New family of ferroelectric materials offers opportunities for improving information and energy storage

A new family of materials that could improve digital information storage and use less energy may be possible thanks to a team of researchers at Penn State who demonstrated ferroelectricity in zinc oxide substituted by magnesium.

Ferroelectric materials are spontaneously electrically polarized because the negative and positive charges in the material tend to opposite sides and with the application of an external electric field reorient themselves. They can be affected by physical force, which is why they are useful for push button igniters such as those found in gas grills. They can also be used for data storage and memory, as they stay in a polarized state without additional power, just like low power digital storage solutions.

“We have identified a new family of materials from which we can make tiny capacitors and we can set their polarization orientation so that their surface charge is more or less,” said Jon-Paul Maria, science professor and materials engineering at Penn State. , and co-author of the article published in the Journal of Applied Physics. “This setting is non-volatile, which means we can set the capacitor to more, and there is more, we can set it to less, there is less. And then we can go back and identify how we set this capacitor, say , an hour ago. ”

This capacity could enable a form of digital storage that does not use as much electricity as other forms.

“This type of storage does not require any additional energy,” said Maria. “And that’s important because many of the computer memories we use today require additional electricity to hold information, and we use a substantial amount of the US energy budget for information.”

The new materials are made with thin films of magnesium substituted zinc oxide. The film was developed by sputtering, a process in which argon ions are accelerated to target materials, impacting it with high enough energy to release the target’s atoms containing magnesium and zinc. The released magnesium and zinc atoms move in the vapor phase until they react with oxygen and come together on a platinum coated aluminum oxide substrate and form the thin films.

Researchers studied magnesium substituted zinc oxide as a method of increasing the bandgap of zinc oxide, a key feature of the material that is important for the creation of semiconductors. However, the material has never been explored for ferroelectricity. Nevertheless, the researchers believed that the material could be made ferroelectric, based on an idea of ​​”ferroelectric everywhere” put forward by Maria and Susan Trolier-McKinstry, professor at Evan Pugh University, Steward S. Flaschen professor of science and ceramics engineering, and co-author on paper.

“Generally speaking, ferroelectricity often occurs in structurally and chemically complex minerals,” said Maria. “And our team came up with the idea about two years ago, that there are other, simpler crystals in which this useful phenomenon could be identified, because some clues have made us come up with this possibility. Say ‘ferroelectric everywhere.’ is a bit of a play on words, but it captures the idea that there were materials around us that gave us clues, and we ignored those clues for a long time. ”

Trolier-McKinstry’s research career has focused on ferroelectrics, including the search for better ferroelectric materials with different properties. She noted that the University of Kiel in Germany found the very first of this surprising type of ferroelectric material in 2019 in nitrides, but that she and Maria demonstrated comparable behavior in an oxide.

Part of the process followed by Trolier-McKinstry and Maria’s group is to develop a figure of merit, an amount used in sciences such as analytical chemistry and materials research that characterizes the performance of a device, of a material or method versus alternatives.

“When we look at any material application, we often design a figure of merit that indicates what combination of material properties we would need for a given application in order to make it as efficient as possible,” said Trolier-McKinstry. “And this new family of ferroelectrics, it gives us whole new possibilities for those figures of merit. It’s very attractive for applications where we haven’t historically had large sets of materials, so this type of development of new materials tend to spark new applications. ”

An additional advantage of magnesium-substituted zinc oxide thin films is the way they can be deposited at much lower temperatures than other ferroelectric materials.

“The overwhelming majority of electronic materials are prepared using high temperatures, and high temperatures mean between 300 and 1,000 degrees Celsius (572 to 1,835 degrees Fahrenheit),” Maria said. “Whenever you make materials at high temperatures, it presents a lot of challenges. Usually these are engineering difficulties, but they make everything more difficult nonetheless. Consider that each capacitor needs two electrical contacts, if I prepare my ferroelectric layer for high temperatures. on at least one of these contacts, at some point, an unwanted chemical reaction will occur. So when you can craft items at low temperature, you can integrate them much more easily. ”

The next step for the new materials is to transform them into capacitors about 10 nanometers in thickness and 20 to 30 nanometers in lateral dimensions, which presents a difficult technical challenge. Researchers must create a way to control the growth of materials so that there are no problems such as imperfections in the materials. Trolier-McKinstry said resolving these issues will be key to whether these materials are usable in new technology – cell phones with chips that consume much less power, allowing sustained operation for a week or more.

“When developing new materials, you have to find out how they fail and then figure out how to mitigate those failure mechanisms,” said Trolier-McKinstry. “And for each application, you have to decide what the essential properties are and how they will change over time. And until you take some measurements on it, you don’t know what the big challenges will be, and the reliability and the ease of manufacture are enormous to know if this material is found in your cell phone in five years. ”