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Silicon now shines with optical potential.

An illuminating rebirth of silicon -- long considered an "optically dead" material -- has delighted scientists who work with this vital semiconductor. After decades of unsuccessful attempts, researchers armed with lasers have finally teased visible light from specially treated silicon, achieving an effect known as photoluminescence.

Mastering silicon's photoluminescence and extending it to electrical stimulation of light emission, or electroluminescence, could revolutionize optical electronics and lead to superior computers. "It's pretty hot stuff," says Subramanian S. Iyer of IBM's Thomas J. Watson Research Center in Yorktown Heights, N.Y.

In May, British and French researchers presented the first evidence that acid-etched silicon wafers can emit light when illuminated. Several groups have since confirmed those observations, but new findings cast doubt on the initial explanation for silicon's puzzling glow.

Luminescence starts in semiconductors when electrons, stimulated by lasers or electricity, jump to the conduction bands from the valence bands within the material, leaving "holes" - the positively charged equivalent of electrons. Many semiconductors will release a photon when the electrons fall back across these energy gaps and combine with the holes.

Silicon, however, is an indirect band-gap material: It rarely produces visible photons when electrons and holes recombine. For this reason, light-emitting diodes, lasers and other optical electronic devices currently rely on gallium arsenide and other direct band-gap semiconductors, which are expensive and unwieldy.

Scientists now know that silicon can mimic a direct band-gap material, but they have yet to figure out what makes it do so. One theory, put forth by the British researchers who initially achieved the effect, holds that bathing silicon in hydrofluoric acid changes its light-emitting behavior. Leigh T. Canham and his colleagues at the Defense Research Agency in Malvern, England, proposed in May that the acid etches a forest of microscopic pillars into the silicon. These small, in effect one-dimensional structures -- called "quantum wires" -- then facilitate the electron-hole recombination by confining the electron's movement, they suggested.

That simple theory now faces a challenge from new images of the acid-treated, light-emitting silicon taken with a transmission electron microscope. "[Canham's] pillars are far too large for quantum confinement," says John M. Macaulay of AT&T Bell Laboratories in Murray Hill, N.J., who led the team that produced the as yet-unpublished images. "[They] are not necessary in photoluminescence."

What luminescence requires, he says, is simply silicon structures of 10 nanometers or less. Many of the micrographs reveal a complex, sponge-like structure and not pillars.

Such minute structures create a "quantum size effect" that appears to broaden silicon's band gap and allow more efficient recombination of electrons and holes, suggests Reuben T. Collins of the Watson Research Center.

But before silicon can replace gallium arsenide, Collins notes, researchers must take the giant step from photoluminescence to electroluminescence.

In May, Canham and his co-workers claimed they had created a working silicon device that accomplishes electroluminescence, but they have refused to release any details because of pending patents, according to frustrated researchers. Such devices -- if practical -- might finally allow construction of the long-awaited optical computer.

Silicon's shining breakthrough has clearly excited a once-dormant filed. Light-emitting silicon "has tremendous potential," says Peter Searson, a materials scientist at Johns Hopkins University in Baltimore. "It's sort of like superconductivity in '86."

-- J. Travis
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Title Annotation:development of photoluminescent silicon with potential applications in optical electronics
Author:Travis, John
Publication:Science News
Date:Aug 31, 1991
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