Cheaper optics-chip link on
tap
By
Eric Smalley,
Technology Research NewsRealizing the full promise of the Internet -- video-on-demand, videoconferencing from your living room and fully-immersive interactive games -- will require a lot more bandwidth than today's cable modems and DSL phone lines offer.
The challenge to speeding up Internet access is finding an affordable way to extend to the home the high-bandwidth fiber-optic lines that make up the bulk of the world's communications infrastructure. Part of the problem is the cost of connecting fiber optics, which carry light signals, to computers, which use electrical signals.
Researchers from the Italian Institute for the Physics of Matter and the University of Rome Three in Italy have discovered an inexpensive, low-temperature method of manufacturing fast photodetectors in the near-infrared light range used in optical communications equipment. Photodetectors convert light pulses to electrical signals.
The device can detect the light pulses that carry digital information at rates as fast as 2.5 gigabits per second, which is a standard speed for communications networks. It detects light at both of the major infrared wavelengths -- 1.3 and 1.5 microns -- used in optical fibers.
The high-speed photodetector can be built into ordinary silicon computer chips because the material it is made of is compatible with silicon, according to Gianlorenzo Masini, an assistant professor of electrical engineering at the University of Rome Three. "Our approach stems from the unique properties of germanium as a material technologically close to silicon," he said.
Infrared light passes through silicon, but is absorbed by germanium. Germanium, however, is not very efficient at converting light to electricity. The researchers found that putting a thin film of germanium on a silicon wafer caused the electrons generated when the germanium absorbed infrared light to pass into the silicon, leading to a faster, more efficient conversion of light to electricity.
The researchers developed a similar device in 1998, but the manufacturing process required temperatures that would have damaged ordinary computer chips. The current device is based on a form of germanium that can be made into thin films at the relatively low temperature of 300 degrees Celsius.
Integrating high-speed photodetectors into silicon chips could make connecting personal computers directly to fiber optic lines economically feasible, according to Masini.
To make the devices, the researchers deposited a mist of polycrystalline germanium onto a monocrystalline silicon substrate to form a 120-nanometer germanium layer. A nanometer is one millionth of a millimeter. The researchers used a light beam to etch raised square areas of germanium, then deposited silver electrical contacts on top of these germanium mesas.
Semiconductors like silicon and germanium can be amorphous, monocrystalline or polycrystalline. The atoms in amorphous materials are arranged randomly. The atoms in monocrystalline materials are arranged in a regular lattice structure. Polycrystalline materials are aggregates of tiny crystals.
Polycrystalline germanium can be manufactured at low temperatures, and monocrystalline silicon is faster than amorphous or polycrystalline silicon at collecting and transporting the energy from the germanium.
The researchers' device is not as responsive to light as many photodetectors, which means the light pulses have to be relatively strong. The researchers are working to increase the device's sensitivity by improving the manufacturing process, said Masini. The responsiveness could also be increased by using waveguides to channel light to the photodetector more efficiently, he said.
The technology could find applications in commercial products in one to three years, he said.
Masini's research colleagues were Lorenzo Colace and Gaetano Assanto. The work appeared in the April 14, 2003 issue of Applied Physics Letters. The research was funded by the Italian Space Agency (ASI) and by the Italian Institute for the Physics of Matter (INFM).
Timeline: 1-3 years
Funding: Government
TRN Categories: Optical Computing, Optoelectronics and Photonics; Internet; Networking
Story Type: News
Related Elements: Technical paper, "2.5 Gbit/s Polycrystalline Germanium-on-Silicon Photodetector Operating from 1.3 to 1.55 Microns," Applied Physics Letters, April 14, 2003.
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July 16/23, 2003
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