Georgia Tech graduate students Yike Hu and John Hankinson observe a high-temperature furnace used to produce graphene on a silicon wafer. (Photo: Gary Meek) |
Move over silicon. There’s a new electronic material in town, and it goes
fast.
That material is graphene—a fancy name for extremely thin layers of ordinary
carbon atoms arranged in a “chicken-wire” lattice. These layers,
sometimes just a single atom thick, conduct electricity with virtually no
resistance, very little heat generation—and less power consumption than
silicon.
With silicon device fabrication approaching its physical limits, many
researchers believe graphene can provide a new platform material that would
allow the semiconductor industry to continue its march toward ever-smaller and
faster electronic devices—progress described in Moore’s Law. Though graphene will likely
never replace silicon for everyday electronic applications, it could take over
as the material of choice for high-performance devices.
And graphene could ultimately spawn a new generation of devices designed to
take advantage of its unique properties.
Since 2001, Georgia Tech has been developing epitaxial graphene. In a recent
paper published in the journal Nature Nanotechnology, Georgia Tech
researchers reported fabricating an array of 10,000 top-gated transistors on a
0.24 square centimeter chip, an achievement believed to be the highest density
reported so far in graphene devices.
In creating that array, they also demonstrated a new approach for growing
complex graphene patterns on templates etched into silicon carbide. The new
technique offered the solution to one of the most difficult issues that had been
facing graphene electronics.
“This is a significant step toward electronics manufacturing with
graphene,” said Walt de Heer, a professor in Georgia Tech’s School of Physics who pioneered the development of
graphene for high-performance electronics.
Unrolled carbon nanotubes
For de Heer, the story of graphene begins with carbon nanotubes. De Heer was
among the researchers excited about the properties of nanotubes, whose unique
arrangement of carbon atoms gave them physical and electronic properties that
scientists believed could be the foundation for a new generation of electronic
devices.
Carbon nanotubes still have attractive properties, but the ability to grow
them consistently—and to incorporate them in high-volume electronics
applications—has so far eluded researchers. De Heer realized before others that
carbon nanotubes would probably never be used for high-volume electronic
devices.
But he also realized that the key to the attractive electronic properties of
the nanotubes was the lattice created by the carbon atoms. Why not simply grow
that lattice on a flat surface, and use fabrication techniques proven in the
microelectronics industry to create devices in much the same way as silicon
integrated circuits?
By heating silicon carbide, de Heer and his colleagues were able to drive
silicon atoms from the surface, leaving just the carbon lattice in thin layers
of graphene large enough to grow the kinds of electronic devices familiar to a
generation of electronics designers.
That process was the basis for a patent filed in 2003, and for initial
research support from chip-maker Intel. Since then, de Heer’s group has
published dozens of papers and helped spawn other research groups also using
epitaxial graphene for electronic devices. Though scientists are still learning
about the material, companies such as IBM have launched research programs based
on epitaxial graphene, and agencies such as the National Science Foundation
(NSF) and Defense Advanced Research Projects Agency (DARPA) have invested in
developing the material for future electronics applications.
Silicon “running out of gas”
A new electronics material is needed because silicon is running out of
miniaturization room.
“Primarily, we’ve gotten the speed increases from silicon by
continually shrinking feature sizes and improving interconnect
technology,” said Dennis Hess, director of the National Science
Foundation-sponsored Materials Research Science and Engineering Center (MRSEC)
established at Georgia Tech to study future electronic materials, starting with
epitaxial graphene. “We are at the point where in less than 10 years, we
won’t be able to shrink feature sizes any farther because of the physics of the
device operation. That means we will either have to change the type of device
we make, or change the electronic material we use.”
It’s a matter of physics. At the very small size scales needed to create
ever more dense device arrays, silicon generates too much resistance to
electron flow, creating more heat than can be dissipated and consuming too much
power.
Graphene has no such restrictions, and in fact, can provide electron
mobility as much as 100 times better than silicon. De Heer believes his group
has developed the roadmap for the future of high-performance electronics—and
that it is paved with epitaxial graphene.
