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| DOI | 10.1126/science.abd1212 |
| A key piece of the ferroelectric hafnia puzzle | |
| Beatriz Noheda; Jorge Íñiguez | |
| 2020-09-11 | |
| 发表期刊 | Science
 |
| 出版年 | 2020 |
| 英文摘要 | The ferroelectrics community is witnessing one of those moments in which serendipity changes the course of science. The story of ferroelectric hafnia (HfO2) resembles that of Cinderella: Not invited to the polar dielectrics ball, nanoscale HfO2 was dismissed as not being a real ferrolectric, a material that has a switchable spontaneous polarization, despite the experimental evidence for this response. On page 1343 of this issue, Lee et al. ([ 1 ][1]) bring us closer to a real-life fairy tale ending with their theoretical calculations, which show that nanoscale HfO2 becomes a ferroelectric through a different mechanism. Polarization manifests in the form of two-dimensional (2D) slices separated by nonpolar spacers, associated with flat polar phonon bands that allow for homogeneous switching of electric dipoles.
The story starts with research that began in 2006 but was not published until 2011 ([ 2 ][2]). Scientists fabricating silicon transistors with HfO2-based insulating layers spent several years trying to explain the origin of a strange peak observed in the capacitance-voltage characteristics. The peak looked very much like the ones observed in ferroelectrics when an applied electric field switches the direction of the spontaneous polarization. This feature has made ferroelectrics one of the oldest nonvolatile semiconductor memory types ([ 3 ][3]).
However, ferroelectricity was unlikely for two reasons: No polar phases had ever been reported in HfO2, a refractory material with a long history of research ([ 4 ][4]), and these HfO2 layers were only a few nanometers thick. Ferroelectricity is not expected at the nanoscale because it is a cooperative phenomenon. The local dipoles in ferroelectric materials, which result from the relative displacement of positive and negative ions, interact electrically with the dipoles of the neighboring cells and have a tendency to align collectively in the same direction, akin to what happens in a ferromagnet with the electron spins. The collective ordering leads to a spontaneous polarization. However, when the dimensions of the ferroelectric sample are small, as needed in microelectronics, a substantial number of dipoles lie on its surface. The stabilization of the ferroelectric phase is hampered by the energy cost of the depolarizing electric field that such dipoles create inside and outside the ferroelectric material, as dictated by Maxwell's equations.
In nature, this electrostatic penalty is reduced by domain formation, in which regions with alternating polarization (up and down) form in the sample. In theory, compensation of the dipolar surface charges can also be achieved by sandwiching the ferroelectric in between two metallic electrodes. The free carriers of the metal should screen the polarization charges and eliminate the depolarizing field, avoiding the need to form domains. In practice, this approach does not work perfectly with real metals, and screening is not complete ([ 5 ][5]). How to work around this issue has been one of the main research focuses of the ferroelectrics community for more than 30 years, driven by the vision of a ferroelectric nonvolatile memories that would be faster, denser, and less power-consuming than their magnetic counterparts ([ 6 ][6]).
Thus, even when the paper reporting on ferroelectric HfO2 was published ([ 2 ][2]), the ferroelectrics community largely dismissed this result as an artifact, assuming that a material that is not polar in bulk would not become polar at the nanoscale. Moreover, at the nanoscale, it is hard to distinguish ferroelectric switching peaks from the voltammetry characteristics that could arise from electrochemical reactions at interfaces ([ 7 ][7]). However, after many subsequent studies from several groups ([ 8 ][8]), the evidence for robust switching became difficult to ignore. The current consensus is that ferroelectric-like switching in HfO2-based ferroelectrics does exist, but its origin is still highly debated. Only one or two reports have shown a ferroelectric phase transition in this material ([ 9 ][9], [ 10 ][10]). In addition, switching requires large applied fields and does not seem to proceed as in other ferroelectrics through movement of domain walls ([ 11 ][11]).
