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  • Graphene on chip closing the gap with germanium

Photonics Spectra
Dec 2013
VIENNA, CAMBRIDGE, Mass., and HONG KONG – 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,” said 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 2-D carbon atoms
Graphene, a 2-D sheet made of carbon atoms, can convert light into electrical current. Photo courtesy of TU Vienna.


Dirk Englund and colleagues from Columbia University, MIT and the IBM T.J. Watson Research Center report an ultrafast graphene light detector with a responsivity approximately 16 times greater than that of previous graphene-based 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 from 1.31 to 1.65 µm that includes all the bands used by optical fiber communication systems. Its responsivity is about eight times higher than that of earlier graphene light detectors.

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

light signal waveguide
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). Photo 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. The 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.


GLOSSARY
germanium
A crystalline semiconductor material that transmits in the infrared.
optical fiber
A thin filament of drawn or extruded glass or plastic having a central core and a cladding of lower index material to promote total internal reflection (TIR). It may be used singly to transmit pulsed optical signals (communications fiber) or in bundles to transmit light or images.  
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
The study of how light interacts with nanoscale objects and the technology of applying photons to the manipulation or sensing of nanoscale structures.
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