For the microelectronics industry, ferroelectric materials are attractive for a variety of applications, including making computer memory.
Random access memory (RAM) made out of ferroelectric materials—materials that have a spontaneous electric polarization that can be switched by applying an electric field—is able to retain its information when the power is turned off, which allows for long-term data storage.
“With most of the computer RAM today, you have to constantly apply power to the device, and as soon as you remove the power, you lose the information,” says Jason Kawasaki, an assistant professor of materials science and engineering. “Ferroelectric materials provide a way to make a nonvolatile memory element, meaning you don’t have to apply power to retain a stored bit.”
The vast majority of materials that display ferroelectricity are oxide-based materials, and Kawasaki says researchers have narrowly focused on oxide materials in their search for ferroelectrics because these materials align with the conventional wisdom of what’s required for a material to be ferroelectric.
However, Kawasaki is challenging convention in his search for new ferroelectric materials. With funding from the Army Research Office Young Investigator Program, Kawasaki will look for ferroelectric properties in material classes beyond the oxides—specifically in a class of intermetallic compounds called hexagonal Heusler compounds.
“Our work will investigate new classes of potentially ferroelectric materials that we can use for nonvolatile, low-energy-consumption, fast-switching memory applications,” Kawasaki says. “The mechanisms for ferroelectricity are predicted to be quite different in Heuslers than in oxides. Demonstration of a ferroelectric Heusler would enhance our understanding of ferroelectricity in general, and enable smaller, faster, and more energy efficient devices than are possible with current materials.”
Graduate student Estiaque Shourov installs an effusion cell, which is essentially a furnace for evaporating (or sublimating) ultrahigh purity elements. Shourov and Patrick Strohbeen (right) are Jason Kawasaki’s materials science and engineering PhD students. Photo credit: Stephanie Precourt.
Researchers have conducted calculations that suggest Heusler compounds could have ferroelectric properties. But, so far, scientists have failed to experimentally demonstrate ferroelectricity in these compounds due to the challenges involved in synthesizing and stabilizing them.
“In some cases the materials we want simply can’t be found in nature. They aren’t stable. For the ones that are stable, controlling the composition and crystalline defects are big challenges,” Kawasaki says.
Kawasaki will attempt to overcome these challenges by using a technique called molecular beam epitaxy, which he likens to spray-painting with atoms, to grow Heusler compounds.
Molecular beam epitaxy a method for growing single crystalline thin films by evaporating high-purity elements in an ultrahigh vacuum chamber. Researchers can observe this process in real time, using tools such as electron diffraction to monitor the growth dynamics at each atomic layer of the crystal. Kawasaki can then refine the process by stabilizing certain phases of growth, therefore ultimately producing high-quality crystalline films with fine-tuned properties.
Kawasaki notes that Heusler compounds have some key advantages over oxide materials for technological applications. Oxide materials can be more difficult to integrate in a conventional semiconductor processing line, because the lattice parameters of many oxide crystals are quite different than traditional semiconductors like silicon or gallium arsenide. As a result, when researchers try to grow one on top of the other, many crystalline defects emerge because the crystal structures don’t match. Additionally, the oxygen in oxide materials will diffuse into a lot of semiconductors, creating electrically dead layers that are undesirable.
In contrast, hexagonal Heusler compounds have symmetry and crystal structures that are very well matched to conventional semiconductors. “So we can grow them nicely, one on top of another, and the Heusler compounds also don’t contain oxygen,” Kawasaki says.
Kawasaki is particularly interested in new phenomena that crop up in artificially layered heterostructures in which layers of different functionality are combined, and new properties emerge at their interfaces.
“Once you grow a crystal with a very desirable property, the really interesting properties emerge when you start mixing and matching layers with different functionality,” he says.
Kawasaki says a prime example is the case of the oxide materials strontium titanate and lanthanum aluminate. Separately these two materials are insulators, but when sandwiched together they form a two-dimensional metal at their interface that even becomes superconducting at low temperature. Kawasaki is searching for similar phenomena in Heusler compounds.
“One of the holy grails in the oxide community has been finding multiferroic materials, materials that are simultaneously magnetic and ferroelectric, and working with Heusler compounds might be another system where we could potentially find these novel counterposed properties,” he says.