Researchers from the Graphene Flagship have discovered a novel approach to developing graphene that has potential in photonics, optoelectronics and electronics applications, specifically data communications. Graphene is a two-dimensional atomic crystal made up of carbon atoms arranged in a hexagonal lattice. Due to its
unique combination of superior properties, graphene is a credible starting
point for new disruptive technologies across a wide range of fields.
Typically, polycrystalline graphene is used in applications which need
to be integrated over wafer scale, however this material presents grain
boundaries between the different crystals that lower the exceptional elec-
trical properties that have been measured in the exfoliated single crystals
that made graphene famous.
The method—developed by Graphene Flagship researchers Camilla Coletti from
Istituto Italiano di Tecnologia (IIT) and Marco Romagnoli from Consorzio Nazionale
Interuniversitario per le Telecomunicazioni (CNIT) both working in Italy—is designed
to grow single graphene crystals by chemical vapor deposition on copper “seeds” deposited using optical lithography.
It results in graphene that is more mobile than graphene grown in a continuous
film and also exhibits electric properties that are comparable to those achieved from
pristine graphene exfoliated from flakes.
How it works
This flexible “seeding approach” produces high quality graphene to be grown in arrays
with different spacing and dimensions. Metallic “seeds,” from which the graphene
crystals grow, are deposited with optical lithography. This growth approach allows
graphene to grow only where it is needed, reducing the amount of the material
necessary. For example, on optoelectronic/photonic devices it is known where a device
will be placed, so the crystals are grown just in the areas where they are needed.
Working with “patches” of graphene rather than a continuous film also reduces
the difficulty associated with transferring a 12-inch, one-atom-thin wafer, including
adhesion issues, strain and wrinkles.
The end result is a process that is simpler and yields higher-quality graphene
crystals that display the same conductive properties found in graphene that has
been mechanically exfoliated from a flake. Furthermore, the process is fast, scalable,
consistent and safe (utilizing less than 1.25 percent explosive gases), making it ideal
for larger scale industrial production in a robust and low-cost manufacturing line.
While further research into the quality and performance of devices using this
type of graphene crystal is necessary, the time to market for this technology is close
enough to match the market maturity for the high-speed transmitters and receivers
that the team is targeting.
Large, single crystal graphene vs polycrystalline graphene
Single-crystal has various advantages over traditional poly crystalline graphene.
While the grain size for polycrystalline graphene is 5-20 μm, single crystals can
have a grain of up to 4cm, and while 95 percent of polycrystalline graphene is a
monolayer, single crystal graphene allows a great deal of control over the thickness.
Carrier mobility for single crystal graphene has been measured at RT 300 000 cm2/
Vs, higher than for polycrystalline graphene which is limited to 1000-3000 cm2/ Vs
by the presence of grain boundaries. The positioning of the single crystals, however,
is not controlled.