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HAFNIA FERROELECTRICS

Ferroelectricity in bulk hafnia

Ferroelectricity in bulk crystals of hafnium oxide demonstrates that these properties are not limited to films prepared by thin-film deposition techniques.

Ferroelectric materials, which exhibit a permanent polarization that can be electrically switched, have long been considered of interest for electronic devices. But the standard ferroelectric materials — perovskites such as barium titanate (BaTiO3 or BTO) or lead zirconium titanate (Pb[ZrxTi1−x]O3 or PZT) — have compatibility issues with silicon technology, the bedrock of electronics. The recent discovery1 of ferroelectricity in doped thin films of silicon-compatible hafnium oxide (HfO2, or hafnia) has led to keen attention from both basic and applied researchers on this system. However, the origin of the ferroelectric properties at the time of discovery in 2011 was a mystery, especially as the ferroelectricity only existed in ultrathin films of several nanometres in thickness. This was contrary to the conventional theories of ferroelectricity that indicated that at such a short length regime, the depolarizing fields caused by the interface should remove the ferroelectric polarization. Although there was speculation from early on that a possible origin of ferroelectricity was the orthorhombic Pbc21 phase, it took several years for this to be experimentally verified. There are many possible factors that can stabilize the ferroelectric phase in hafnium oxide (see Fig. 1, left). Surface energy was identified as an important factor2, explaining why the effect was most pronounced in thin films in the 10-nm range. To match the expectations of this surface energy model, kinetic aspects needed to be considered as well3. Moreover, additional factors such as dopants and oxygen vacancies have been observed to stabilize the orthorhombic phase4. However, all of these factors have only explained ferroelectricity in ultrathin films.

Fig. 1: Factors involved in stabilizing the ferroelectric orthorhombic phase in hafnium oxide.
figure 1

The factors that have been generally used for thin films in the literature are shown on the left, and the ones explicitly used to obtain the bulk crystals in ref. 5 are shown on the right (dark-shaded). The lighter-shaded factors are expected to be influenced by doping, or fast quenching, or both. For example, trivalent doping with yttrium (Y) will generate oxygen vacancies. At the top, the three stable polymorphs of hafnium oxide are illustrated, while the bottom picture shows the desired ferroelectric orthorhombic crystal. Red and blue spheres both denote oxygen atoms, olive spheres hafnium atoms. Crystal structures reproduced with permission from ref. 5, Springer Nature Ltd.

Now, writing in Nature Materials, a research team led by Sang-Wook Cheong has reported ferroelectricity in bulk crystals of Y-doped hafnium oxide5. The team achieved this by using a special variant of the float-zone crystallization technique6, in which a liquid zone is moved through the material to produce a single crystal. In the version used here (the laser-diode-heated float-zone method, LDFZ), the laser diodes are used to heat the samples up to a high temperature of about 3,000 °C. This allows a steep temperature gradient during cool down after the crystallization. In this way, the researchers prepared transparent crystals of up to 50 mm in length and up to 2 g in mass. Previous work demonstrated ferroelectricity in hafnium oxide in polycrystalline films up to a thickness of 1 μm with high remanent polarization values7. But the key difference here is that with the float-zone technique, sizable bulk crystals rather than deposited films are produced, further extending the upper size limit. The possibility of achieving ferroelectricity in bulk HfO2 crystals widens the application fields of ferroelectric hafnium oxide into regions where typically perovskite materials like PZT or BTO are used8.

In their work, Cheong and colleagues have used a combination of Y doping and very fast quenching, applying the LDFZ technique to achieve a material with a large proportion of orthorhombic phase, having domain sizes of the order of 100 nm. The remanent polarization values of about 6 μC cm−2 are much lower than what is usually observed in nanoscale thin films, where values of about 20 μC cm−2 are achieved for Y-doped hafnium oxide films. Although the authors show that they can model such low values in their density functional theory calculations based on the expected crystal structure, this may still indicate a substantial non-polar or pinned phase portion in the bulk material.

While doping is well known as the most effective way of stabilizing the metastable orthorhombic phase, fast quenching after the last high-temperature crystallization step has been identified as an important ingredient to avoid the stabilization of the paraelectric monoclinic phase after the crystallization3,9. Both methods were combined by Cheong and colleagues to achieve ferroelectricity in their bulk crystals. We itemize in Fig. 1 (left side) the factors that are currently discussed in the literature for the stabilization of the ferroelectric orthorhombic phase and compare these to the direct measures taken by Cheong and colleagues (right side). Note that for the thin-film case, a selection of these factors and not a combination of all of them has been used in different studies. There is also an interdependence of the factors applied to stabilize ferroelectric hafnium oxide (for example, Y-doping is expected to produce oxygen vacancies). In other words, doping is expected to generate oxygen vacancies and strain, while rapid quenching can generate strain and modulate surface energy. Therefore, this observation of the ferroelectric phase in the bulk gives an additional data point to explore the currently discussed physical origins of ferroelectric phase stabilization in hafnium oxide.

The results reported here extend the observation of ferroelectricity in hafnium oxide to bulk crystals, which will enable additional applications and fundamental studies of this class of materials. At the same time, this work underlines that there is not just one decisive variable but a whole toolbox of parameters to stabilize the ferroelectric phase that could be adjusted to the boundary conditions of the desired material, the fabrication technique and its application.

More work, however, will be required to confirm whether the rather low polarization values are indeed connected to the achieved structure or whether non-polar phase fractions and pinned domains are also relevant. The broad switching peaks in the hysteresis measurements point towards such effects. Measuring the polarization stability as a function of temperature and time will shed more light on this topic and is a logical next step towards understanding and using these bulk ferroelectric crystals. Moreover, the key parameters for piezoelectric and pyroelectric properties, which are connected to ferroelectricity, need to be determined to understand the possible benefits of this bulk system. The properties of the observed antipolar phase also need to be explored as the basis for possible applications. Finally, devices based on the piezo-, pyro- and ferroelectricity need to be constructed and compared with their counterparts based on other ferroelectric bulk crystals, to substantiate the application-relevant importance of this bulk ferroelectric.

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Mikolajick, T., Schroeder, U. Ferroelectricity in bulk hafnia. Nat. Mater. 20, 718–719 (2021). https://doi.org/10.1038/s41563-020-00914-z

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