“We
have shown that we can make the graphene material,
that we can pattern it, and that its transport properties
are very good,” said Walt de Heer, a professor in
Georgia Tech's School of Physics. “The material has
high electron mobility, which means electrons can
move through it without much scattering or resistance.
It is also coherent, which means electrons move through
the graphene much like light travels through waveguides.”
The results should encourage further development
of graphene-based electronics, though de Heer cautions
that practical devices may be a decade away.
“This is really the first step in a very long path,” he
said. “We are at the proof-of principle stage, comparable
to where transistors were in the late 1940s. We have
a lot to do, but I believe this technology will advance
rapidly.”
The research, begun by de Heer's team in 2001, is
supported by the U.S. National Science Foundation
and the Intel Corporation.
In their paper, the researchers report seeing evidence
of quantum confinement effects in their graphene
circuitry, meaning electrons can move through it
as waves. “The graphene ribbons we create are really
like waveguides for electrons,” de Heer said.
Because carbon nanotubes conduct electricity with
virtually no resistance, they have attracted strong
interest for use in transistors and other devices.
However, the discrete nature of nanotubes – and variability
in their properties – pose significant obstacles
to their use in practical devices. By contrast, continuous
graphene circuitry can be produced using standard
microelectronics processing techniques.
“Nanotubes are simply graphene that has been rolled
into a cylindrical shape,” de Heer explained. “Using
narrow ribbons of graphene, we can get all the properties
of nanotubes because those properties are due to
the graphene and the confinement of the electrons,
not the nanotube structures.”
De Heer envisions using the graphene electronics
for specialized applications, potentially within
conventional silicon-based systems.
“We have shown that we can interconnect graphene,
put current into it, and take current out,” he said. “We
have a very promising electronic material. We see
graphene as a platform, a canvas on which we can
work.”
De Heer and collaborators Claire Berger, Zhimin
Song, Xuebin Li, Xiaosong Wu, Nate Brown, Tianbo
Li, Joanna Hass, Alexei Marchenkov, Edward Conrad
and Phillip First of Georgia Tech and Didier Mayou
and Cecile Naud of CNRS start with a wafer of silicon
carbide, a material made up of silicon and carbon
atoms. By heating the wafer in a high vacuum, they
drive silicon atoms from the surface, leaving a thin
continuous layer of graphene.
Next, they spin-coat onto the surface a photo-resist
material of the kind used in established microelectronics
techniques. Using electron-beam lithography, they
produce patterns on the surface, then use conventional
etching processes to remove unwanted graphene.
“We are doing lithography, which is completely familiar
to those who work in microelectronics,” said de Heer. “It's
exactly what is done in microelectronics, but with
a different material. That is the appeal of this
process.”
Using electron beam lithography in Georgia Tech's
Microelectronics Research Center, they've created
feature sizes as small as 80 nanometers. The graphene
circuitry demonstrates high electron mobility – up
to 25,000 square centimeters per volt-second, showing
that electrons move with little scattering. The researchers
expect to see ballistic transport at room temperature
when they make structures small enough.
So far, they have built an all graphene planar field-effect
transistor. The side-gated device produces a change
in resistance through its channel when voltage is
applied to the gate. However, this first device has
a substantial current leak, which the team expects
to eliminate with minor processing adjustments.
The researchers have also built a working quantum
interference device, a ring-shaped structure that
would be useful in manipulating electronic waves.
The key to properties of the new circuitry is the
width of the ribbons, which confine the electrons
in a quantum effect similar to that seen in carbon
nanotubes. The width of the ribbon controls the material's
band-gap. Other structures, such as sensing molecules,
could be attached to the edges of the ribbons, which
are normally passivated by hydrogen atoms.
Beyond coherence and high electron mobility, the
researchers note that the speed of electrons through
the graphene is independent of energy – just like
light waves. The electrons also possess the properties
of Dirac particles, which allow them to travel significant
distances without scattering.
Among the challenges ahead is improving the techniques
for patterning the graphene, since electron transport
is affected by the smoothness of edges in the circuitry.
Researchers will also have to understand the material's
fundamental properties, which could still contain “show-stoppers” that
might make the material impractical.
De Heer has seen hints that graphene may offer some
surprises. “We already have indications of some new
and surprising electronic properties of this material,” he
said. “It is doing things that we have never seen
in two-dimensional materials before.”
Graphics: Photographs of researchers with laboratory
equipment, scanning electron microscope images of
graphene circuitry, prototype graphene device.
Writer: John Toon |
