Researchers have discovered a technique using which graphene nanoribbons with desirable semiconducting properties can directly be grown on a conventional germanium semiconductor wafer.

The discovery will enable manufacturers to easily use graphene nanoribbons in hybrid integrated circuits, which promise to significantly boost the performance of next-generation electronic devices. Researchers reveal that their technology could also have specific uses in industrial and military applications, such as sensors that detect specific chemical and biological species and photonic devices that manipulate light.

The study, published in the journal Nature Communications, describes the method for producing graphene nanoribbons and according to the authors their technique can easily be scaled for mass production and is compatible with the prevailing infrastructure used in semiconductor processing.

Progressively zoomed-in images of graphene nanoribbons grown on germanium. The ribbons automatically align perpendicularly and naturally grow in what is known as the armchair edge configuration. Credit: Arnold Research Group and Guisinger Research Group
Progressively zoomed-in images of graphene nanoribbons grown on germanium. The ribbons automatically align perpendicularly and naturally grow in what is known as the armchair edge configuration. Credit: Arnold Research Group and Guisinger Research Group

A sheet of carbon atoms with only one-atom thickness, graphene, is capable of conducting electricity and dissipating heat much more efficiently than silicon owing to which it has been pegged as one of the most promising replacement candidate; however, to realise graphene’s potential, the material’s nanoribbons need to be less than 10 nanometers wide, which is phenomenally narrow.

Further, these nanoribbons must have smooth, well-defined “armchair” edges in which the carbon-carbon bonds are parallel to the length of the ribbon.

Though current methodologies can be used to fabricate graphene nanoribbons, the lithographic methods also known as “top-down” fabrication approach lacks precision and produces nanoribbons with very rough edges.

Another strategy for making nanoribbons is to use a “bottom-up” approach such as surface-assisted organic synthesis, where molecular precursors react on a surface to polymerize nanoribbons. Arnold says surface-assisted synthesis can produce beautiful nanoribbons with precise, smooth edges, but this method only works on metal substrates and the resulting nanoribbons are thus far too short for use in electronics.

To overcome these hurdles, the UW-Madison researchers pioneered a bottom-up technique in which they grow ultra-narrow nanoribbons with smooth, straight edges directly on germanium wafers using a process called chemical vapor deposition. In this process, the researchers start with methane, which adsorbs to the germanium surface and decomposes to form various hydrocarbons. These hydrocarbons react with each other on the surface, where they form graphene.

The team made its discovery when it explored dramatically slowing the growth rate of the graphene crystals by decreasing the amount of methane in the chemical vapor deposition chamber. They found that at a very slow growth rate, the graphene crystals naturally grow into long nanoribbons on a specific crystal facet of germanium. By simply controlling the growth rate and growth time, the researchers can easily tune the nanoribbon width be to less than 10 nanometers.

“What we’ve discovered is that when graphene grows on germanium, it naturally forms nanoribbons with these very smooth, armchair edges,” said Michael Arnold, an associate professor of materials science and engineering at UW-Madison. “The widths can be very, very narrow and the lengths of the ribbons can be very long, so all the desirable features we want in graphene nanoribbons are happening automatically with this technique.”

The nanoribbons produced with this technique start nucleating, or growing, at seemingly random spots on the germanium and are oriented in two different directions on the surface. Arnold says the team’s future work will include controlling where the ribbons start growing and aligning them all in the same direction.