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Super Silicon: Top semiconductor turns into a superconductor

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11:37am, November 21, 2006
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While not yet leaping over tall buildings in a single bound, silicon is doing something pretty super these days—conducting electricity with zero resistance.

The material achieved that triumph when physicists in France crammed unprecedented numbers of boron atoms into a silicon wafer's surface. When cooled to less than 0.4 kelvin, the boron-laden silicon permitted electrons to flow unimpeded, the scientists report in the Nov. 23 Nature.

As the stuff of microchips, "silicon has become the technologically most important material of the past 50 years," notes superconductivity researcher Robert J. Cava of Princeton University in a commentary in the same journal issue. Silicon's characteristics as a semiconductor—a substance with electrical properties midway between those of a conductor and an insulator—make it the dominant material of microelectronics.

In experiments in the 1980s, other teams fleetingly made silicon a superconductor when they squeezed it to about 100,000 times atmospheric pressure.

Cava calls the new, more lasting superconductor a "breakthrough." He adds, however, that "it's too early to tell [whether it's] a herald of more and better devices and materials."

Indeed, making silicon so profoundly cold would be commercially impractical, admits Étienne Bustarret of the National Center for Scientific Research in Grenoble, a member of the research team. However, he adds, it's possible that further modifications could boost the superconducting temperature.

"This discovery suggests a possibility of producing new . . . devices," particularly if researchers could make a second silicon superconductor with phosphorus, comments Yoshihiko Takano of the National Institute for Materials Science in Tsukuba, Japan. The boron- and phosphorus-infused superconductors could be combined to form electronic components, he says.

To make commercial transistors, manufacturers put atoms of boron, phosphorus, or other elements into silicon. Bustarret and his colleagues injected millions of times greater concentrations of boron into 10 spots on a silicon wafer.

To do so, the team put the wafer in a chamber of boron chloride gas and blasted each spot with a powerful ultraviolet laser that emitted 200 bursts, each lasting 25 nanoseconds. Each pulse melted the silicon surface. Ultimately, boron atoms took more than 8 percent of crystal locations normally occupied by silicon atoms.

Besides pursuing a phosphorus version of the silicon superconductor, Bustarret and his team are force-feeding elements to other semiconductors—such as aluminum nitride—that they expect to superconduct at higher temperatures.

"The treatment they meted out to silicon to force its conversion," Cava quips, "can only be termed abusive."

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