Unruly Electrons Quieted

Noise in graphene-based semiconductor devices can, in fact, be suppressed, it has been discovered.

IBM scientists said they have found a way to suppress unwanted interference of electrical signals created when shrinking graphene -- a two-dimensional, single-atomic layer thick form of graphite -- to dimensions just a few atoms long.

One problem in using these devices is the inverse relationship between the size of the device and the amount of uncontrolled electrical noise that is generated. As they are made smaller and smaller, the noise -- electrical charges that bounce around the material causing all sorts of interference that impede their usefulness -- increases inversely. This trend, known as Hooge's rule, occurs in traditional silicon-based devices as well as in graphene nano-ribbons and carbon nanotube-based devices.

At left is a single layer, or sheet of carbon molecules known as graphene. The noise that occurs from electrical signals in the material as a current is passed through increases as the device is made smaller and smaller, impeding performance for nanoscale electronics. At right, as IBM scientists demonstrated, adding a second sheet of graphene reduces the noise significantly, making it potentially useful in nanoelectronics.

"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," said IBM Researcher Phaedon Avouris, who leads IBM's exploration into carbon nanotubes and graphene. "To quote the famous physicist Rolf Landauer, at the nanoscale 'the noise is your signal'; in other words, you cannot produce any useful electronic device at the nanoscale if the noise is comparable to the signal you are trying to switch on and off."

The researchers reported (Nano Letters; "Strong Suppression of Electrical Noise in Bilayer Graphene Nanoribbons," by Yu-Ming Lin and Phaedon Avouris of IBM's T.J.Watson Research Center) that the noise in graphene-based semiconductor devices can, in fact, be suppressed. They first used a single layer, or sheet, of graphene to build a transistor and noted that the device does in fact follow Hooge's Rule: As they are made smaller and smaller, the noise created increases.

However, when they built the same device with two sheets of graphene instead of one -- one stacked on top of the other -- they noted that the noise is suppressed, and that it is weak enough that these so-called bilayer graphene ribbons could be useful for building semiconductor devices such as sensors, communications devices and computing systems. The noise is inhibited because of the strong electronic coupling between the two graphene layers that counteracts the influence of the noise sources -- the system acts as a noise insulator.

The findings provide exciting opportunities for graphene bilayers in a variety of applications, the researchers said. They added that more detailed analysis and studies are required to better understand the phenomena.

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