New approach makes silicon's red glow blue.

Research done more than a century ago sheds new light on the recent discovery of luminescence in silicon, a semiconductor previously thought to lack the ability to emit a strong glow. Just two years ago, British scientists obtained luminescence at room temperature from silicon samples made porous by etching in acid. That announcement and subsequent reports that light or electricity will make porous silicon emit light prompted a flurry of research because of the possibility of making silicon-based optoelectronic devices (SN: 2/15/92 p.103).

But silicon in those applications may not require acid etching at all, says Martin S. Brandt of the Max Planck Institute for Solid-State Science in Stuttgart, Germany. In combing the German scientific literature, he and his colleagues discovered reports from 1863 and the 1920s discussing the conversion of a silicon-calcium compound to a strongly luminescing silicon material called siloxene.

Following up on that research, the Stuttgart team has now demonstrated the potential for incorporating siloxene thin films onto silicon computer chips. "It would be, in principle, compatible with existing silicon technology," Brandt says.

He and his colleagues first grow the calcium-silicon compound on silicon and then create a thin film of siloxene by replacing the calcium with hydroxyl groups. With this technique, they have made a 400-nonometer-thick siloxene film that "has the same properties as porous silicon," Brandt reported this month at the Materials Research Society's spring meeting, held in San Francisco.

"That's an exciting possibility," comments Ulrich M. Gosele, a solid-state physicist at Duke University in Durham, N.C. Siloxene presents a better surface for attaching metal contacts than does the fragile, much-pitted porous silicon. But to really be useful, siloxene also needs to luminesce when subjected to electrical current, he says. Moreover, researchers have not demonstrated that siloxene can be doped and fashioned into practical devices, says Reuben T. Collins of IBM's Thomas J. Watson Research Center in Yorktown Heights, N.Y.

Using a charge-coupled device, the Stuttgart group showed that siloxene materials can emit a rainbow of colors, including porous silicon's red and blue, a color not yet reported from porous silicon and one crucial for creating color displays or signs, Brandt says.

A siloxene molecule groups six silicon atoms as a hexagonal ring with three oxygen and six hydrogen atoms attached. These rings link up, using an oxygen atom to bridge two rings, and arrange in flat layers, Brandt explains. He adjusts the color of the luminescence by replacing the other attached atoms with different chemical side groups.

Siloxene that forms on surfaces of porous silicon during acid etching could cause that material's luminescence, says Brandt, noting similarities in the optical properties of the two types of silicon materials.

Leigh T. Canham of the Defense Research Agency in Malvern, England, who first described luminescing porous silicon, proposes a different mechanism. He suggests that luminescence occurs because etching creates silicon crystals so thin that an effect called quantum confinement occurs. After light or electrical current excites electrons in these 1- to 5-nonometer-thick "quantum wires," these "confined" electrons can calm down again only by emitting light.

But for both these explanations, "the arguments are a bit indirect," says Collins. Other researchers have proposed different mechanisms for luminescence, and while many have some evidence to back up their ideas, no single model seems to explain all the observations. Indeed, a combination of mechanisms may lead to silicon's bright glow.

"It could be quantum confinement and siloxene," Gosele suggests.