March 31, 2010

IBM has Graphene and Germanium Photodetectors That Will Enable Low Power Ultrafast Terahertz OnChip Communication

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1. IBM Research has demonstrated an optical link using a graphene photodetector fabricated on a silicon-on-insulator (SOI) substrate.

The 10 Gbit per second receiver uses a novel interdigitated source and drain on a field-effect transistor (FET) with graphene as the channel.

The vertical-incidence metal-graphene-metal photodetector achieved 6.1 milliamps per watt at the communications wavelength of 1.55 microns, but was shown to be useful over a very wide bandwidth of 300 nanometers to 6 microns, making the graphene optical link a promising candidate not only for communications, but for remote sensing, environmental monitoring and surveillance.

IBM demonstrated that graphene could be used to fabricate photodetectors that run as fast as 40 Gbps and predicted that by going to palladium electrodes near terahertz speed could eventually be achieved.

To achieve the world's first optical data link using graphene, IBM fabricated an asymetrical metal-graphene-metal FET that used palladium and titanium as the source and drain electrodes, respectively, and graphene as the channel.





2. IBM recently developed the nanophotonic avalanche photodetector which is based on Germanium

The IBM device explores the “avalanche effect” in Germanium, a material currently used in production of microprocessor chips. Analogous to a snow avalanche on a steep mountain slope, an incoming light pulse initially frees just a few charge carriers which in turn free others until the original signal is amplified many times. Conventional avalanche photodetectors are not able to detect fast optical signals because the avalanche builds slowly.

“This invention brings the vision of on-chip optical interconnections much closer to reality,” said Dr. T.C. Chen, vice president, Science and Technology, IBM Research. “With optical communications embedded into the processor chips, the prospect of building power-efficient computer systems with performance at the Exaflop level might not be a very distant future.”

The avalanche photodetector demonstrated by IBM is the world’s fastest device of its kind. It can receive optical information signals at 40Gbps (billion bits per second) and simultaneously multiply them tenfold. Moreover, the device operates with just a 1.5V voltage supply, 20 times smaller than previous demonstrations. Thus many of these tiny communication devices could potentially be powered by just a small AA-size battery, while traditional avalanche photodetectors require 20-30V power supplies.

In IBM’s device, the avalanche multiplication takes place within just a few tens of nanometers (one-thousandths of a millimeter) and that happens very fast. The tiny size also means that multiplication noise is suppressed by 50% - 70% with respect to conventional avalanche photodetectors. The IBM device is made of Silicon and Germanium, the materials already widely used in production of microprocessor chips. Moreover it is made with standard processes used in chip manufacturing. Thus, thousands of these devices can be built side-by-side with silicon transistors for high-bandwidth on-chip optical communications.

The Avalanche Photodetector achievement, which is the last in a series of prior reports from IBM Research, is the last piece of the puzzle that completes the development of the “nanophotonics toolbox” of devices necessary to build the on-chip interconnects.



FURTHER READING
IBM photonics research website

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