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Scientists Try a Solar Sell

Light Reading
News Analysis
Light Reading
8/24/2001

Solar cell technology has given scientists new ideas about how to make efficient silicon light-emitting devices. Writing in the journal Nature yesterday, Martin Green and his colleagues from the University of New South Wales, Australia, report on a way of improving the emission efficiency of bulk silicon by a factor of 100.

Silicon light emitters are thought to be crucial to the development of future integrated circuits. As transistors shrink in size, the electrons spend proportionately more time in the wires that connect them. In the end, the speed of the connections between circuits becomes a bottleneck that puts a halt to any further improvements in chip speed and density.

Using light instead of electrons to send signals between different parts of a chip could overcome that bottleneck. Right now, however, scientists are still searching for a way of making a light-emitting device that's compatible with silicon electronics. In other words, one that's actually made out of silicon.

Researchers have tried lots of approaches to coax light out of silicon (see, for example, Light From Silicon), but these all have very low efficiency. In devices reported so far, only about 0.01 percent of the electrical power put into the device gets turned into optical output power, says Green in his paper.

Green and his colleagues have managed to achieve a power conversion efficiency of about 1 percent with their device. That may not sound like a lot, he notes, but it's about the same as what was possible with semiconductor materials such as gallium arsenide about a decade ago.

It might seem strange that a team of researchers specializing in solar cells should have made this advance. But there is a link. Solar cells are designed to absorb as much light as possible and turn it into electricity -- the reverse of the process occurring in a light-emitting diode.

There's a law of physics involved here. Kirchoff's Law states that anything that absorbs light well also emits light well. As a result, the solar cell designs that Green and his co-workers are developing should also be good for making light emitters.

A simple slab of semiconductor is bad at emitting light because of what's called the "escape cone." Owing to the large difference in refractive index between silicon and air, only light that's nearly perpendicular to the interface can escape. To increase the amount of light that can get out, the researchers use a well-known technique of breaking up the surface into many different angles.

Green contends the design that he reports is way more efficient than other solar cell designs, though exactly why was not made clear. It involves etching inverted pyramids into the surface using a special "anisotropic" technique that preferentially etches certain crystal planes and not others. This, along with other design tweaks, is enough to improve the efficiency by a factor of 10.

To get the second 10x improvement, Green and co-workers worked to make the atomic structure of the crystal as near perfect as possible. Imperfections in the crystal kill light emission because they allow energy to be dissipated as heat rather than as light.

The Australian team claims that the performance of the existing device is "acceptable", and they plan to start working on the applications straight away. "We are [currently] developing versions of the device suitable for integrated circuits as well as developing high-speed integrated modulators required for high speed signalling with such devices," Green wrote in an email to Light Reading.

A potential drawback to this approach is that it would be difficult to turn the design into a laser. That's because lasers rely on mirror-like surfaces to bounce light to and fro. Green's etching results in exactly the opposite sort of surface.

— Pauline Rigby, Senior Editor, Light Reading
http://www.lightreading.com

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