Chips with Your Fiber

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The team's electro-optic fiber was developed by taking advantage of the geometry of conventional optic lines, which are made of hollow, capillary-like glass tubes bundled together. One of the challenges of depositing semiconductors into the tubes -- which can range in shape from circular to hexagonal, with diameter from nanometers to micrometers -- is to uniformly apply the material down the length of the tube without clumps or defects, Badding says. Sending semiconductor compounds into these tubes is "the equivalent of filling a garden hose over a mile long," he says.

To overcome this challenge, the group modified the well-known deposition technique called chemical vapor deposition (CVD). In this process, compounds of silicon, germanium, or other semiconductors are vaporized and typically sprayed onto flat substrates. Badding says his team used CVD, but forced the vaporized material through the long, thin capillaries at pressures as high as 1,000 times atmospheric pressure and many times more than conventional CVD. While the material filled the tubes, the researchers heated the fiber so that the material assembled into a crystallized semiconductor.

Another group that's also working to integrate electronics into fiber is led by Mehmet Bayindir at MIT. This group adds semiconductor materials, metals, and polymers to raw glass before the fiber is stretched into its eventual length (see Smart Fibers). The advantage is that "our technique uses traditional fiber drawing technology" and is completed in one step, says Ayman Abouraddy, a postdoc researcher at MIT who works with Bayindir. In contrast, using CVD to build electronics in fiber is a two-step process, in which the fiber is drawn and then the devices are deposited.

It would be challenging, Abouraddy suspects, to form a semiconductor the entire length of a meters-long fiber by using CVD. However, the technique could have applications where short fiber is needed, such as for a splice to replace an external switch or other electronic device, he says.

Abouraddy also notes that the crystalline semiconductors made by the Southampton and Penn State researchers produce faster detectors and light modulators than the slower, so-called amorphous semiconductors used by the MIT group. While slower detectors are good enough for medical applications, such as detecting concentrations of a chemical in the body, crystalline semiconductors are required in telecommunications networks, where information needs to be processed quickly.