Inside view of the vacuum chamber where 'pulsed laser deposition' is used to create hafnium oxide crystals. On the left, the glowing substrate on which the film is growing with atomic precision; in the center, the blue ionic plasma is created by shooting a laser at a target with a specific chemical composition (target visible on the right side of the photo). Photo: Henk Bonder, University of Groningen.Using ferroelectricity instead of magnetism in computer memory saves energy. If ferroelectric bits were nanosized, this would also save space. But conventional wisdom dictates that ferroelectric properties disappear at nanoscales.
Reports that hafnium oxide can be used to make a nanoscale ferroelectric have not yet convinced the field. But physicists at the University of Groningen in the Netherlands have now gathered evidence that could persuade the sceptics, which they report in a paper in Nature Materials.
Ferroelectric materials have a spontaneous dipole moment that can point up or down, which can be used to store information, just like the magnetic bits on a hard disk. The advantage of ferroelectric bits is that they can be written at a low voltage and power. Magnetic bits require large currents to create a magnetic field for switching, and thus use more power. The disadvantage of ferroelectrics is that the aligned dipoles are only stable in fairly large groups, which means that as the crystals of a ferroelectric material get smaller, the dipole moment eventually disappears.
“Reducing the size of ferroelectric materials has been a research topic for more than 20 years,” says Beatriz Noheda, professor of functional nanomaterials at the University of Groningen. Some eight years ago, a breakthrough was announced by researchers at the Nanoelectronic Materials Laboratory in Dresden, Germany. They claimed that hafnium oxide thin films were ferroelectric when thinner than 10nm and that thicker films actually lost their ferroelectric properties.
“This went against everything we knew, so most scientists were sceptical, including me,” admits Noheda. Some of this scepticism was due to the fact that the ferroelectric hafnium samples used in these studies were polycrystalline and showed multiple phases, obscuring any clear fundamental understanding of such an unconventional phenomenon.
Noheda and her group decided to investigate. They wanted to study these crystals by growing clean (single-phase) films on a substrate. Using X-ray scattering and high-resolution electron microscopy techniques, they observed that very thin films (under 10nm) grow with an entirely unexpected and previously unknown polar structure, which is necessary for ferroelectricity. Combining these observations with meticulous transport measurements, Noheda and her group confirmed that the material was indeed ferroelectric. “In the substrate that we used, the atoms were a little bit closer than those in hafnium oxide, so the hafnium crystals would be a little strained,” Noheda explains.
To their surprise, they noticed that the crystal structure changed when the layers exceeded 10nm, thus reproducing the results of the Dresden lab. “We used a totally different method, but we reached similar conclusions,” says Noheda. “This confirmed that ferroelectricity in nanosized hafnium oxide crystals is indeed real and unconventional. And that begged the question: why does this happen?”
The common denominator in both studies was size. Small crystals became ferroelectric, whereas larger crystals lost this property. This led the scientists to study the phase diagrams of hafnium oxide. At very small scales, particles of hafnium oxide have a very large surface energy, which creates pressures of up to 5 gigapascals in the crystal. The phase diagrams show a different crystal arrangement at such a pressure. “This pressure, along with the substrate-imposed strain, induces a polar phase, which is in line with the observation that these crystals are ferroelectric,” concludes Noheda.
Another important finding was that, in contrast to the thin films from Dresden, the new crystals do not need a 'wake-up' cycle to become ferroelectric. “The previously studied thin films only became ferroelectric after going through a number of switching cycles,” Noheda explains. “This increased the suspicion that ferroelectricity was some sort of artefact. We now believe that the wake-up cycles were necessary to align the dipoles in ‘unclean’ samples grown via other techniques. In our material, the alignment is already present in the crystals.”
In Noheda's opinion, the results are conclusive: hafnium oxide is ferroelectric at the nanoscale. This means that very small bits can be constructed from this material, with the added advantage that they can switch at a low voltage. Furthermore, the particular substrate used in this study is magnetic, and this combination of magnetic and ferroelectric bits brings an extra degree of freedom, allowing each bit to store double the information.
Now that the mechanism of nanosized ferroelectricity is clear, it seems likely that other simple oxides should have similar properties. Noheda expects that this will spark a lot of new research.
This story is adapted from material from the University of Groningen, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.