Understanding the Role of Strain, Stress, Defects and Dopants in the Ferroelectric, Switching and Phase Stability Properties of Epitaxial Hafnia and Zirconia Films

Hafnia (HfO₂) and Zirconia (ZrO₂) ferroelectric (FE) thin films are at the forefront in the quest for novel devices for the next computational paradigm. They find very promising applications in non-volatile memories, energy storage capacitors and neuromorphic devices, due to their capability to retain ferroelectricity in the very thin film limit [1] and low crystallization temperature, which makes them readily integrable in back end-of-line CMOS devices [2]. Hf/ZrO₂ have a rich phase diagram and ferroelectricity is currently explained by the presence of the metastable orthorhombic (o) or rhombohedral (r) phases that can be stabilized due to different effects, such as strain, stress, doping, etc. [2].
It is difficult to discriminate between these effects on polycrystalline films due to the coexistence of polar and non-polar polymorphs. Therefore, epitaxial Hf/ZrO₂ films are ideal systems to investigate these effects separately. To disentangle the role of each of these effects, density functional theory (DFT) calculations can play an important role as they allow to accurately simulate the materials at the atomistic level unraveling the connection between structure, composition and functionality. Herein, we will employ DFT calculations to understand how these factors influence key aspects of FE thin films such as relative phase stability, FE polarization and switching barriers in La doped HfO₂ films [3]. In addition, the role of strain and stress effects will be discussed in terms of phase stability and ferroelectric polarization magnitude [4].
We will also demonstrate that the presence of oxygen vacancies can stabilize the ferroelectric phase while potentially providing an electrochemical source for ferroelectricity [5]. Finally, we will show how different dopants affect the phase stability, FE polarization magnitude in ZrO₂ films, and reveal how dopants shift T_C and how this could lead to large –up to sevenfold- improvement of the piezoelectric response compared to undoped systems [6].
Overall, these results deepened our understanding of Hf/ZrO₂ FE thin films, largely accelerating the development of devices based upon them.

[1] S.S. Cheema, D. Kwon, N. Shanker, R. dos Reis, S.-L. Hsu, J. Xiao, et al., Enhanced ferroelectricity in ultrathin films grown directly on silicon, Nature 580 (2020) 478–482. https://doi.org/10.1038/s41586-020-2208-x.
[2] J.P.B. Silva, R. Alcala, U.E. Avci, N. Barrett, L. Bégon-Lours, M. Borg, et al., Roadmap on ferroelectric hafnia- and zirconia-based materials and devices, APL Materials 11 (2023) 089201. https://doi.org/10.1063/5.0148068.
[3] A. Silva, I. Fina, F. Sánchez, J.P.B. Silva, L. Marques, V. Lenzi, Unraveling the ferroelectric switching mechanisms in ferroelectric pure and La doped HfO2 epitaxial thin films, Materials Today Physics 34 (2023) 101064. https://doi.org/10.1016/j.mtphys.2023.101064.
[4] T. Song, V. Lenzi, J.P.B. Silva, L. Marques, I. Fina, F. Sánchez, Disentangling stress and strain effects in ferroelectric HfO2, Applied Physics Reviews 10 (2023) 041415. https://doi.org/10.1063/5.0172259.
[5] V. Lenzi, J.P.B. Silva, B. Šmíd, V. Matolín, C.M. Istrate, C. Ghica, et al., Ferroelectricity induced by oxygen vacancies in rhombohedral ZrO2 thin films, ENERGY & ENVIRONMENTAL MATERIALS 7 (2024) e12500. https://doi.org/10.1002/eem2.12500.
[6] A. Silva, R. Ganser, J.P.B. Silva, A. Kersch, V. Lenzi, L. Marques, Tuning the ferro- and piezoelectric
properties of ZrO2 with doping, submitted.