“We have basically developed a whole scheme for making electronics out
of graphene,” he said. “We have set down what we believe will be the
ground rules for how that will work, and we have the key patents in
place.”
Silicon, of course, has matured over many generations through constant
research and improvement. De Heer and Hess agree that silicon will always be
around, useful for low-cost consumer products such as iPods, toasters, personal
computers, and the like.
De Heer expects graphene to find its niche doing things that couldn’t
otherwise be done.
“We’re not trying to do something cheaper or better; we’re going to do
things that can’t be done at all with silicon,” he said. “Making
electronic devices as small as a molecule, for instance, cannot be done with
silicon, but in principle could be done with graphene. The key question is how
to extend Moore’s
Law in a post-CMOS world.”
Making the best graphene
Since beginning the exploration of graphene in 2001, de Heer and his research
team have made continuous improvements in the quality of the material they
produce, and those improvements have allowed them to demonstrate a number of
physical properties—such as the Quantum Hall Effect—that verify the unique
properties of the material.
“The properties that we see in our epitaxial graphene are similar to
what we have calculated for an ideal theoretical sheet of graphene suspended in
the air,” said Claire Berger, a research scientist in the Georgia Tech
School of Physics who also has a faculty appointment at the Centre National de
la Recherche Scientifique in France.
“We see these properties in the electron transport and we see these
properties in all kinds of spectroscopy. Everything that is supposed to be
occurring in a single sheet of graphene we are seeing in our systems.”
Key to the material’s future, of course, is the ability to make electronic
devices that work consistently. The researchers believe they have almost
reached that point.
“All of the properties that epitaxial graphene needs to make it viable
for electronic devices have been proven in this material,” said Ed Conrad,
a professor in Georgia Tech’s School
of Physics who is also a
MRSEC member. “We have shown that we can make macroscopic amounts of this
material, and with the devices that are scalable, we have the groundwork that
could really make graphene take off.”
Reaching higher and higher device density is also important, along with the
ability to control the number of layers of graphene produced. The group has
demonstrated that in their multilayer graphene, each layer retains the desired
properties.
“Multilayer graphene has different stacking than graphite, the material
found in pencils,” Conrad noted. “In graphite, every layer is rotated
60 degrees and that’s the only way that nature can do it. When we grow graphene
on silicon carbide, the layers are rotated 30 degrees. When that happens, the
symmetry of the system changes to make the material behave the way we want it
to.”
Epitaxial versus exfoliated
Much of the world’s graphene research involved the study of exfoliated
graphene: layers of the material removed from a block of graphite, originally
with tape. While that technique produces high-quality graphene, it’s not clear
how that could be scaled up for industrial production.
While agreeing that the exfoliated material has produced useful information
about graphene properties, de Heer dismisses it as “a science project”
unlikely to have industrial electronics application.
“Electronics companies are not interested in graphene flakes,” he
said. “They need industrial graphene, a material that can be scaled up for
high-volume manufacturing. Industry is now getting more and more interested in
what we are doing.”
De Heer says Georgia Tech’s place in the new graphene world is to focus on
electronic applications.
“We are not really trying to compete with these other groups,” he
said. “We are really trying to create a practical electronic material. To
do that, we will have to do many things right, including fabricating a scalable
material that can be made as large as a wafer. It will have to be uniform and
able to be processed using industrial methods.”
Resolving technical issues
Among the significant technical issues facing graphene devices has been
electron scattering that occurs at the boundaries of nanoribbons. If the edges
aren’t perfectly smooth—as usually happens when the material is cut with
electron beams—the roughness bounces electrons around, creating resistance and
interference.
To address that problem, de Heer and his team recently developed a new
“templated growth” technique for fabricating nanometer-scale graphene
devices. The technique involves 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 of specific widths without the use of e-beams or other destructive
cutting techniques. Graphene nanoribbons produced with these templates have
smooth edges that avoid electron-scattering problems.
“Using this approach, we can make very narrow ribbons of interconnected
graphene without the rough edges,” said de Heer. “Anything that can
be done to make small structures without having to cut them is going to be
useful to the development of graphene electronics because if the edges are too
rough, electrons passing through the ribbons scatter against the edges and reduce
the desirable properties of graphene.”
