discovered to be ferroelectric in 2011, is a fluorite-structure binary oxide (a face-centered-cubic oxygen sublattice with cations in eightfold coordination) that can be grown by atomic layer deposition (ALD, a gas-phase, self-limiting growth technique) at low temperature conformally on silicon, making it compatible with CMOS processing. The solid solution Hf0.5Zr0.5O2 (HZO) is the most widely used composition. In films thicker than about 5 nm, ferroelectricity arises from a polar orthorhombic phase (conventionally indexed as Pca21Pca2_1) competing with the stable monoclinic and tetragonal polymorphs, and device behavior is dominated by 'wake-up' (the progressive change in switching under field cycling attributed to phase and defect redistribution). Phase-stability models for this regime are built around grain size, surface energy, doping, and electrode-imparted capping stress.

Cheema et al. (Nature 580, 478 (2020)) showed that HZO grown by ALD directly on Si/SiO2 retains inversion-symmetry breaking and switchable polarization down to 1 nm (about two fluorite unit cells, roughly 10 ALD cycles), evidenced by piezoresponse force microscopy (PFM, a scanning-probe technique sensing local piezoelectric strain), grazing-incidence X-ray diffraction, and scanning transmission electron microscopy. Counter to perovskite trends, structural gauges of polar distortion are enhanced at reduced thickness: the d111d_{111} lattice spacing (spacing between consecutive (111) crystallographic planes), the orthorhombic aspect ratio (a structural proxy for polarization), spectroscopic orbital polarization from X-ray linear dichroism (XLD, the polarization-dependence of X-ray absorption, which senses orbital asymmetry and thus inversion-symmetry breaking), and tetrahedral and rhombic crystal-field splittings (energy separations between electronic orbitals set by the local oxygen coordination) all grow substantially in the ultrathin regime. The authors attribute part of this enhancement to confinement strain (the anisotropic in-plane thermal stress imparted by the capping metal during post-deposition rapid thermal annealing) and observe texturing (preferred crystallite orientation) below 25 ALD cycles. They further demonstrate ~1 nm HZO ferroelectric tunnel junctions (FTJs, metal/ferroelectric/metal devices whose tunnel current depends on the polarization state) showing tunneling electroresistance (TER, the resistance ratio between the two polarization states) that is voltage-polarity-independent, i.e. polarization-driven rather than filamentary.

The paper explicitly leaves four connected issues open, which together define the problem addressed here: (i) macroscopic quantification of PrP_r is blocked because 'prohibitive tunnel currents prevent accurate quantification of polarization from traditional P-V (polarization-voltage) measurements'; (ii) 'further studies should explore how the current understanding of film synthesis, phase competition and polar distortion in HZO developed for thicker films (> 5 nm) evolves in the ultrathin regime (< 2 nm)', including how confinement strain and interface templates determine orientation and orbital polarization in sub-25-cycle films; (iii) 'optimizing FTJ behavior employing HZO the ultrathin regime (~1 nm) will need to be carefully studied', since the initial TER results only establish feasibility; and (iv) harnessing confinement strain 'to tailor electronic structure and engineer polarization at the orbital-level' was suggested but not mechanistically resolved.

Problem Statement

Establish a quantitative, mechanism-level, and device-validated account of ferroelectricity in fluorite-structure Hf0.5Zr0.5O2 (and the closely related HfO2-ZrO2 solid-solution family) films of about 1 nm thickness (the sub-2 nm, sub-25-ALD-cycle regime) grown directly on silicon, as left open by Cheema et al. (Nature 2020). The problem has three linked components that jointly serve this one objective: (1) Quantification — determine the remanent polarization of ~1 nm HZO on silicon as a directly measured macroscopic value — conventional polarization-voltage loops being blocked by prohibitive tunnelling currents — so that the polarization inferred from structural and spectroscopic gauges (d111 spacing, orthorhombic aspect ratio, XLD orbital polarization, crystal-field splittings) is checked against that measured value. (2) Microscopic origin — determine how the phase-competition picture developed for thicker (> 5 nm) HZO evolves below 2 nm: the interplay of confinement strain, surface energy, texturing, and phase competition — all four factors, addressed jointly — in producing the enhanced polar distortion; which polar phase(s) and orientations are stabilized and why; and how confinement strain and interface templates determine the orbital polarization and crystal-field splitting at the electronic level, connecting atomic displacements to orbital-level electronic structure. (3) Device physics — determine how these mechanisms set the performance limits of ~1 nm HZO ferroelectric tunnel junctions: tunneling electroresistance magnitude, on-current, endurance, retention, and variability as functions of composition, synthesis, and device scaling — all five figures of merit, addressed jointly across these three axes — beyond the proof-of-feasibility TER results of the source work. The three components are jointly linked: a consistent quantitative picture in which the measured polarization, the microscopic mechanism, and the tunnel-junction behavior are mutually consistent — the mechanism accounts for the measured polarization and for the tunnel-junction behavior — for films grown directly on silicon.

