Episode 9: Stacking materials induces ferroelectricity into wurtzites

In this podcast episode, MRS Bulletin’s Sophia Chen interviews Beth Dickey from Carnegie Mellon University about her new approach to inducing ferroelectricity into a material. Dickey’s research group worked with a class of materials known as wurtzites. The researchers specifically studied aluminum nitride and zinc oxide, which are not ferroelectric in their pristine form at room temperature. However, alloys of these materials are ferroelectric. When the researchers stacked the ferroelectric alloy with a non-ferroelectric wurtzite and applied electric fields to the material, they found that the crystal lattice of the ferroelectric layer began to invert, then switching propagated into the pristine wurtzite, confirming that the entire material was ferroelectric. The results of this study could lead to development of ferroelectric materials for computers where memory and computation can be brought together into a single device, saving energy. This work was published in a recent issue of Nature. 

Transcript

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Sophia Chen. Your laptop and most computers today use what’s known as a von Neumann architecture. They’ve got memory, where you store your data, and your CPU, which fetches the data from memory to then perform computations. Basically, memory and computing are separate in a von Neumann machine. But this type of computer architecture has its drawbacks. Shuttling data back and forth between CPU and memory takes a lot of energy. Beth Dickey, a materials researcher at Carnegie Mellon University, is researching a class of materials known as ferroelectrics. Ferroelectric materials could enable a different type of computer architecture that’s more energy efficient.

BETH DICKEY: Although there are many devices that could be enabled through this type of material, one of the key ones is having an in-compute memory, where you bring memory and computation together into a single device, and so there are energy benefits associated with that.

SOPHIA CHEN: Ferroelectric materials have a distinctive property. They are electrically polarized in a particular direction, which mean that the electric charges are not symmetrically distributed throughout the material, which leads to charge accumulation at the surfaces. But notably, when you apply an electric field to the material, you can reverse the direction of the asymmetry, or polarization. This occurs because the electric field inverts the crystal lattice structure. This causes the electric charge distribution to become inverted in the opposite direction, thereby switching the polarization of the material. So ferroelectric materials can act as a switch. 

BETH DICKEY: One of the applications that we're looking at is using it as a memory device. So you can imagine, in its simplest form, having an up state and a down state and being able to access that for a memory application.

SOPHIA CHEN: Typically, researchers have created and developed ferroelectric materials by manipulating their chemical composition through alloying. In new work, Dickey and her colleagues have discovered a new way to design ferroelectric materials. Instead of working with chemical composition, they found that they can make a material that by itself is not ferroelectric become ferroelectric. They do this by stacking the material that is not ferroelectric with a ferroelectric material. They call it “proximity ferroelectricity.”

BETH DICKEY: Now what we can think about is, well, we don't have to change the underlying material compositionally. We can rely on an adjacent material to assist it and therefore we can have a broader, I think, design space for devices.

SOPHIA CHEN: They worked with a class of materials known as wurtzites, which are defined by their crystal structure. They studied aluminum nitride and zinc oxide, which are both wurtzites but are not ferroelectric in their pristine form at room temperature. But alloys of these materials are ferroelectric. Specifically, they used the ferroelectric alloys aluminum boron nitride, aluminum scandium nitride, and zinc magnesium oxide. They layered a ferroelectric alloy with a non-ferroelectric wurtzite via a vapor deposition process involving sputtering to create thin films of around 100 nanometers thick. Then they used a scanning transmission electron microscope to apply electric fields to the material to study whether the material's polarization switched direction.

BETH DICKEY: We did it by producing inhomogeneous fields with the electron microscope.

SOPHIA CHEN: Under the applied electric field, they saw that the crystal lattice of the ferroelectric layer began to invert first. That switching then propagated into the pristine wurtzite, which is normally not ferroelectric. They confirmed the entire alloy switched polarization using the microscope. This switching confirmed that the entire material was ferroelectric.

BETH DICKEY: This was key in the paper, to really confirm that the entire material is switching all the way through, from the known ferroelectric layers through the nonferroelectric or polar layers.

SOPHIA CHEN: The significance of their work is that it presents a new approach to inducing ferroelectricity into a material. It means that to design a ferroelectric material with the desired property, they could find a polar but non-ferroelectric material that already exists and layer it with a ferroelectric. Dickey’s team has more work ahead of them to continue developing these materials for devices. One issue is that some of the energy applied to the material produces undesired electric current in the material, known as leakage current. Ideally, you’d want all the energy to go to switching the device’s polarization. To understand and ultimately reduce this leakage current, Dickey says they need to study the defects in the material.

BETH DICKEY: There has to be much more greater attention to controlling other types of defects in the material, controlling the interfaces and really controlling leakage current.

SOPHIA CHEN: This work was published in a recent issue of Nature. My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.