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Steamed, gallium arsenide guides light.

Sometimes, a little rust can do a lot of good. Electrical engineers have discovered that by steaming gallium arsenide chips they can oxidize the surface of this semiconductor and create microscopic patterns useful for guiding light, and possibly electrons, in lasers and other electronic and optical devices.

This finding puts gallium arsenide on a more equal footing with silicon, which reigns as the material of choice in electronics, in part because it forms a stable oxide. This allows engineers to etch circuits into the surface easily, says Nick Holonyak Jr. at the University of Illinois at Urbana-Champaign.

He and his colleagues made a similar oxide on gallium arsenide, then tapped this semiconductor's ability to emit light when subjected to an electrical current to create laser diodes. They report their work in the March 30 APPLIED PHYSICS LETTERS.

"The semiconductor is a [brighter] light source than any other light source, but we haven't had a good way to move photons," says Holonyak.

The Illinois team starts with a chip of pure gallium arsenide sandwiched between layers of gallium arsenide in which aluminum atoms have replaced some gallium atoms. "It's the aluminum that makes it possible for us to make a good oxide," Holonyak says. The researchers put a chemical mask with the desired pattern atop the chip surface, steam the chip, remove the mask and coat the chip with a thin conductor.

The oxidized parts insulate, so current reaches the light-emitting core only where the mask prevented oxidation. The oxidized parts also have a lower index of refraction, so they deflect light, confining it to the pattern.

The researchers discovered they could make this smooth, hard oxide while studying the decade-long effects of exposure to air and moisture on aluminum gallium arsenide. In 1989, Holonyak and graduate student John M. Dallesasse tried to speed up decay by heating the material above 400[degrees]C in the presence of water vapor. But instead of crumbling, the chips formed a hard oxide. Since then, the group has made lines, rectangles and rings. They expect they can guide light along any pathway they want.

Because the oxide won't peel off, processing these chips into useful devices becomes relatively straightforward, says Holonyak. In addition, the technique appears to work for other compound semiconductors that contain aluminum in their top layers, says Russell D. Dupuis of the University of Texas at Austin.

He and Holonyak expect that companies using laser diodes in compact disk players, telecommunications and other applications will want to use this simpler approach. "And as we go toward integrated optoelectronic devices, this could be the enabling technology," Dupuis says.

Scientists first need to learn how well the oxide can channel electrons, as well as photons, and whether the oxide changes its characteristics over time, says Dupuis. "The snapshot we have now, though, is that it's working well," he adds.
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Author:Pennisi, Elizabeth
Publication:Science News
Date:Apr 4, 1992
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