‘Bulk’ Silicon Emits Visible Light

With silicon entrenched as the material of choice for the electronics industry, augmenting its optical properties so it could be integrated into photonic circuitry would make consumer-level applications of the technology more feasible. Now, for the first time, “bulk” silicon has been shown to emit broad-spectrum, visible light, opening up the possibility of using the element in devices that have both electronic and photonic components.

Some semiconductors, when imparted with energy, emit light, directly producing photons instead of heat. This phenomenon is commonplace and used in LEDs. To get the desired photonic properties, however, means finding the right semiconductor material.

Semiconducting materials — especially silicon — form the backbone of modern electronics and computing, but, unfortunately, silicon is an especially poor emitter of light and belongs to a group of semiconducting materials that turns added energy into heat. This makes integrating electronic and photonic circuits a challenge.

“The problem is that electronic devices are made of silicon, and photonic devices are typically not,” said University of Pennsylvania associate professor Ritesh Agarwal. “Silicon doesn’t emit light, and the materials that do aren’t necessarily the best materials for making electronic devices.”

A schematic of silicon nanowire integrated with an omega-shaped metal nanocavity. Images courtesy of the University of Pennsylvania.

Scientists have tried to address the problem by doping silicon with other materials, Agarwal said, but the light emission is in the very long wavelength range, which degrades its electronic properties. “Another approach is to make silicon devices that are very small — five nanometers in diameter or less,” he said. “At that size, you have quantum confinement effects, which allows the device to emit light, but making electrical connections at that scale isn’t currently feasible, and the electrical conductivity would be very low.”

To get elemental, “bulk” silicon to emit light, Agarwal and colleagues drew upon previous research they had conducted on plasmonic cavities, whereby they wrapped a cadmium sulfide nanowire in a layer of silicon dioxide and then in a layer of silver. (See: All-Optical Nanoswitch Promises Faster Computers) The silver coating supports surface plasmons, which are highly confined to the surface where the silicon dioxide and silver layers meet. For certain nanowire sizes, the silver coating creates pockets of resonance and highly confined electromagnetic fields of light within the nanostructure.

Under normal conditions, semiconductors must first cool down after excitation, relasing energy as heat, before “jumping” back to the ground state and releasing the remaining energy as light. Coupled with plasmonic nanocavities, however, the investigators’ semiconductor nanowires jumped directly from a high-energy excited state to the ground state, all but eliminating the heat-releasing cool-down period. This ultrafast emission time opens the possibility of producing light from semiconductors such as silicon that might otherwise produce only heat.

“If we can make the carriers recombine immediately, then we can produce light in silicon,” Agarwal said.

White light emission from silicon coupled with a nanocavity under laser excitation.

In their latest work, the researchers wrapped pure silicon nanowires in a similar fashion, first with a coating of glass and then one of silver. In this case, however, the silver did not wrap completely around the wire, as the researchers first mounted the glass-coated silicon on a separate pane of glass. Tucking under the curve of the wire but unable to go between it and the glass substrate, the silver coating took on the shape of the Greek letter omega — Ω — while still acting as a plasmonic cavity.

Critically, the transparent bottom of the omega allowed the researchers to impart energy to the semiconductor with a laser and then examine the light silicon emitted. Even though the silicon nanowire is excited at a single energy level, which corresponds to the wavelength of the blue laser, it produces white light that spans the visible spectrum. This translates into a broad bandwidth for possible operation in a photonic or optoelectronic device.

In the future, it should also be possible to excite these silicon nanowires electrically.

“If you can make the silicon emit light itself, you don’t have to have an external light source on the chip,” Agarwal said. “We could excite the silicon electrically and get the same effect, and we can make it work with wires from 20 to 100 nanometers in diameter, so it’s very compatible in terms of length scale with current electronics.”

The work was published in Nature Photonics (doi: 10.1038/nphoton.2013.25).

For more information, visit: www.upenn.edu