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  • Amorphous silicon makes better optical fibers

Photonics Spectra
Mar 2012
Ashley N. Paddock,

COLLEGE PARK, Pa. – A first-of-its-kind technique deposits a noncrystalline form of silicon into the long, ultrathin pores of optical fibers, making more flexible and efficient fibers. This method uses high-pressure chemistry to make well-developed films and wires from this particular kind of silicon semiconductor.

Hydrogenated amorphous silicon is ideal for solar cell applications and could also be useful for the light-guiding cores of optical fibers, said John Badding, a chemistry professor at Pennsylvania State University. He added, however, that depositing the silicon compound into a tiny optical fiber presents a challenge.

“Traditionally, hydrogenated amorphous silicon is created using an expensive laboratory device known as a plasma reactor,” Badding said. “Such a reactor begins with a precursor called silane – a silicon-hydrogen compound. Our goal was not only to find a simpler way to create hydrogenated amorphous silicon using silane, but also to use it in the development of an optical fiber.”

A bed of amorphous hydrogenated silicon wires that were prepared in the pores of optical fibers. The wires have been chemically etched out of the optical fiber to reveal them. Scale bar = 100 µm. Inset: An array of amorphous hydrogenated silicon tubes deposited in an optical fiber. The optical fiber has been cleaved in half to reveal the array of tubes. The very thin glass walls of the fiber surrounding each tube are largely obscured. Courtesy of John Badding Lab, Pennsylvania State University.

The low-pressure plasma reactor technique works well for depositing hydrogenated amorphous silicon onto a surface to make solar cells. However, the method cannot be used for depositing hydrogenated amorphous silicon into fiber – so the team had to rethink the approach.

“The trick was to develop a high-pressure technique that could force the molecules of silane all the way down into the fiber and then also convert them to amorphous hydrogenated silicon,” said Pier J.A. Sazio of the University of Southampton in the UK, a team leader. “The high-pressure chemistry technique is unique in allowing the silane to decompose into the useful hydrogenated form of amorphous silicon, rather than the much less useful nonhydrogenated form that otherwise would form without a plasma reactor. Using pressure in this way is very practical because the optical fibers are so small.”

Optical fibers with a noncrystalline form of silicon have many applications, such as in telecommunication devices or for changing laser light into different infrared wavelengths. IR light could be used to improve surgical techniques, military countermeasure devices and chemical-sensing tools, such as those that detect pollutants or environmental toxins. The team members also hope that their research will be used to improve existing solar cell technology.

“What’s most exciting about our research is that, for the first time, optical fibers with hydrogenated amorphous silicon are possible,” Badding said. “However, our technique also reduces certain production costs, so there’s no reason it could not help in the manufacturing of less expensive solar cells as well.”

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