In nanometer-scale graphene ribbons, quantum confinement makes the material
behave as a semiconductor suitable for creation of electronic devices. But in
ribbons a micron or so wide, the material acts as a conductor. Controlling the
depth of the silicon carbide template allows the researchers to create these
different structures simultaneously, using the same growth process.
“The same material can be either a conductor or a semiconductor
depending on its shape,” noted de Heer. “One of the major advantages
of graphene electronics is to make the device leads and the semiconducting
ribbons from the same material. That’s important to avoid electrical resistance
that builds up at junctions between different materials.”
After formation of the nanoribbons, the researchers apply a dielectric
material and metal gate to construct field-effect transistors. While successful
fabrication of high-quality transistors demonstrates graphene’s viability as an
electronic material, de Heer sees them as only the first step in what could be
done with the material.
“When we manage to make devices well on the nanoscale, we can then move
on to make much smaller and finer structures that will go beyond conventional
transistors to open up the possibility for more sophisticated devices that use
electrons more like light than particles,” he said. “If we can factor
quantum mechanical features into electronics, that is going to open up a lot of
new possibilities.”
Collaborations with other groups
Before engineers can use epitaxial graphene for the next generation of
electronic devices, they will have to understand its unique properties. As part
of that process, Georgia Tech researchers are collaborating with scientists at
the National Institute of Standards and Technology (NIST). The collaboration
has produced new insights into how electrons behave in graphene.
In a recent paper published in the journal Nature Physics, the
Georgia Tech-NIST team described for the first time how the orbits of electrons
are distributed spatially by magnetic fields applied to layers of epitaxial
graphene. They also found that these electron orbits can interact with the
substrate on which the graphene is grown, creating energy gaps that affect how
electron waves move through the multilayer material.
“The regular pattern of magnetically-induced energy gaps in the
graphene surface creates regions where electron transport is not allowed,”
said Phillip N. First, a professor in the Georgia Tech School of Physics and
MRSEC member. “Electron waves would have to go around these regions,
requiring new patterns of electron wave interference.
Understanding this interference would be important for some bi-layer
graphene devices that have been proposed.”
Earlier NIST collaborations led to improved understanding of graphene
electron states, and the way in which low temperature and high magnetic fields
can affect energy levels. The researchers also demonstrated that atomic-scale
moiré patterns, an interference pattern that appears when two or more graphene
layers are overlaid, can be used to measure how sheets of graphene are stacked.
In a collaboration with the U.S. Naval Research Laboratory and Univ. of Illinois at Urbana-Champaign, a group of
Georgia Tech professors developed a simple and quick one-step process for
creating nanowires on graphene oxide.
“We’ve shown that by locally heating insulating graphene oxide, both
the flakes and the epitaxial varieties, with an atomic force microscope tip, we
can write nanowires with dimensions down to 12 nm,” said Elisa Riedo, an
associate professor in the Georgia Tech School of Physics and a MRSEC member.
“And we can tune their electronic properties to be up to four orders of
magnitude more conductive.”
A new industrial revolution?
Though graphene can be grown and fabricated using processes similar to those of
silicon, it is not easily compatible with silicon. That means companies
adopting it will also have to build new fabrication facilities—an expensive
investment. Consequently, de Heer believes industry will be cautious about moving
into a new graphene world.
“Silicon technology is completely entrenched and well developed,”
he admitted. “We can adopt many of the processes of silicon, but we can’t
easily integrate ourselves into silicon. Because of that, we really need a
major paradigm shift. But for the massive electronics industry, that will not
happen easily or gently.”
He draws an analogy to steamships and passenger trains at the dawn of the
aviation age. At some point, it became apparent that airliners were going to
replace both ocean liners and trains in providing first-class passenger
service. Though the cost of air travel was higher, passengers were willing to pay
a premium for greater speed.
“We are going to see a coexistence of technologies for a while, and how
the hybridization of graphene and silicon electronics is going to happen
remains up in the air,” de Heer predicted. “That is going to take
decades, though in the next ten years we are probably going to see real
commercial devices that involve graphene.”
