25-01-2022 | | By Liam Critchey
Hafnia (HfO2) is a well-known and well-used material in the electronics and technology space and possess several properties that make it very versatile in many different situations. Hafnia was first used in the electronics space by Intel as a dielectric material for field-effect transistors (FETs).
Since its first inception as a dielectric material, it was found that hafnia also exhibits ferroelectricity. This opened the gates for hafnia being used as an easily polarisable material for memory-based applications within FETs and other similar devices. A lot of research has gone into the ferroelectric properties of hafnia, and in recent years, it has become one of the most promising ferroelectric materials in the electronic space.
Now, even though commercial applications are still being developed which utilise the ferroelectric properties of hafnia, researchers are starting to look at other potential properties of hafnia that can be harnessed for electronic devices. One of these is piezoelectric properties, and it has been discovered that hafnia could exhibit a piezoelectric effect. If the piezoelectric effect could be harnessed in hafnia, then there is the potential to further extend the use and application scope of hafnia materials in electronic devices.
The piezoelectric effect is the generation of an electrical charge under an applied stress/load—such as the squeezing/compression of a material. Piezoelectricity is found in a range of materials—especially solid-state materials—including in a number of insulating materials that have specific (noncentrosymmetric) symmetries within the unit cell of their solid-state lattice (the unit cell is the fundamental building block of any crystal lattice and represents how the different atoms sit within the material).
The piezoelectric effect is a reversible phenomenon, so when the applied load is removed from a material, the applied charge, and in turn the piezoelectricity, is removed from across the material. Piezoelectricity can also work the other way round, in that if an electrical charge is applied to a material that can exhibit piezoelectricity, the material will deform, and physical stress will be induced on the material.
A material generates piezoelectricity at the molecular level. The generation of an electrical current is achieved at the macro level because the regular repeating atom arrays within the material deform and rearrange themselves when a stress is applied across the material. When a stress is applied to the material, the oppositely charged ions within the material move from their usual orientation and into a position where they lie closer to each other. This atomic change also alters the charge balance within the material and causes an external electric field to be generated. The effects of the altered charge balance then spread throughout the material so that a net charge (either positive or negative) forms on the outer face of the material. This, in turn, generates a voltage across the oppositely charged material faces, and this voltage is piezoelectricity.
Compared to other inorganic ferroelectric materials that are similar from a material perspective, e.g. perovskite oxide materials, hafnia exhibits some peculiar features that scientists are only just starting to understand and harness. For example, current studies have indicated that the ferroelectricity in hafnia may not be proper, but it could be switchable. This is different to many other ferroelectric materials used in commercial applications today.
On another note, hafnia exhibits anti-polar instabilities, which results in very narrow electric domains and domain walls, so in essence, it makes hafnia a quasi 2D-ferroelectric material. However, these properties mean that hafnia also exhibits some other unusual properties, such as a better ferroelectric effect when the material is on a smaller scale (e.g., thin films or nanoscales), but bulk ferroelectricity has been realised using hafnia as well. The interesting and unusual ferroelectric properties are also thought to be a part of why hafnia could exhibit a piezoelectric effect as well.
It has been theorised through computational chemistry and now confirmed with experimental approaches that hafnia could exhibit the piezoelectric effects under the right conditions. At the fundamental level, first principle computational calculations have predicted that the unique ferroelectric phase of hafnia could also generate a negative longitudinal piezoelectric response—where compressing the material along the polarisation direction will increase the polar distortion across the material. Some thought that a positive piezoresponse could be possible (where it behaves more like a perovskite piezoresponse). Still, recent calculations have confirmed the presence of a negative longitudinal piezoelectric effect.
It was found that the chemical coordination of the active oxygen atoms within hafnia is responsible for the observed negative longitudinal piezoelectric effect. More specifically, it was found that when hafnia is strained along its polar axis, the material responds by shifting the lattice positions of the oxygen anions that are responsible for hafnia’s spontaneous polarisation. This is done to preserve the equilibrium distance for the Hf–O bonds in the lattice, but the atomic rearrangement also affects the polarisation in the material, and it does so in such a way the polarisation grows when the strain is compressive. This leads to the generation of a negative longitudinal piezoelectric effect.
It was also deduced that the ferroelectric phase of hafnia can also have a positive or a detrimental effect on the piezoresponse if you change the epitaxial strain across the material—which is typically done by changing the environment of the active oxygen anions. In some cases, if the change is enough, then the sign of the response can also be changed, and a positive piezoelectric response can be generated instead.
Given that other studies have observed a positive piezoelectric effect for hafnia and that the material can be tuned to change its piezoresponse from negative to positive without switching the polarisation, it’s thought that the way in which hafnia is made and how thick the formed material is, could have an impact on the type of piezoresponse exhibited. So, it could theoretically be possible to create hafnia materials with differing thicknesses that exhibit oppositely charged piezoresponses and tailor the material response to an application environment in this way.
The recent study has shown that the ferroelectric properties of hafnia help to induce a negative piezoelectric effect in hafnia (unlike other, chemically similar materials), but it could also be possible to generate a positive piezoresponse by controlling the fabrication conditions. The findings not only offer insights into hafnia itself, but they may also open new ways of looking at how to control piezoelectricity in different ferroelectric materials, and look at how this piezoelectricity could theoretically be harnessed for other applications.