![Figure][12]
Figuring out a thin ferroelectric
The theoretical study by Lee et al. explains how an ultrathin material, hafnia, can be a ferroelectric by comparing it with a classical three-dimensional (3D) ferroelectric. In both cases, alternating domains of polarization (arrows representing ferroelectric dipoles along the z crystal direction) eliminate net surface charges and prevent depolarization.
GRAPHIC: JOSHUA BIRD/ SCIENCE
How HfO2 becomes ferroelectric at the nanoscale and how it screens polarization charges at surfaces are the main questions to resolve. The former has been explained by a combination of effects (surface energy, ordered dopants, and oxygen vacancies) that favor the occurrence of the polar phase ([ 3 ][3], [ 7 ][7]). The latter could be explained by the much lower dielectric permittivity of HfO2 compared with other ferroelectrics, but why is it so low?
Theoretical calculations by Lee et al. now show that ferroelectricity in HfO2 is of a different type (see the figure). The polar features of HfO2 are associated with a nearly flat phonon band (similar frequency of the different modulations along the energy band). Thus, a homogeneous polar order, in which all electric dipoles align parallel as in a regular 3D ferroelectric phase, is as likely as any transversally modulated inhomogeneous order in which an arbitrary sequence of ferroelectric domains are separated by 180° domain walls. Put differently, the domain walls in HfO2 have essentially zero energy cost and a negligible width.
This situation, which is reminiscent of the effect called pressure-induced amorphization ([ 12 ][13]), has two important consequences: HfO2 has essentially 2D polar instabilities, meaning that a polar 2D plane (polarization within the plane) can in principle appear by itself, even if the rest of the material remains nonpolar. The polarization of such 2D slices has a very small electrostatic penalty (depolarizing field) associated with it, much smaller than that for 3D polar order, which helps explain why ferroelectricity can occur in HfO2 at the nanoscale.
Also, the 2D polar slices are all but decoupled from each other, so in HfO2, the switching of one domain has no effect on its surrounding domains. Lee et al. argue that this process must have dramatic effects in how ferroelectric switching proceeds in this material because nucleation of reversed domains is not followed by growth, which should yield very large coercive fields, as is indeed observed. The occurrence of individual switching of 2D polar planes offers the possibility of multilevel polarization switching with ideally as many intermediate states as the number of unit cells. This capability is of much interest for adaptable electronics and brain-inspired computing applications.
Lee et al. have found that a flat phonon band gives rise to dipolar localization, a phenomenon reminiscent of localization effects for electrons, photons, and other particles but whose implications in the case of ferroelectrics have not been fully explored. In this way, dipolar order can occur without the need for cooperative 3D behavior, allowing miniaturization and multivalued nonvolatile storage. The next step will be to use this knowledge to engineer lower switching voltages for memory applications in this material that is already compatible with silicon electronics.
1. [↵][14]1. H.-J. Lee
, Science 369, 1343 (2020).
[OpenUrl][15][Abstract/FREE Full Text][16]
2. [↵][17]1. T. S. Böscke,
2. J. Müller,
3. D. Bräuhaus,
4. U. Schröder,
5. U. Böttger
, Appl. Phys. Lett. 99, 102903 (2011).
[OpenUrl][18][CrossRef][19]
3. [↵][20]1. J. Handy
, “FRAM turns 68,” The Memory Guy 10 July 2020; |
| 领域 | 气候变化 ; 资源环境 |
| URL | 查看原文 |
| 引用统计 | |
| 文献类型 | 期刊论文 |
| 条目标识符 | http://119.78.100.173/C666/handle/2XK7JSWQ/294087 |
| 专题 | 气候变化 资源环境科学 |
| 推荐引用方式 GB/T 7714 | Beatriz Noheda,Jorge Íñiguez. A key piece of the ferroelectric hafnia puzzle[J]. Science,2020. |
| APA | Beatriz Noheda,&Jorge Íñiguez.(2020).A key piece of the ferroelectric hafnia puzzle.Science. |
| MLA | Beatriz Noheda,et al."A key piece of the ferroelectric hafnia puzzle".Science (2020). |
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