References

  1. Suraj S. Cheema, Daewoong Kwon, Nirmaan Shanker, Roberto dos Reis, Shang-Lin Hsu, Jun Xiao, Haigang Zhang, Ryan Wagner, Adhiraj Datar, Margaret R. McCarter, Claudy R. Serrao, Ajay K. Yadav, Golnaz Karbasian, Cheng-Hsiang Hsu, Ava J. Tan, Li-Chen Wang, Vishal Thakare, Xiang Zhang, Apurva Mehta, Evguenia Karapetrova, Rajesh V. Chopdekar, Padraic Shafer, Elke Arenholz, Chenming Hu, Roger Proksch, Ramamoorthy Ramesh, Jim Ciston, Sayeef Salahuddin, 'Enhanced ferroelectricity in ultrathin films grown directly on silicon', Nature 580, 478-482 (2020), DOI: 10.1038/s41586-020-2208-x, https://doi.org/10.1038/s41586-020-2208-x
  2. Suraj S. Cheema, Nirmaan Shanker, Shang-Lin Hsu, Yoonsoo Rho, Cheng-Hsiang Hsu, Vladimir A. Stoica, Zhan Zhang, John W. Freeland, Padraic Shafer, Costas P. Grigoropoulos, Jim Ciston, Sayeef Salahuddin, 'Emergent ferroelectricity in subnanometer binary oxide films on silicon', Science 376, 648-652 (2022), DOI: 10.1126/science.abm8642, https://doi.org/10.1126/science.abm8642
  3. Suraj S. Cheema, Nirmaan Shanker, Li-Chen Wang, Cheng-Hsiang Hsu, Shang-Lin Hsu, Yu-Hung Liao, Matthew San Jose, Jorge Gomez, Wriddhi Chakraborty, Wenshen Li, Jong-Ho Bae, Steve K. Volkman, Daewoong Kwon, Yoonsoo Rho, Gianni Pinelli, Ravi Rastogi, Dominick Pipitone, Corey Stull, Matthew Cook, Brian Tyrrell, Vladimir A. Stoica, Zhan Zhang, John W. Freeland, Christopher J. Tassone, Apurva Mehta, Ghazal Saheli, David Thompson, Dong Ik Suh, Won-Tae Koo, Kab-Jin Nam, Dong Jin Jung, Woo-Bin Song, Chung-Hsun Lin, Seunggeol Nam, Jinseong Heo, Narendra Parihar, Costas P. Grigoropoulos, Padraic Shafer, Patrick Fay, Ramamoorthy Ramesh, Souvik Mahapatra, Jim Ciston, Suman Datta, Mohamed Mohamed, Chenming Hu, Sayeef Salahuddin, 'Ultrathin ferroic HfO2-ZrO2 superlattice gate stack for advanced transistors', Nature 604, 65-71 (2022), DOI: 10.1038/s41586-022-04425-6, https://doi.org/10.1038/s41586-022-04425-6
  4. Suraj S. Cheema, Nirmaan Shanker, Cheng-Hsiang Hsu, Adhiraj Datar, Jongho Bae, Daewoong Kwon, Sayeef Salahuddin, 'One nanometer HfO2-based ferroelectric tunnel junctions on silicon', arXiv:2007.06182 (2020), https://arxiv.org/abs/2007.06182
  5. Sangjun Lee, Hyangsook Lee, Hyun Hwi Lee, Han-Koo Lee, Jung-Hwa Kim, Seontae Park, Hyun Jae Lee, Hagyoul Bae, Sanghyun Jo, Musarrat Hasan, Yongho Ha, Bong Jin Kuh, Jinseong Heo, Duk-Hyun Choe, Eunha Lee, 'Emergence of unconventional ferroelectric phase in ultrathin Hf0.5Zr0.5O2 films', Science Advances 12 (2026), DOI: 10.1126/sciadv.adz8245, https://doi.org/10.1126/sciadv.adz8245
  6. Yihao Shen, Xiaochun Ma, Yuxiao Liu, Hongzheng Wang, Shuxian Wang, Fei Liang, Pengfei Nan, Binghui Ge, Shujun Zhang, Haohai Yu, Huaijin Zhang, 'Atomic-Scale Strain-Confined Ferroelectricity in Fluorite Hafnium Dioxide', Advanced Functional Materials (2026), DOI: 10.1002/adfm.75899, https://doi.org/10.1002/adfm.75899
  7. Jingchao Xiao, Yating Cao, Wei Zhang, Yubao Li, 'Sub-nanometer ZrO2 capping Hf0.5Zr0.5O2 thin films for BEOL-compatible ferroelectric tunnel junction with giant electroresistance', Applied Physics Letters (2025), DOI: 10.1063/5.0284014, https://doi.org/10.1063/5.0284014
