27 January 2010—A new method for producing light in silicon could lead to densely integrated optical sensors .
"It could be a new angle to solve this ongoing problem of getting light out of silicon," says Jurriaan Schmitz, a professor of electrical engineering and head of the semiconductor components research group at the University of Twente, in the Netherlands. "We might have a good one in our hands here."
The technology, described by researchers at Twente and at the FOM Institute in the January issue of Nature Materials, uses oscillations of electrons known as surface plasmons to carry an electromagnetic wave through a silicon chips and emit it as visible light. The process is electrically pumped, instead of being powered by light, as many silicon photonics schemes are. And it is potentially compatible with standard production processes for complementary-metal-oxide semiconductors (CMOS). Both these attributes make the technology appealing for commercialization.
The key to the Dutch team's breakthrough is a complex waveguide for plasmons. Using atomic layer deposition, the team lays down five layers of alumina on a silicon wafer, with a layer of silicon quantum dots deposited by low-pressure chemical-vapor deposition between each layer, making a semi-insulating layer cake about 100 nanometers thick. The quantum dots are nanocrystals of silicon. When stimulated, the dots emit light with wavelengths that are determined by their diameters. The researchers place the alumina/quantum dot structure, in turn, between two layers of gold cladding, each 200 nm thick.
When the researchers apply a voltage across the insulator layer, some electrons can tunnel through the semi-insulating alumina. They transfer their excess energy to the quantum dots, which in turn re-emit it. But because the waveguide is physically too thin to contain a photon, the dots instead emit a surface plasmon polariton (SPP), a charge density wave that travels along the interface between the metal and the insulator.
Depending on the material and how it's been engineered, the SPP can travel for up to about 10 micrometers, like a wave rippling over the surface of a pond. When it reaches a grating inscribed in the gold by the engineers, it exits the device, emerging as a photon with a wavelength determined by the size of the quantum dot.
The Dutch device still needs a lot of development before it is really useful. Its external power efficiency—less than 10-6—was extremely low and will need to be improved by a few orders of magnitude to be practical. But Schmitz says that engineering work aimed at optimizing the layer thickness, improving the quantum dots, and redesigning the surface structure of the component materials may overcome that.
Schmitz says it could take four or five years to develop the technology for practical applications, the first of which will likely be chemical sensors, which require only a modest performance improvement. Higher-power optical interconnects for high-speed data transfer could be further down the road. "I would be extremely reluctant to say we have really solved the problem today," Schmitz admits. However, he says, that the waveguide "does offer a new opportunity to generate light."
About the Author
Neil Savage writes about science and technology from Lowell, Mass. In December 2009, he wrote about how quantum dots can enhance LED lighting.