Silicon nanocrystals glow

By Kimberly Patch, Technology Research News

While a silicon computer chip uses electricity to operate its logic circuits, fiber-optic communications lines use light to transfer information at blazingly fast speeds. Bridging the gap between electron and photon would be smoother if engineers could find one medium that speaks both languages.

Researchers from the Italian universities of Trento and Catania have taken a large step in this direction by demonstrating that silicon, in the form of densely packed silicon nanocrystals embedded in silicon oxide, can emit a useful amount of light. "Once you reduce silicon to nanometer dimensions it is able to amplify light," said Lorenzo Pavesi, an associate professor of experimental physics at the University of Trento.

The work may pave the way for silicon lasers, light-based connections for computer chip components and better integration of silicon-based computers and fiber-optic telecommunications lines.

Silicon lasers would also be potentially much cheaper than today's gallium arsenide based lasers because silicon is 200 times cheaper to produce than gallium arsenide, according to Pavesi. Adding a silicon-based laser to a computer chip would also be a much cheaper manufacturing operation then the mostly manual placement required to combine non-silicon optoelectronic devices with silicon chips today, said Pavesi.

The silicon nanocrystals are a type of quantum dot. Quantum dots are microscopic structures that contain, or trap small numbers of electrons. The nanocrystals are produced by shooting silicon atoms into a quartz substrate, then heating the material to force the atoms to conglomerate into very small balls.

The key to the researchers' work is the silicon nanocrystals are densely packed, which makes for many surfaces. It is these surfaces that glow, or emit photons of light when they're stimulated by light or electricity. The gain also has to do with the properties of the bond between the silicon oxide and the surface of the nanocrystals. The researchers showed a gain of more than one percent.

In order to be practical, the light emitting efficiency of a material must be at least a few percent. Before this, silicon's efficiency had always been below one percent.

"The idea was born to us after 10 years of work on silicon light emitting materials," said Pavesi. "We realized that to have a good emitter one needs both quantum confinement in low dimensional silicon and good surface [protection using] silicon oxide," said Pavesi.

The work is novel and potentially useful, said Philippe M. Fauchet, professor and chairman of electrical computer engineering at the University of Rochester.

If lasers can ultimately be made out of the material, it would solve a big problem for the electronics industry -- how to take advantage of the very fast, very tightly packed devices found in today’s chips, he said. The idea of using fast optical interconnects "has been in part hampered by a lack of suitable laser sources that can be integrated and fabricated using silicon technology," said Fauchet. "A silicon laser would provide a powerful, elegant solution -- everything could be made of silicon."

There are two major steps to go in order to make a useful laser from the silicon nanocrystal material, said Pavesi. First, the mirrors that allow laser light to build up by bouncing back and forth must be integrated into the nanocrystal silicon oxide material.

More difficult, the device must be pumped electrically, meaning electrical current must somehow be channeled into the material in order to power the laser action. This will be difficult because the nanocrystals are buried in silicon oxide, which is an insulator. New forms of injecting current must be invented for this type of material, said Pavesi.

"To make a laser in which pumping is provided by electrical current will be a most daunting task, but one that may be tackled by many groups," said Fauchet.

The researchers also plan to change the light emission wavelength of the device. Currently the nano crystals emit red light. Blue, green and infrared lasers may be possible by using different size nanocrystals or introducing impurities to the mix, said Pavesi.

The engineering required to make useful silicon lasers will likely take five to ten years, said Pavesi.

Pavesi's research colleagues were L. Dal Negro, C. Mazzoleni, G. Franzo and F. Priolo. They published their research in the November 23, 2000 issue of Nature. The research was funded by the Italian National Institute for the Physics of Matter (INFM).

Timeline:   5-10 years
Funding:   Government
TRN Categories:  Optical Computing, Optoelectronics and Photonics; Semiconductors and Materials
Story Type:   News
Related Elements:   Technical paper, "Optical Gain in Silicon Nanocrystals," Nature, November 23, 2000


December 13, 2000

Page One

Nanotube fabric looms large

Silicon nanocrystals glow

Exploratory robot hops and rolls

Lasers flex gel muscles

Atomic scale wire speeds electrons


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