Today's transistors and light emitting diodes (LED) are based on silicon and
gallium arsenide semiconductors, which have fixed electronic and optical properties.
Now, University of California,
Berkeley, researchers have shown that a form of carbon called graphene has
an electronic structure that can be controlled by an electrical field, an effect
that can be exploited to make tunable electronic and photonic devices.
While such properties were predicted for a double layer of graphene, this is
the first demonstration that bilayer graphene exhibits an electric field-induced,
broadly tunable bandgap, according to principal author Feng Wang, UC Berkeley
assistant professor of physics.
The bandgap of a material is the energy difference between electrons residing
in the two most important states of a material - valence band states and conduction
band states - and it determines the electrical and optical properties of the
"The real breakthrough in materials science is that for the first time
you can use an electric field to close the bandgap and open the bandgap. No
other material can do this, only bilayer graphene," Wang said.
Because tuning the bandgap of bilayer graphene can turn it from a metal into
a semiconductor, a single millimeter-square sheet of bilayer graphene could
potentially hold millions of differently tuned electronic devices that can be
reconfigured at will, he said.
Wang, post-doctoral fellow Yuanbo Zhang, graduate student Tsung-Ta Tang and
their UC Berkeley and Lawrence Berkeley National Laboratory (LBNL) colleagues
report their success in the June 11 issue of Nature.
"The fundamental difference between a metal and a semiconductor is this
bandgap, which allows us to create semiconducting devices," said coauthor
Michael Crommie, UC Berkeley professor of physics. "The ability to simply
put a material between two electrodes, apply an electric field and change the
bandgap is a huge deal and a major advance in condensed matter physics, because
it means that in a device configuration we can change the bandgap on the fly
by sending an electrical signal to the material."
Graphene is a sheet of carbon atoms, each atom chemically bonded to its three
neighbors to produce a hexagonal array that looks a lot like chicken wire. Since
it was first isolated from graphite, the material in pencil lead, in 2004, it
has been a hot topic of research, in part because solid state theory predicts
unusual electronic properties, including a high electron mobility more than
10 times that of silicon.
However, the property that makes it a good conductor - its zero bandgap - also
means that it's always on.
"To make any electronic device, like a transistor, you need to be able
to turn it on or off," Zhang said. "But in graphene, though you have
high electron mobility and you can modulate the conductance, you can't turn
it off to make an effective transistor."
Semiconductors, for example, can be turned off because of a finite bandgap
between the valence and conduction electron bands.
While a single layer of graphene has a zero bandgap, two layers of graphene
together theoretically should have a variable bandgap controlled by an electrical
field, Wang said. Previous experiments on bilayer graphene, however, have failed
to demonstrate the predicted bandgap structure, possibly because of impurities.
Researchers obtain graphene with a very low-tech method: They take graphite,
like that in pencil lead, smear it over a surface, cover with Scotch tape and
rip it off. The tape shears the graphite, which is just billions of layers of
graphene, to produce single- as well as multi-layered graphene.
Wang, Zhang, Tang and their colleagues decided to construct bilayer graphene
with two voltage gates instead of one. When the gate electrodes were attached
to the top and bottom of the bilayer and electrical connections (a source and
drain) made at the edges of the bilayer sheets, the researchers were able to
open up and tune a bandgap merely by varying the gating voltages.
The team also showed that it can change another critical property of graphene,
its Fermi energy, that is, the maximum energy of occupied electron states, which
controls the electron density in the material.
"With top and bottom gates on bilayer graphene, you can independently
control the two most important parameters in a semiconductor: You can change
the electronic structure to vary the bandgap continuously, and independently
control electron doping by varying the Fermi level," Wang said.
Because of charge impurities and defects in current devices, the graphene's
electronic properties do not reflect the intrinsic graphene properties. Instead,
the researchers took advantage of the optical properties of bandgap materials:
If you shine light of just the right color on the material, valence electrons
will absorb the light and jump over the bandgap.
In the case of graphene, the maximum bandgap the researchers could produce
was 250 milli-electron volts (meV). (In comparison, the semiconductors germanium
and silicon have about 740 and 1,200 meV bandgaps, respectively.) Putting the
bilayer graphene in a high intensity infrared beam produced by LBNL's Advanced
Light Source (ALS), the researchers saw absorption at the predicted bandgap
energies, confirming its tunability.
Because the zero to 250 meV bandgap range allows graphene to be tuned continuously
from a metal to a semiconductor, the researchers foresee turning a single sheet
of bilayer graphene into a dynamic integrated electronic device with millions
of gates deposited on the top and bottom.
"All you need is just a bunch of gates at all positions, and you can change
any location to be either a metal or a semiconductor, that is, either a lead
to conduct electrons or a transistor," Zhang said. "So basically,
you don't fabricate any circuit to begin with, and then by applying gate voltages,
you can achieve any circuit you want. This gives you extreme flexibility."
"That would be the dream in the future," Wang said.
Depending on the lithography technique used, the size of each gate could be
much smaller than one micron - a millionth of a meter - allowing millions of
separate electronic devices on a millimeter-square piece of bilayer graphene.
Wang and Zhang also foresee optical applications, because the zero-250 meV
bandgap means graphene LEDs would emit frequencies anywhere in the far- to mid-infrared
range. Ultimately, it could even be used for lasing materials generating light
at frequencies from the terahertz to the infrared.
"It is very difficult to find materials that generate light in the infrared,
not to mention a tunable light source," Wang said.
Crommie noted, too, that solid state physicists will have a field day studying
the unusual properties of bilayer graphene. For one thing, electrons in monolayer
graphene appear to behave as if they have no mass and move like particles of
light - photons. In tunable bilayer graphene, the electrons suddenly act as
if they have masses that vary with the bandgap.
"This is not just a technological advance, it also opens the door to some
really new and potentially interesting physics," Crommie said.
Wang, Zhang, Tang and their colleagues continue to explore graphene's electronic
properties and possible electronic devices.