Ferroelectric everywhere? | Pennsylvania State University
UNIVERSITY PARK, Pa .– 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 have demonstrated ferroelectricity in zinc oxide substituted with magnesium.
Ferroelectric materials possess a spontaneous electrical polarization resulting from displacements of negative and positive charges within the material which can be reoriented through the application of an external electric field. 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 either more or less,” said Jon-Paul Maria, professor. of Materials Science and Engineering, 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 this is important because most of the computer memory we use today requires 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” posed 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 there were clues that made us come up with this possibility. Saying “ferroelectric everywhere” is a bit of a pun, 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 imagine 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 these figures of merit. It’s very attractive for applications where we haven’t always had large sets of materials, so this type of new material development tends to trigger 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, there are 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 high temperature ferroelectric layer. on at least one of these contacts, at some point, an unwanted chemical reaction will occur. So when you can do things at a 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 thick 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 a few measurements on it, you don’t know what the big challenges will be, and the reliability and manufacturing capacity is huge to see if this material ends up in your cellphone in five years.
Other Penn State researchers on the project include Kevin Ferri, recent PhD in materials science and engineering and first author of the paper; Saiphaneendra Bachu, graduate student; Wanlin Zhu, assistant research professor; and John Hayden, graduate student, Department of Materials Science and Engineering and Materials Research Institute; and also Mario Imperatore, graduate student, and Chris Giebink, associate professor, from the School of Electrical and Computer Engineering.
The US Department of Energy and the National Science Foundation supported this research.