AN ITALIAN company has worked out how to make silicon emit light when an electric current is passed through it. This promises to revolutionise electronics by freeing microchips of the layers upon layers of connecting wires that slow them down and lead to overheating. Instead, circuits on chips will be able to beam signals to each other using light.
Researchers have been trying to squeeze light out of silicon for years, but the results announced by ST Microelectronics, the world鈥檚 third largest semiconductor maker, appears to put others鈥� efforts in the shade. The company claims it has achieved a 鈥渜uantum efficiency鈥� of 10 per cent at room temperature, which means that for every ten electrons injected into the device, one photon is emitted. This efficiency is a hundred times better than has ever been achieved before. And ST says it will deliver the first light-emitting silicon chips before the year ends.
But some rival researchers who鈥檝e spent years striving to perfect light-emitting silicon are sceptical of ST鈥檚 claims. 鈥淚f they鈥檝e got 10 per cent efficiency, I鈥檒l take up farming,鈥� jokes one.
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Light-emitting diodes built from other semiconductors often exceed 30 per cent efficiency, but it is difficult to persuade silicon to emit light. In semiconductors such as gallium arsenide, photons are produced when an electron recombines with a 鈥渉ole鈥� in the crystal lattice. But in silicon, the two are reluctant to recombine and the energy of the electrons and holes is dissipated as heat. It鈥檚 just too inefficient a process to rely on for light emission. Salvo Coffa and his colleagues at ST鈥檚 lab in Catania, Sicily, have created a silicon-based semiconductor in which ions of erbium, a rare-earth metal, emit light.
The erbium ions are implanted into a layer of silicon dioxide that is densely packed with tiny nanocrystals of pure silicon (see Graphic). ST is not giving much away, but it鈥檚 thought that electrons are injected into this layer and travel through the network of nanocrystals until they encounter and excite an erbium ion. When the erbium relaxes from this excited state into its ground state it spits out a photon. The energy change of this transition 鈥� and hence the energy of the photon 鈥� falls conveniently within the range that鈥檚 used in telecommunications.
ST鈥檚 first light-emitting silicon products will use optical signals to electrically isolate high and low-voltage parts of circuits. In a conventional chip, there鈥檚 a danger that the two will short-circuit, frying the components. With optical rather than electrical connection between them, the risk of a dangerous short disappears.
Eventually, the electrical signals that criss-cross computer chips on hot stacked wires could also become optical.