Toward more-efficient optical devices
Context: Silicon is good at shuttling electrons around chips but much worse than most other semiconductors at manipulating light. This shortcoming has kept optical chips, which transmit information more efficiently than electrical chips, from wider use. Silicon “nanocrystals,” a few atoms of silicon covered with an oxide layer, emit light more efficiently than bulk silicon, but devices incorporating them wear out quickly and are still too inefficient for most applications. Now, a team led by Harry Atwater of Caltech has improved silicon’s ability to emit light, giving a boost to an industry looking for new ways to make faster chips.
Methods and Results: In a conventional light-emitting diode (LED), electrons traveling through a semiconducting crystal meet electron “holes” – or gaps left in the crystal by absent electrons – and lose energy, which is emitted as light. But this approach doesn’t work well with silicon nanocrystal LEDs, where electrons moving toward the holes can collide with atoms in the crystal and displace them, degrading performance.
Previous silicon LEDs used separate electrodes to inject holes and electrons into silicon nanocrystals. But Atwater and colleagues figured out how to inject both from a single electrode. In their device, a thin layer of silicon nanocrystals sits atop an electrode that alternates between adding electrons and adding holes. This keeps electrons from rocketing violently across the crystal and damaging it. Also, by eliminating one of the entry and exit points for electrons, the Caltech group has made devices that are easier to fabricate and more consistent in performance.
Why It Matters: The new LED can be built using standard equipment that could be integrated into a chip-manufacturing line. Its performance, however, is still low enough to limit its use. To improve the processing speed of silicon chips, the LED would have to switch on and off more quickly; to be of use in a display, it would have to consume less power. Nonetheless, the semiconductor industry has much practice improving the performance of silicon chips. The problems of speed and power may not remain unsolved for long.
Source: Walters, R. J., G. I. Bourianoff, and H. A. Atwater. 2005. Field-effect electroluminescence in silicon nanocrystals. Nature Materials 4:143–146.