Damaged goods: Probing the depths

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  • Published: Aug 1, 2012
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
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Optical damage

Norman Tolk, left, and Andrew Steigerwald in the Tolk lab. (Joe Howell / Vanderbilt) 
Tolk (left) and Steigerwald on optical damage
(Credit: Joe Howell/Vanderbilt)

Electronic devices are susceptible to damage from radiation, potentially cosmic radiation and even ultraviolet radiation. A study using coherent acoustic phonon spectroscopy (CAPS) reveals that the amount of optical damage, as opposed to structural damage, that can be caused may be more than ten times greater than studies suggested. The work putatively explains why certain experimental devices might fail.

Researchers at Vanderbilt University in Nashville, Tennessee have used CAPS to probe solid materials to pinpoint the size and location of defects within at an unprecedented level of precision. Andrew Steigerwald, Norman Tolk and colleagues Anthony Hmelo, Kalman Varga and Leonard Feldman report details in the Journal of Applied Physics.

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

Earlier work on damage assessment in materials used in electronics have done little more than scan the atomic lattice for defects and deformations. CAPS, however, can pinpoint when those atoms have been displaced and so determine whether a material is likely to display errant electrical or optical properties. "We are looking at optical effects from atoms that have been displaced as opposed to ionised, although a semi-permanent ionisation may contribute," Steigerwald told SpectroscopyNOW. To detect these electron dislocations, the team upgraded the 15-year-old method of CAPS, a technique akin to the seismic techniques used by oil exploration companies to find underground deposits. CAPS generates a pressure wave that passes through a piece of semiconductor using ultrashort laser pulses. The researchers use a second laser to measure the impact of the pulses. If there are defects and deformities within the material, the reflections are disturbed, just as oil deposits disturb the seismic reflection in oil exploration. The researchers can then pinpoint individual defects.

The team looked at a layer of the common semiconductor gallium arsenide, which they had irradiated with high-energy (400 keV) neon atoms. They found that the structural damage caused by an embedded neon atom spread over a volume containing 1000 atoms, which is a much wider range than is revealed by other techniques, the team says.

"This is significant because today people are creating nanodevices that contain thousands of atoms," says Steigerwald. Devices such as solar collectors comprising quantum dots may have just a few thousand atoms in each dot. "Our results may explain recent studies that have found that these quantum-dot solar collectors are less efficient than predicted," he adds. If radiation damage is embedding defects in such devices at a much higher rate than previously anticipated, then the device will not be operating optimally and may fail given a particularly high level of damage.

"The fact is that we really don't understand how any atomic-scale defect affects the performance on an optoelectronic device," explains 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."

Optical vs structural

"These are somewhat new ideas, especially in my opinion the concept of 'optical damage' versus 'structural damage' where as people normally focus on the later, I believe the former is a very much unexplored area with important implications for actual, real, operating nanodevices," Steigerwald told SpectroscopyNOW. "Considering that, I hope in the future that our results can be extended to more materials (e.g. silicon) and perhaps coupled with a sensitive microscopy technique like xSTM so that we can compare spectroscopy results with mapping of electronic states."

Related Links

  • J Appl Phys, 2012, 112, 013514: "Determination of optical damage cross-sections and volumes surrounding ion bombardment tracks in GaAs using coherent acoustic phonon spectroscopy"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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