Researchers at the Georgia Institute of Technology’s School of Physics have developed what they call a templated growth technique for fabricating nanoribbons of epitaxial graphene (see the figure). An allotrope of carbon, graphene can be simply described as carbon sheet measuring about one atom thick. The technique can render structures measuring 15 to 40 nm wide with virtually zero resistance to current flow.
“We can now make very narrow, conductive nanoribbons \\[from graphene\\] that have quantum ballistic properties,” says professor Walt de Heer. “These narrow ribbons become almost like a perfect metal. Electrons can move through them without scattering, just like they do in carbon nanotubes.”
In addition to creating ever smaller and lighter electronics, these graphene structures could usher in a unique generation of components and products that can fully exploit the quantum properties of electrons.
The technique starts with the researchers employing conventional microelectronics techniques to etch patterns or contours into a flat silicon-carbide wafer. These contours and patterns control the direction of growth for the graphene structures.
Acting like templates, they enable the formation of graphene nanoribbons of specific shapes and sizes. When etching is complete, the team heats the contoured wafer to approximately 1500°C. The melting process that ensues polishes any rough edges left by the etching process. Apparently this technique eliminates the need for cutting the wafer, an approach that normally creates rough edges.
“We have essentially eliminated the edges that take away from the desirable properties of graphene,” de Heer says. “The edges of the epitaxial graphene merge into the silicon carbide, producing properties that are really quite interesting.”
The next step involves employing methods for growing graphene from silicon carbide by driving off the silicon atoms from the surface of the wafer. The researchers limit heating time so the graphene will only grow on portions of the patterns or contours, an approach that does not exactly produce a consistent layer of graphene across the full surface of the wafer. However, the grown nanoribbons exhibit a width that is proportional to the depth of the contours.
In addition to “providing a mechanism for precisely controlling the nanoribbon structures,” de Heer says, the technique allows the researchers to avoid the complicated e-beam lithography steps that people have been using to create structures in epitaxial graphene.
“We are seeing very good properties that show these structures can be used for real electronic applications,” de Heer says.
One of those applications is the development of high-frequency transistors operating, possibly, in the terahertz range. Since the material can transport electrons with no resistance, epitaxial graphene may be ideal for such devices.
“The way we will be doing graphene electronics will be different,” de Heer forecasts. “We will not be following the model of using standard field-effect transistors, but will pursue devices that use ballistic conductors and quantum interference. We are headed straight into using the electron wave effects in graphene.”
Essentially, the quantum graphene devices will be smaller than conventional transistors and operate at lower power. The researchers hope to create a functional switch using the quantum interference principle before the end of the year.
Other Graphene Candidates
We recently looked at some research involving the creation of transparent electrodes using graphene (see “Transparent Electrodes Forecast Flexible Electronics As Graphene Abets The Effort,” March 10, p. 72).
Led by UCLA assistant professor of materials science and engineering Suneel Kodambaka, research teams from UCLA Engineering, Sandia National Laboratories, and the Colorado School of Mines found that the electronic properties of graphene rely on its crystalline properties and a metal contact. Its extremely thin makeup and high carrier ability make it more than viable for use in low-power applications as well as a material for creating transparent electrodes.