Part of the process of creating ferroelectric magnesium-substituted zinc oxide thin films includes: (left) Image showing thin film being sputter-deposited from metal sources; (center) ferroelectric hysteresis loops of thin-film capacitors showing two remanent polarization states at zero field; (right) atomic force microscope image showing a smooth surface at the nanometer scale and a very fine-grained and fiber-textured microstructure. Credit: Materials Research Institute, Penn State
A new family of materials that could result in improved digital information storage and uses less energy may be possible thanks to a team of Penn State researchers who demonstrated ferroelectricity in magnesium-substituted zinc oxide.
Ferroelectric materials can be spontaneously electric polarized because negative and positive charge in the material tend to opposite sides. However, external electric fields can reorient the material. They can be affected physically, which makes them useful for push-button ignitions such as those used in gas grills. They are also useful for data storage and memory because they can be kept in one polarized condition without any additional power. Low-energy digital storage solutions are also possible.
Jon-Paul Maria, Penn State professor of materials sciences and engineering and coauthor of the paper in the Journal of Applied Physics, said that "we've identified a family of materials from which to make tiny capacitors" and that they can adjust their polarization orientation so their surface charges are either plus or minus. This setting is nonvolatile. It allows us to set the capacitor to plus and it stays plus. We can also set it to minus and it stays minus. We can then go back to the setting that we made an hour ago and find out how it was set.
This could allow digital storage to be created that uses less electricity than other forms.
Maria stated that this type of storage does not require additional energy. Maria said that this is important because many computer memories we use today need additional electricity to maintain the information. We also use a significant amount of the American energy budget for information.
The new materials are made with magnesium-substituted zinc oxide thin films. Sputter deposition was used to grow the film. This is a process in which argonions are directed towards target materials and impact them with enough energy to cause atoms to be released from target material that contains zinc or magnesium. The magnesium and zinc atoms that have been liberated travel in a phase called vapor until they react with oxygen. They then collect on a platinum-coated aluminumoxid substrate, forming thin films.
Researchers have studied magnesium-substituted zinc oxide as a method of increasing zinc oxide's band gap, a key material characteristic that is important for creating semiconductors. The material was not explored for ferroelectricity. The researchers thought that the material could be made to ferroelectricity, based upon the idea of "ferroelectrics all around" as posited by Maria and Susan Trolier McKinstry, Evan Pugh University Professors, Steward S. Bottlen Professors of Ceramic Science and Engineering and co-authors of the paper.
Maria stated that ferroelectricity is often found in minerals that are complex from both a structure and chemical point of views. Maria said that the idea of ferroelectricity was first proposed by Maria's team two years ago. There are simpler crystals where this phenomenon might be found, and that we had some clues to support this possibility. Although the term "ferroelectrics everywhere" is a bit of a joke, it conveys the idea that there are materials all around us that could be giving us clues, but we ignored them for a long while.
Trolier-McKinstry has been a researcher in ferroelectrics. This includes the search for ferroelectric materials that have different properties. Maria and she noted that while the University of Kiel, Germany, had discovered the first of these surprising ferroelectric materials in 2019, it was in nitrides.
Trolier-McKinstry's and Maria's groups followed a process of developing a figure for merit. This is a measure used in sciences like analytical chemistry or materials research to characterize the performance of a device/material or method relative with alternatives.
Trolier-McKinstry said, "As we examine any application for material we often devise figures of merit that tells us what combination of properties we would need to make it as efficient as possible." This new family of ferroelectrics opens up new opportunities for these figures of merit. This is a great choice for applications where we don't have the best materials sets. Therefore, this type of new material development can spark new ideas.
An added benefit of the magnesium-substituted zinc oxide thin films is how they can be deposited at much lower temperatures than other ferroelectric materials.
Maria stated that "the overwhelming majority of electronic materials were prepared using high temperatures. This means anything from 300 to 1000 degrees Celsius (or 572 to 1835 Fahrenheit)." Material made at high temperatures can present many difficulties. These are engineering problems, but they can make things more difficult. You should consider that each capacitor requires two electrical contacts. If I heat my ferroelectric layer to high temperatures on at most one of these contacts then an undesirable chemical reaction will occur. You can easily integrate things if you can make them at low temperatures.
Next, the researchers will make the new materials into capacitators. These capacitators are about 10 nanometers thick with 20 to 30 nanometers lateral dimension. This is a challenging engineering challenge. Researchers must find a way to control growth so that there are no imperfections. Trolier-McKinstry stated that these problems will determine whether these materials can be used in new technologies. This includes phones that have chips that consume less energy and that can sustain operation for up to a week.
Trolier-McKinstry stated that when developing new materials, it is important to understand what causes failures and how to avoid them. You must determine the essential properties of each application and how they will change over time. Until you have taken measurements, you won't know the biggest challenges and how reliable and durable this material will be in your phone five years from now.
Continue reading First flexible memory device with oxide ferroelectric material
More information: Kevin Ferri et. al. Ferroelectrics everywhere. Ferroelectricity is magnesium substituted zinc oxide thin film, Journal of Applied Physics (2021). Journal Information: Journal of Applied Physics Kevin Ferri et., Ferroelectrics everywhere, Ferroelectricity In Magnesium Substituted Zinc Ox Thin Films, (2021). DOI: 10.1063/5.0053755
