Bottleneck. Chips could communicate better if light beams replaced sluggish wires.

Silicon Lights the Way

Computer engineers can design souped-up chips all they want, but their wizardry is in vain unless they can also speed the flow of information between chips and other computer components. One way to do that is to replace today's sluggish metal wires with higher speed optical connections. Now, researchers have devised a light-emitting diode (LED) based on silicon that could be a big step toward that goal.

LEDs convert electricity into light. Researchers inject the LED with negatively charged electrons and positively charged "holes"; when the particles collide, they give off photons with wavelengths similar to the ones used in optical communications. Unfortunately, the best light-emitting semiconductors, such as gallium arsenide (GaAs), are hard to integrate with the silicon of which chips are made. And the ideal material for the job--silicon itself--has been a poor light emitter: Only about 0.01% to 0.1% of the photons ever get out. The rest just create unwanted heat.

To change that, physicist Martin Green and colleagues at the University of New South Wales in Sydney borrowed a trick for making solar cells. In recent years, the researchers had found that they could make solar cells absorb much more light by texturing their top and bottom. Because the best light-absorbing semiconductors are also the best light emitters, Green hit upon the idea that texturing silicon could improve the efficiency of silicon LEDs as well. Indeed, when his team created an array of pyramid-shaped wells on the silicon's top surface and a mirrorlike flat bottom surface, many more newly created photons bounced around inside the device until they escaped, the team reports in the 23 August issue of Nature. That raised the efficiency to more than 1%, a 10- to 100-fold increase.

The all-silicon LED still isn't as bright as LEDs made of GaAs. But researchers hope further improvements will bring it up to snuff and make it a useful component for microcircuitry. "If it can interact with transistors and memory, it would probably be really important," says Daniel Radack, who oversees advanced computing issues for the Defense Advanced Research Projects Agency in Arlington, Virginia.

Related sites

Photovoltaic Engineering, University of New South Wales
Darpa VLSI Program