Nanocrystal lights way to faster circuits

Applied physicists at the California Institute of Technology have invented a light-emitting transistor that could potentially bypass a major bottleneck that slows down electronic circuitry.

The new device could pave the way for on-chip optical interconnections that would enable the marriage of two modern technologies–communications based on the transmission of photons, and computing with silicon-based devices that are driven by electric currents. A successful optical interconnection technology would allow information to move around inside a silicon chip at the speed of light while creating substantially less heat, leading to faster computers.

Reporting in a recent issue of the journal Nature Materials, Caltech graduate student in applied physics Robb Walters and his faculty adviser, Professor Harry Atwater, described their success in building a nanophotonic device that employs a novel method of turning an electric signal into a light pulse.

“It’s been difficult to combine silicon-based integrated circuits and optical devices,” says Walters, who invented the device and is the first author of the Nature Materials paper. “Our new device brings us one step closer to a silicon-based light source that may ultimately lead to the light-emitting devices needed for on-chip optical interconnections.”

The device Walters has invented contains at its core a tiny spherical bead called a silicon nanocrystal that absorbs an electron and a positive-charge carrier called a “hole.” Inside this nanocrystal, the electron and the hole can be combined to release energy as a photon of near-infrared light that shines out of the transparent side of the bead. In effect, this pulse of light, when launched into a waveguide, takes the place of an electrical signal travelling down a wire in a chip, increasing the speed of data transmission.

The dimensions of the bead are responsible for the wavelength of the light emitted, due to quantum effects. The bead size can be used to “tune” the frequency of the photons, a slightly smaller bead emitting slightly higher photon energy and a larger bead, lower energy. The fact that one nanocrystal absorbs one electron and hole, and emits one photon, could also conceivably be useful for future single-photon technologies, says Atwater.

“Eventually, the photons from the nanocrystals will go to a photodetector in a complete, photonic integrated circuit,” says Atwater. “The device might also be useful for visible displays. However, this is still basic research and development. We have not yet integrated the device with waveguide detection; but in principle, it will work.”

The new device is different from existing silicon light-emitting diodes and other nanocrystal structures in that there is not a constant driving current required for light emission. The new approach, based on field-driven carrier injection, may be far more efficient than any existing technology.

“The current external power efficiency record for a silicon-based LED is about one percent,” Atwater says. “We hope that our new device will allow that record to be substantially improved.”