Apr 4, 2014
Graphene synthesis: Joining the dots
Graphene has been touted as the material to usurp silicon in next-generation electronics, but it remains unlikely without an economic approach to fabricating high-quality, wafer-scale graphene layers. Now, researchers in Korea have demonstrated a way of growing several patches of oriented graphene until they coalesce seamlessly, providing a route to producing high-quality single-crystal graphene over large areas.
"We are now quite close to mass production of wafer-scale single-crystal graphene, like silicon," says Dongmok Whang at Sungkyunkwan University, one of the researchers behind this latest work. "I think these single-crystal graphene monolayers can be cheaper than silicon wafers eventually."
Whang’s team, together with researchers from the Samsung Advanced Institute of Technology, has fabricated simple transistor devices from the material it has grown, and shown that they do indeed deliver the enhanced properties expected for single-crystal silicon.
Large-scale graphene layers have typically been produced using chemical vapour deposition (CVD). However, this process typically creates polycrystalline graphene, with single crystal grains separated by grain boundaries that act as defects and compromise the material’s electrical and mechanical properties.
Dongmok Whang and colleagues at Sungkyunkwan University and Samsung Advanced Institute of Technology in Korea tried growing graphene on a germanium surface and noticed that the grains of graphene seemed to grow with a preferred direction of orientation.
"We thought the structure and symmetry of the substrate surface might be related to the grain orientation, which made us think of using the germanium surface for large-area single-crystal graphene growth," explains Whang.
The germanium advantage
Whang points out that most researchers have grown graphene on metal surfaces. However, since he and his colleagues have been studying nanomaterials of group-IV materials, such as carbon, silicon and germanium for the past decade, he explains: "Studying synergetic effects of carbon and germanium for obtaining new nanomaterials was a natural choice for us."
In fact, germanium has a number of advantages as a substrate for graphene growth. First, it is highly catalytic, which lowers the energy barrier for forming graphitic carbon. Second, well defined germanium crystal surfaces with atoms arranged along one direction are readily available for growing aligned graphene grains. Carbon is extremely insoluble in germanium, which means that carbon grains can grow into a complete monolayer, while graphene and germanium share very similar thermal expansion coefficients to help to prevent the formation of wrinkles.
Optimised conditions for optimum growth
To grow the graphene, the researchers first deposited a single-crystal germanium surface on a silicon wafer, and then flowed low-pressure methane (CH4) over the germanium layer at 900–930 °C in a CVD chamber. Years of previous research with CVD growth helped them to optimize the conditions.
High-resolution electron microscopy images and electron diffraction data confirmed the presence of high-quality monolayer graphene with no defects, no wrinkles and a single orientation for the crystal lattice throughout. Experimenting with a different face of the germanium crystal relative to its lattice structure still yielded good-quality graphene, but in this case it was polycrystalline.
Economies of scale
After lifting off the graphene the germanium substrate could be re-used five times, with no apparent deterioration in the quality of the graphene grown – with the potential for recycling the substrate and no technological limit for mass production. Whang believes the approach could in time be more cost effective than silicon-based electronics.
The researchers have not yet tried using the approach to produce other materials, but point out further opportunities to explore. "Hexagonal crystalline boron hydride might be another possible choice we can try, because it has a structure that is quite similar to graphene," says Whang.
The researchers are also planning to use the approach to develop applications that require orientation-dependent graphene properties, such as in graphene nanoribbon devices.
Full details are reported in Science Express.
About the author
Anna Demming is online editor for nanotechweb.org.