Strain Creates Duet of Change in Semiconductor Material

University of Arkansas and College of William and Mary researchers have found that strain can induce changes in the optical, electromechanical and polar properties of a semiconductor material. These changes could mean that one material could be used to serve multiple functions in place of two or more materials, creating cheap, fast, efficient lasers, cell phones, computers military sonar devices and medical ultrasound devices.

Vivek Ranjan, a research associate in physics and Laurent Bellaiche, associate professor of physics at the University of Arkansas and Eric J. Walter of the College of William and Mary in a collaboration through the Center for Piezoelectrics by Design report their findings in an upcoming issue of Physical Review Letters.

Two classes of compounds of current interest to scientists have different but significant technological properties. Semiconductors have important optical properties of use in lasers and computers. Ferroelectric materials convert small changes in mechanical energy into electrical energy, called a piezoelectric response, and are used in military sonar and medical ultrasound. These properties occur in different materials, requiring the use of more than one material in a device that makes use of both.

The researchers found that growing atomic layers of some materials on smaller surfaces creates a strain that induces both a large energy conversion and emits light in the entire range of the visible spectrum.

"Using this finding, you can design a new multifunctional material that exhibits both properties," Ranjan said.

Ranjan, Bellaiche and Walter looked at Scandium nitride (ScN), a semiconductor material. They showed that by "squeezing" or putting a strain on the system at the atomic level, the material goes through optical changes that take it through the width of the visible spectrum. Further, the material also changes from non-polar to polar, exhibiting a leap in piezoelectric response characteristic of ferroelectric materials.

The piezoelectric response in this material represents the process of changing shape as the ScN compound "squeezes" itself to match atom-for-atom the smaller atomic surface.

The researchers plan to investigate other semiconductors to see if this trait presents itself given the same circumstances.

"We think that there may be other semiconductors that will have this trait," Ranjan said.