Process Adds New Properties to Ferroelectric Materials

L-R: Vengadesh Mangalam, Karthik Jambunathan, and Lane Martin (Brian Stauffer, University of Illinois)

Materials scientists at University of Illinois in Urbana developed a new type of thin metal oxide film with a built-in electric field, useful  for semiconductor devices such as computer memory. The team led by Illinois professor Lane Martin published their findings online in a recent issue of the journal Advanced Materials (paid subscription required).

Lane, with postdoctoral researcher Vengadesh Mangalam and graduate student Karthik Jambunathan (pictured left), engineered new properties of thin films made from lead zirconate titanate, a material known by its chemical initials PZT, with piezoelectric properties that can generate electricity from pressure or stress. PZT is a ferroelectric material, which means it has a spontaneous or built-in polarity that can be changed through the application of an electric current. Ferroelectric materials are found in computer memories, actuator motors, capacitors for storing electric power, and aerospace applications because of their lower sensitivity to radiation.

A lot of the current research on ferroelectric materials focuses on altering their crystal structures by applying pressure or strain to thin films of the materials. The research often involves making strained thin films with alternating layers only a few nanometers thick of materials with slightly different crystal structures.

PZT, while a widely used ferroelectric material, has not been the target of this recent research, because of its resistance to crystalline strain. The material’s zirconium and titanium determine its crystal structure, say the researchers, and too much strain applied to PZT causes it to revert to its original crystal structure, not an altered structure.

Lane and colleagues devised a different process for creating thin flims from PZT by adding successive layers of material with slightly different concentrations of zirconium and titanium, by slightly changing the strain with each layer. The researchers report their process makes it possible to begin with PZT made of 80 percent zirconium on the top layer and end with an 80 percent titanium form of PZT on the bottom.

“With our method, we’ve been able to extend our ability to strain these materials,” says Martin. “We go to the nanoscale so we can pull on these films and dramatically change the shape, and that affects the properties.”

The new type of PZT, reports the researchers, comes with new kinds of electric properties, in particular a built-in electric field without adding an external current, called a preferred polarity. To read a bit of data in computer memories made with a traditional ferroelectric material, its polarity is switched, which means that every time the bit is read, it has to be re-written and compared to a reference bit.

With the new type of PZT, however, bits of data would not need to have their polarity switched to be read, so computer components made with the new material could be smaller, faster, and longer lasting. “Otherwise you have to engineer similar effects using features not native to the materials to have the same thing happen,” says graduate student and co-author Jambunathan “Here,” he adds, “it’s grown into the material to begin with.”

The process devised by the researchers uses a smooth transition from one layer to the other, but Martin says altering that kind of transition can perhaps result in materials with even more novel and unexpected properties. “Each one of these,” notes Martin, “is going to give its own set of structures and potential properties that we haven’t even begun to scratch the surface of.”

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