23 Jul 2012

Microelectronics radiation damage much greater than thought

The amount of damage that radiation causes in electronic materials may be at least ten times greater than previously thought, report US researchers from Vanderbilt University.

The researchers' observations follow the development of a new characterisation method that uses a combination of lasers and acoustic waves to provide scientists with a capability tantamount to X-ray vision.

"The ability to accurately measure the defects in electronic materials becomes increasingly important as the size of microelectronic devices continues to shrink," says Professor Norman Tolk, Vanderbily University. "When an individual transistor contains millions of atoms, it can absorb quite a bit of damage before it fails. But when a transistor contains a few thousand atoms, a single defect can cause it to stop working."

Previous methods used to study damage in electronic materials have been limited to looking at defects and deformations in the atomic lattice.

The new method is the first that is capable of detecting disruption in the positions of the electrons that are attached to the atoms. This is particularly important because it is the behavior of the electrons that determine a material's electrical and optical properties.

To detect the electron dislocations, the physicists upgraded a 15-year-old method called coherent acoustic phonon spectroscopy (CAPS).

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," says Tolk's colleague, Dr Andrew 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 predicted."

"The fact is that we really don't understand how any atomic-scale defect affects the performance on an optoelectronic device," adds Tolk. "Techniques like the one that we have developed will give us the detailed information we need to figure this out and so help people make nanodevices that work properly."

• The research is published online in the Journal of Applied Physics.

Most popular news in Electronics manufacture