A NEW way of tweaking slivers of silicon could let researchers create microchips with tiny, built-in lasers. The technique could be used to make on-chip sensors, or computers using light instead of electrons.
Efforts to get silicon to emit light have intensified over the past 10 years, but even the best methods are still woefully lacking, with an efficiency of only 1 per cent. Other semiconductors, such as gallium arsenide, can produce light with more than 30 per cent efficiency.
The commonest way to get light out of semiconductors is to apply a voltage that bumps electrons in the atoms from a low energy level called a “valence band” to a higher energy level called a “conduction band”. The electrons then fall back into the valence band, emitting the energy as light. The problem with silicon is that when electrons fall back, most of the energy they release is absorbed by the material as heat, so little light gets out.
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Now computer simulations by Shengbai Zhang and his team at the National Renewable Energy Laboratory in Golden, Colorado, show that putting a single layer of oxygen atoms on top of a few layers of silicon alters the way electrons move in the silicon. Their results, published in Physical Review Letters (vol 89, p 076802-1), suggest that silicon with such an oxygen layer should emit light as efficiently as other semiconductors.
To mass-produce real devices, light-emitting silicon would eventually have to be made with the technology already in place at existing chip fabrication plants. Zhang believes the current technology is up to it. “The beauty of our structural design is that it is fully compatible with current technology,” he says.
Albert Polman, an expert in opto-electronic materials at the FOM Institute for Atomic and Molecular Physics in Amsterdam, agrees that the structures could be built, leading to a new range of silicon-based optical devices. But Kevin Homewood, who is working on applications of light-emitting silicon at the University of Surrey in Guildford, is less confident. Producing a layer of oxygen with that level of precision across whole silicon wafers is likely to prove extremely tough, he says.