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Graphene on chip closing the gap with germanium

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Graphene-based photodetectors can efficiently convert IR light into electrical signals, three independent studies have reported. The work “makes it very likely that graphene will soon replace germanium and compound semiconductors in high-performance light detectors,” report editors at Nature Photonics, which published the papers.

Graphene – a single layer of carbon atoms arranged in a honeycomb lattice – has exceptional electrical and optical properties and is being pursued as a more attractive alternative to germanium or compound semiconductors for silicon-based photonics. Attempts to integrate photodetectors made of materials such as germanium onto a chip have resulted in bandgap-limited detectors that can process light of only a specific wavelength range. But graphene, a zero-bandgap material, has been shown to convert all wavelengths used in telecommunications equally well, and recent graphene integration work has yielded high-performance optoelectronic devices such as modulators, polarizers and photodetectors.

graphene sheet
The light signal arrives through a waveguide (left) in the 2-µm-wide graphene sheet, and electrical current is generated. GND = ground (the ground electrode). Courtesy of TU Vienna.


Dirk Englund and colleagues at Columbia University, MIT and the IBM T.J. Watson Research Center report an ultrafast graphene light detector with a responsivity about 16 times greater than that of previous graphene detectors over a broad bandwidth of 1.45 to 1.59 µm.

Although that amount of current still lags behind germanium photodetectors, “the gap is closing very, very quickly,” Englund said. His team’s work appeared in Nature Photonics (doi: 10.1038/nphoton.2013.253).

Thomas Müller and colleagues at Vienna University of Technology, working with teammates from Johannes Kepler University in Linz, Austria, describe a graphene light detector with multigigahertz operation for a 1.31- to 1.65-µm range that includes all bands used by optical fiber communication systems. Responsivity is about eight times higher than that of earlier graphene light detectors.

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“A narrow waveguide with a diameter of about 200 by 500 nanometers carries the optical signal to the graphene layer,” Müller said. “There the light is converted into an electrical signal, which can then be processed in the chip.”

2-D sheet carbon atoms
Graphene, a 2-D sheet made of carbon atoms, can convert light into electrical current. Courtesy of TU Vienna.


Not only is Müller’s photodetector extremely fast, but it can also be extremely compact, with 20,000 detectors fitting onto a single chip with a surface area of 1 sq cm. In theory, such a chip could receive data separately through each of those 20,000 channels. This research was published in Nature Photonics (doi: 10.1038/nphoton.2013.240).

Xiaomu Wang and colleagues at The Chinese University of Hong Kong recently reported fabricating a high-responsivity graphene photodiode that operates at mid-IR frequencies, with potential applications in environmental monitoring and on-chip IR spectroscopy for medical testing. The work was published in Nature Photonics (doi: 10.1038/nphoton.2013.241).

“In addition to having a very wide wavelength detection range, high-speed operation, a low dark current, a good internal quantum efficiency and a small device footprint, this emerging technology also benefits from the mass production of graphene film and its compatibility with CMOS and other aspects of the mature silicon industry,” wrote Ming Liu and Xiang Zhang of the University of California, Berkeley, in a “News & Views” article in Nature Photonics accompanying the research.

Published: December 2013
Glossary
germanium
A crystalline semiconductor material that transmits in the infrared.
optical fiber
Optical fiber is a thin, flexible, transparent strand or filament made of glass or plastic used for transmitting light signals over long distances with minimal loss of signal quality. It serves as a medium for conveying information in the form of light pulses, typically in the realm of telecommunications, networking, and data transmission. The core of an optical fiber is the central region through which light travels. It is surrounded by a cladding layer that has a lower refractive index than...
photodiode
A two-electrode, radiation-sensitive junction formed in a semiconductor material in which the reverse current varies with illumination. Photodiodes are used for the detection of optical power and for the conversion of optical power to electrical power. See avalanche photodiode; PIN photodiode.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
nanophotonics
Nanophotonics is a branch of science and technology that explores the behavior of light on the nanometer scale, typically at dimensions smaller than the wavelength of light. It involves the study and manipulation of light using nanoscale structures and materials, often at dimensions comparable to or smaller than the wavelength of the light being manipulated. Aspects and applications of nanophotonics include: Nanoscale optical components: Nanophotonics involves the design and fabrication of...
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