Georgia Tech graduate students Yike Hu and John Hankinson observe a high temperature furnace used to produce graphene on a silicon carbide wafer. Georgia Tech researchers have explained the details of their “confinement controlled sublimation” technique for producing high-quality graphene. Photo: Gary Meek |
Scientists from the Georgia Institute of Technology have for the first time
provided details of their “confinement controlled sublimation” technique for
growing high-quality layers of epitaxial graphene on silicon carbide wafers.
The technique relies on controlling the vapor pressure of gas-phase silicon in
the high-temperature furnace used for fabricating the material.
The basic principle for growing thin layers of graphene on silicon carbide
requires heating the material to about 1,500 C under high vacuum. The heat
drives off the silicon, leaving behind one or more layers of graphene. But
uncontrolled evaporation of silicon can produce poor quality material useless
to designers of electronic devices.
“For growing high-quality graphene on silicon carbide, controlling the
evaporation of silicon at just the right temperature is essential,” says Walt
de Heer, a professor who pioneered the technique in the Georgia Tech School of
Physics. “By precisely controlling the rate at which silicon comes off the
wafer, we can control the rate at which graphene is produced. That allows us to
produce very nice layers of epitaxial graphene.”
De Heer and his team begin by placing a silicon carbide wafer into an
enclosure made of graphite. A small hole in the container controls the escape
of silicon atoms as the one-square-centimeter wafer is heated, maintaining the
rate of silicon evaporation and condensation near its thermal equilibrium. The
growth of epitaxial graphene can be done in a vacuum or in the presence of an
inert gas such as argon, and can be used to produce both single layers and
multiple layers of the material.
“This technique seems to be completely in line with what people might one
day do in fabrication facilities,” de Heer says. “We believe this is quite
significant in allowing us to rationally and reproducibly grow graphene on
silicon carbide. We feel we now understand the process, and believe it could be
scaled up for electronics manufacturing.”
The technique for growing large-area layers of epitaxial graphene was
described in an Early Edition of the Proceedings of the National Academy of
Sciences. The research has been supported by the National Science
Foundation through the Georgia Tech Materials Research Science and Engineering
Center (MRSEC), the Air Force Office of Scientific Research, and the W.M. Keck
Foundation.
The paper also describes a technique for growing narrow graphene ribbons, a process
de Heer’s group has called “templated growth.” That technique, which could be
useful for making graphene interconnects, was first described in October 2010
in Nature Nanotechnology.
The templated growth technique involves etching patterns into silicon
carbide surfaces using conventional nanolithography processes. The patterns
serve as templates directing the growth of graphene structures on portions of
the patterned surfaces. The technique forms nanoribbons of specific widths
without the use of electron beams or other destructive cutting techniques.
Graphene nanoribbons produced with these templates have smooth edges that avoid
problems with electron scattering.
Together, the two techniques provide researchers with the flexibility to
produce graphene in forms appropriate to different needs, de Heer notes.
Large-area sheets of graphene may be grown on both the carbon-terminated and
silicon-terminated sides of a silicon carbide wafer, while the narrow ribbons
may be grown on the silicon-terminated side. Because of different processing
techniques, only one side of a particular wafer can be used.
The Georgia Tech research team—which includes Claire Berger, Ming Ruan, Mike
Sprinkle, Xuebin Li, Yike Hu, Baiqian Zhang, John Hankinson, and Edward Conrad—has
so far fabricated structures as narrow as 10 nm using the templated growth
technique. These nanowires exhibit interesting quantum transport properties.
“We can make very good quantum wires using the templated growth technique,”
de Heer says. “We can make large structures and devices that demonstrate the
Quantum Hall Effect, which is important for many applications. We have
demonstrated that templated growth can go all the way down to the nanoscale,
and that the properties get even better there.”
Development of the sublimation technique arose from efforts to protect the
growing graphene from oxygen and other contaminants in the furnace. To address
the quality concerns, the research team tried enclosing the wafer in a graphite
container from which some silicon gas was permitted to leak out.
“We soon realized that graphene grown in the container was much better than
what we had been producing,” de Heer recalls. “Originally, we thought it was
because we were protecting it from contaminants. Later, we realized it was
because we were controlling the evaporation of silicon.”
Epitaxial graphene may be the basis for a new generation of high-performance
devices that will take advantage of the material’s unique properties in
applications where higher costs can be justified. Silicon, today’s electronic
material of choice, will continue to be used in applications where
high-performance is not required, de Heer says.
Though researchers are still struggling to design nanometer-scale epitaxial
graphene devices that take advantage of the material’s unique properties, de
Heer is confident that will ultimately be done.
“These techniques allow us to make accurate nanostructures and seem to be
very promising for making the nanoscale devices that we need,” he says. “While there
are serious challenges ahead for using graphene in electronics, we have
overcome roadblocks before.”
