Georgia Tech graduate student Baiqian Zhang and undergraduate student Holly Tinkey observe a high-temperature furnace used to produce epitaxial graphene on a silicon carbide wafer. Credit: Georgia Tech |
A new “templated growth” technique for fabricating nanoribbons of
epitaxial graphene has produced structures just 15 to 40 nm wide that conduct
current with almost no resistance. These structures could address the challenge
of connecting graphene devices made with conventional architectures—and set the
stage for a new generation of devices that take advantage of the quantum
properties of electrons.
“We can now make very narrow, conductive nanoribbons that have quantum
ballistic properties,” said Walt de Heer, a professor in the School of Physics at the Georgia Institute of
Technology. “These narrow ribbons become almost like a perfect metal.
Electrons can move through them without scattering, just like they do in carbon
nanotubes.”
First reported Oct. 3 in the advance online edition of the journal Nature
Nanotechnology, the new fabrication technique allows production of
epitaxial graphene structures with smooth edges. Earlier fabrication techniques
that used electron beams to cut graphene sheets produced nanoribbon structures
with rough edges that scattered electrons, causing interference. The resulting
nanoribbons had properties more like insulators than conductors.
“In our templated growth approach, we have essentially eliminated the
edges that take away from the desirable properties of graphene,” de Heer
explained. “The edges of the epitaxial graphene merge into the silicon
carbide, producing properties that are really quite interesting.”
The templated growth technique begins with etching patterns into the silicon
carbide surfaces on which epitaxial graphene is grown. The patterns serve as
templates directing the growth of graphene structures, allowing the formation
of nanoribbons and other structures of specific widths and shapes without the
use of cutting techniques that produce the rough edges.
A team of Georgia Tech researchers led by Professor Walt de Heer has pioneered techniques for fabricating epitaxial graphene nanoribbons using a templated growth technique. The equipment shown behind de Heer is used to characterize graphene properties. Credit: Georgia Tech |
In creating these graphene nanostructures, de Heer and his research team
first use conventional microelectronics techniques to etch tiny
“steps”—or contours—into a silicon carbide wafer whose surface has
been made extremely flat. They then heat the contoured wafer to approximately
1,500 degrees Celsius, which initiates melting that polishes any rough edges
left by the etching process.
Established techniques are then used for growing graphene from silicon
carbide by driving the silicon atoms from the surface. Instead of producing a
consistent layer of graphene across the entire surface of the wafer, however,
the researchers limit the heating time so that graphene grows only on portions
of the contours.
The width of the resulting nanoribbons is proportional to the depth of the
contours, providing a mechanism for precisely controlling the nanoribbon
structures. To form complex structures, multiple etching steps can be carried
out to create complex templates.
“This technique allows us to avoid the complicated e-beam lithography
steps that people have been using to create structures in epitaxial
graphene,” de Heer noted. “We are seeing very good properties that
show these structures can be used for real electronic applications.”
Since publication of the Nature Nanotechnology paper, de Heer’s
team has been refining its technique. “We have taken this to an extreme—the
cleanest and narrowest ribbons we can make,” he said. “We expect to
be able to do everything we need with the size ribbons that we are able to make
right now, though we probably could reduce the width to 10 nm or less.”
While the Georgia Tech team is continuing to develop high-frequency
transistors—perhaps even at the terahertz range—its primary effort now focuses
on developing quantum devices, de Heer said. Such devices were envisioned in
the patents Georgia Tech holds on various epitaxial graphene processes.
“This means that the way we will be doing graphene electronics will be
different,” he explained. “We will not be following the model of
using standard field-effect transistors (FETs), but will pursue devices that
use ballistic conductors and quantum interference. We are headed straight into
using the electron wave effects in graphene.”
Georgia Tech graduate students Yike Hu and John Hankinson observe a high-temperature furnace used to produce epitaxial graphene on a silicon carbide wafer. Credit: Georgia Tech |
Taking advantage of the wave properties will allow electrons to be
manipulated with techniques similar to those used by optical engineers. For
instance, switching may be carried out using interference effects—separating
beams of electrons and then recombining them in opposite phases to extinguish
the signals.
Quantum devices would be smaller than conventional transistors and operate
at lower power. Because of its ability to transport electrons with virtually no
resistance, epitaxial graphene may be the ideal material for such devices, de
Heer said.
“Using the quantum properties of electrons rather than the standard
charged-particle properties means opening up new ways of looking at
electronics,” he predicted. “This is probably the way that
electronics will evolve, and it appears that graphene is the ideal material for
making this transition.”
De Heer’s research team hopes to demonstrate a rudimentary switch operating
on the quantum interference principle within a year.
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 said.
“This is an important step in the process,” he added. “There
are going to be a lot of surprises as we move into these quantum devices and
find out how they work. We have good reason to believe that this can be the
basis for a new generation of transistors based on quantum interference.”