IBM Cuts Noise Ratio in Bilayer Graphene
David Lammers, News Editor -- Semiconductor International, 3/6/2008 8:02:00 AM
Scientists at IBM’s T. J. Watson Research Center (Yorktown Heights, N.Y.) have discovered that bilayers of graphene suppress noise much more effectively than single layers, leading to increased hope for graphene as a contender for post-silicon CMOS devices.
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| Phaedon Avouris, IBM Fellow |
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| Yu-Ming Lin, researcher, IBM Watson Laboratory |
While graphene is a zero-bandgap semiconductor, researchers have learned to apply an external voltage to create a tunable bandgap of as much as ~300 MeV, sufficient for room temperature operation.
However, transistors made with a single layer of graphene suffer from external disturbances caused by impurities that scatter the carriers in the monolayer channel, Lin said. In the case of bilayer graphene, noise is reduced by a coupling between the layers, in which the two sheets form a noise suppression system by responding to each other in ways that cancel out the external disturbance.
“The interaction between the two layers gives rise to efficient noise cancellation,” Lin said, with the bilayer system having a 10× reduction compared with the monolayer devices.
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| IBM scientists found that bilayers of graphene have a 10× improvement in signal-to-noise ratio relative to monolayers. |
Noise is a major problem in nanoscale devices of any material stack, including silicon. In the atomic-scale dimensions of graphene, the signal-to-noise ratio can limit its usefulness as the noise overwhelms the signal. “As we scale down transistors, the problem is noise,” Avouris said. “Noise inevitably becomes larger, and that eventually limits the usefulness of all these nanoscale devides.”
“The effect of noise from Hooge’s rule is exaggerated at the nanoscale because the dimensions are approaching the nearly smallest limits, down to only a handful of atoms, and the noise that is created can overwhelm the electrical signal that needs to be achieved to be useful,” Avouris said.
While IBM is ultimately pursuing digital switching with carbon nanotubes (CNTs) and graphene, the zero-bandgap properties of graphene may open up early opportunities in interconnects, sensors and communications applications.
“The signal-to-noise ratio is one of most important figures of merit in terms of the smallest signal that can be detected. With reduced noise in bilayer graphene, we believe we can improve the performance and applicability of nanoscale devices. Graphene has a zero bandgap. There is no way to change that. It will always have a zero gap. That can be good for certain applications, though in logic we need a large on-off ratio,” Avouris said.
Although IBM may first try to apply CNTs and graphene to analog and RF applications where the bandgap is less important, Avouris said, “We are patient, and we are in this for the long term. We may come up first with solutions for RF and analog, but that doesn’t mean we are going to abandon digital. And we understand that what we do must have some advantage over what is already out there, or we are wasting our time and IBM’s money.”
With high carrier mobility and a desirable effective mass, graphene “makes people very excited. Now we can use it in the lab and say it is exciting. However, there are many, many technological problems before we can use it in a real-world technology,” Avouris said.
Lin said the biggest challenge for graphene-based transistors is to achieve a uniform and well-controlled growth of the material over a large area. Some research groups start with a silicon carbon substrate and use very high temperatures to remove silicon atoms, leaving the carbon. This decomposition approach has shown “very promising preliminary results,” Lin said.
At IBM, the research group uses the mechanical exfoliation approach, which basically involves using tape to peel off a layer of graphene that is then transferred directly onto a silicon substrate. While Lin said this approach “does not work 100% of the time, it gives us enough samples to study.”
“Right now, we can make single transistors from single and bilayer nanoribbons. In the future, we need to optimize the materials and figure out the best parameters so we can show the best performance. For this device to be viable, noise is only one of the issues. Other parameters have to be optimized as well,” Lin said.
