To detect the electron dislocations, the physicists upgraded a 15-year-old method called coherent acoustic phonon spectroscopy (CAPS).
"CAPS is similar to the seismic techniques that energy companies use to search for underground oil deposits, only on a much smaller scale," said Steigerwald.
Oil explorers set off a series of small explosions on the surface and measure the sound waves that are reflected back to the surface. That allows them to identify and map the layers of different types of rock thousands of feet underground.
Similarly, CAPS generates a pressure wave that passes through a chunk of semiconductor by blasting its surface with an ultrafast pulse of laser light. As this happens, the researchers bounce a second laser off the pressure wave and measure the strength of the reflection. As the pressure wave encounters defects and deformities in the material, its reflectivity changes and this alters the strength of the reflected laser light. By measuring these variations, the physicists can detect individual defects and measure the effect that they have on the material's electrical and optical properties.
The physicists tested their technique on a layer of gallium arsenide semiconductor that they had irradiated with high-energy neon atoms. They found that the structural damage caused by an embedded neon atom spread over a volume containing 1,000 atoms considerably more extensive than that shown by other techniques.
"This is significant because today people are creating nanodevices that contain thousands of atoms," said Steigerwald. One of these devices is a solar collector made from quantum dots, tiny semiconductor beads that each contains a few thousand atoms. "Our results may explain recent studies that have found that these quantum-dot solar collectors are less efficient than pr
|Contact: David Salisbury|