  8. Jingchao Xiao, Huiping Wang, Wei Liu, Wei Zhang, Yubao Li, 'Recoverable ultrathin Hf0.5Zr0.5O2-based ferroelectric tunnel junction', Applied Physics Letters (2026), DOI: 10.1063/5.0313762, https://doi.org/10.1063/5.0313762
  9. Laeyong Jung, Hojung Jang, Jongseon Seo, Hyunsang Hwang, 'Sub-Nanometer Ferroelectric Tunnel Junctions With Record-High on-Current Density Through Synergistic Microwave Annealing and High-Field Activation', Small (2026), DOI: 10.1002/smll.73730, https://doi.org/10.1002/smll.73730
  10. Chao Zhou, Sizhe Huang, Yangyang Si, Zhongqi Ren, Jianyuan Zhao, Hailin Wang, Jingxuan Li, Xianlong Cheng, Haoliang Huang, Shi Liu, Sujit Das, Shiqing Deng, Zuhuang Chen, 'Decoupling effects of the resistive-switching behavior on the polarization reversal in ultrathin ferroelectric Hf0.5Zr0.5O2 films', Matter (2026), DOI: 10.1016/j.matt.2026.102726, https://doi.org/10.1016/j.matt.2026.102726
  11. Chao Zhou, Yanpeng Feng, Liyang Ma, Haoliang Huang, Yangyang Si, Hailin Wang, Sizhe Huang, Jingxuan Li, Chang-Yang Kuo, Sujit Das, Yunlong Tang, Shi Liu, Zuhuang Chen, 'Fatigue-free ferroelectricity in Hf0.5Zr0.5O2 ultrathin films via interfacial design', Nature Communications (2025), DOI: 10.1038/s41467-025-63048-3, https://doi.org/10.1038/s41467-025-63048-3
  12. Jiangheng Yang, Jiajia Liao, Jin Huang, Ren-Ci Peng, Qiong Yang, Min Liao, Yichun Zhou, 'The origin of ferroelectricity in HfO2 from orbital hybridization and covalency', Applied Physics Letters (2024), DOI: 10.1063/5.0228932, https://doi.org/10.1063/5.0228932
  13. Yusuke Tamura, Kairi Masuda, Yu Kumagai, 'On-demand phase-field modeling: Three-dimensional Landau energy for HfO2 through machine learning', arXiv:2512.16207 (2025), https://arxiv.org/abs/2512.16207
  14. Junghyeon Hwang, Youngin Goh, Sanghun Jeon, 'Physics, Structures, and Applications of Fluorite-Structured Ferroelectric Tunnel Junctions', Small (2023), DOI: 10.1002/smll.202305271, https://doi.org/10.1002/smll.202305271
  15. Eunjin Lim, Dahye Kim, Jongmin Park, Minsuk Koo, Sungjun Kim, 'Recent advances in the mechanism, properties, and applications of hafnia ferroelectric tunnel junctions', Journal of Physics D: Applied Physics 57, 473001 (2024), DOI: 10.1088/1361-6463/ad7036, https://doi.org/10.1088/1361-6463/ad7036
  16. Haoyu Lu, Yu Li, Jiyuan Han, Geng Huangfu, Guan Feng, Shuaishuai Yin, Yingfen Wei, Hao Jiang, Changlin Zheng, Qi Liu, Ming Liu, 'Interface-Modulated Antiferroelectric-to-Ferroelectric-Like Transition in Ultrathin Hf0.5Zr0.5O2 Films', Advanced Functional Materials (2024), DOI: 10.1002/adfm.202414187, https://doi.org/10.1002/adfm.202414187
  17. So Yeon Shin, Yeon-Je Yu, Ae Rim Choi, Dohee Kim, Ja-Yong Kim, Seung Wook Ryu, Il-Kwon Oh, 'Rapid cooling process-driven enhancement of an orthorhombic phase in ferroelectric HfZrOx of sub-3 nm ultrathin films by atomic layer deposition', Applied Surface Science Advances (2025), DOI: 10.1016/j.apsadv.2025.100728, https://doi.org/10.1016/j.apsadv.2025.100728
  18. Yiqian Liu, Yuan-Hua Lin, 'Ferroelectricity survives in the sub-nanometer scale', Science China Materials 66, 2105-2106 (2023), DOI: 10.1007/s40843-023-2465-y, https://doi.org/10.1007/s40843-023-2465-y