Compositionally graded ferroelectric films show new properties

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Ferroelectric materials are used in a number of advanced electronics applications. However, their properties can be boosted by applying strain and now a research team at the University of Illinois at Urbana-Champaign has come up with a way to make ferroelectric thin films with twice the strain.

"It turns out that if you put pressure on certain types of materials, the properties change completely," said Professor Lane Martin. "In our case, we administer pressure by straining or stretching thin versions of these materials like one would stretch plastic wrap to fit on a bowl. You can induce things that don't exist at ambient conditions; you can make phases and properties that don't exist." The researchers grew thin films of lead zirconate titanate layered upon each other. However, the concentration of zirconium and titanium were changed slightly from layer to layer so the bottom film featured 80% zirconium, while the top layer featured 80% titanium. This gradual change is said to create small localised strain, but large overall strain. These so called compositionally graded films not only have improved properties, but also new properties, said Prof Martin. Most notably, the films have a built in electric field, which allows some functions to be performed without the need for an external current or field. A potential application is in memories. Currently, each time a bit is read, it has to be rewritten and compared to a reference bit. But with a built in electric potential, the bits would not need their polarity switched to be read, so components could be smaller, faster and last longer.