Fabrication Technique Gives Graphene the Flexibility for Future Transistors

In 2012, the IBM Watson Research Center's 3D transistors featured in the Ivy Bridge design. Researchers worldwide are looking to another medium to take silicon's place -- something that has superior conductivity, produces lower noise, and can operate at astronomical frequencies that silicon could never achieve.

That material is poised to be grapheme -- a one-atom-thick sheet of pure carbon arranged in a honeycomb lattice of bonded carbon atoms. It's relatively hard to produce in large quantities but has an incredibly high electron mobility rate.

Concept of measuring graphene in silicon carbide trenches, ultimately showing a large electrical band-gap.
(Source: Georgia Tech)

Attempts to use or implement graphene in electronic components such as transistors has been frustratingly slow. The material's intrinsic physical properties make turning off electrical current flow difficult, which negates the band-gap (on/off state) needed to make the transistor function in a stable manner. A research team led by Dr. Zakaria Moktadir from the University of Southampton overcame this problem in 2011 by introducing sharp bends or corners (singularities) into bilayer graphene nanowires. This enabled the team to control the current in the on/off state. The team etched the singularities into the graphene using a focused gallium-ion beam. However, this method places limits on how small the transistors can become before the band-gap disappears entirely.

This was the challenge presented to research scientists from the Georgia Institute of Technology, who created a stable transistor using narrow nano-ribbons. The team, led by Professor Ed Conrad, got around the band-gap issue by including the singularities in a more simplified fashion.

The process begins by using e-beams (electron beam lithography) to cut small, rough trenches into highly polished silicon-carbide wafers. The wafers are put into a high-temperature furnace. This facilitates the growth of thousands of graphene nano-ribbons across the engraved trenches using photolithography. Light transfers geometric patterns using a photomask (an opaque plate with holes that allow light to pass through) to the light-sensitive chemical resist, acting like a glue.

During the growth process, the nano-trench edges become smooth as the material attempts to cure back into a flat surface. Careful monitoring is critical at this point to keep the wafer from melting and control the growth direction of the nano-ribbons, so they form the steps needed to facilitate the band-gap.

The researchers say this method allowed them to create a band-gap with 0.5 electron-volts in 1.4-nm bent sections of the graphene ribbons. This creates not only a fully functional band-gap in the graphene material, but also the possibility of fabricating an entire circuit out of the graphene material. Their method could be utilized in ultra-fast electronics that use all-carbon integrated circuits in the near future.