Published: 15.06.2009

Bilayer graphene gets a bandgap

 
A tunable graphene bandgap opens the way to nanoelectronics and nanophotonics.

BERKELEY, CA – Graphene is the two-dimensional crystalline form of carbon, whose extraordinary electron mobility and other unique features hold great promise for nanoscale electronics and photonics. But there's a catch: graphene has no bandgap.

"Having no bandgap greatly limits graphene's uses in electronics," says Feng Wang of the U.S. Department of Energy's Lawrence Berkeley National Laboratory, where he is a member of the Materials Sciences Division. "For one thing, you can build field-effect transistors with graphene, but if there's no bandgap you can't turn them off! If you could achieve a graphene bandgap, however, you should be able to make very good transistors."

Wang, who is also an assistant professor in the Department of Physics at the University of California at Berkeley, has achieved just that. He and his colleagues have engineered a bandgap in bilayer graphene that can be precisely controlled from 0 to 250 milli-electron volts (250 meV, or .25 eV).

Moreover, their experiment was conducted at room temperature, requiring no refrigeration of the device. Among the applications made possible by this breakthrough are new kinds of nanotransistors and – because of its narrow bandgap – nano-LEDs and other nanoscale optical devices in the infrared range.

The researchers described their work in the June 11, 2009, issue of Nature.

Constructing a bilayer graphene transistor

As with monolayer graphene, whose carbon atoms are arranged in "chickenwire" configuration, bilayer graphene – which consists of two graphene layers lying one on the other – also has a zero bandgap and thus behaves like a metal. But a bandgap can be introduced if the mirror-like symmetry of the two layers is disturbed; the material then behaves like a semiconductor.

Previously, in 2006, researchers at Berkeley Lab's Advanced Light Source (ALS) observed a bandgap in bilayer graphene in which one of the layers was chemically doped by adsorbed metal atoms. But such chemical doping is uncontrolled and not compatible with device applications.

"Creating and especially controlling a bandgap in bilayer graphene has been an outstanding goal," says Wang. "Unfortunately chemical doping is difficult to control."

Researchers then tried to tune the bilayer graphene bandgap by doping the substrate electrically instead of chemically, using a perpendicularly applied, continuously tunable electrical field. But when such a field is applied with a single gate (electrode), the bilayer becomes insulating only at temperatures below one degree Kelvin, near absolute zero – suggesting a bandgap value much lower than predicted by theory.

Says Wang, "With these results it was hard to understand exactly what was happening electronically, or why."

Wang and his colleagues made two key decisions that led to their successful attempt to introduce and determine a bandgap in bilayer graphene. The first was to build a two-gated bilayer device, fabricated by Yuanbo Zhang and Tsung-Ta Tang of the UC Berkeley Department of Physics, which allowed the team to independently adjust the electronic bandgap and the charge doping.

The device was a dual-gated field-effect transistor (FET), a type of transistor that controls the flow of electrons from a source to a drain with electric fields shaped by the gate electrodes. Their nano-FET used a silicon substrate as the bottom gate, with a thin insulating layer of silicon dioxide between it and the stacked graphene layers. A transparent layer of aluminum oxide (sapphire) lay over the graphene bilayer; on top of that was the top gate, made of platinum.

The other key decision the researchers made was to get a better grasp of what was really going on in the device as they varied the voltage. Rather than try to measure the bandgap by measuring the device's electrical resistance, or transport, they decided to measure its optical transmission.

"The problem with transport measurements is that they are too sensitive to defects," says Wang. "A tiny amount of impurity or defect doping can create a big change in the resistance of the graphene and mask the intrinsic behavior of the material. That's why we decided to go with optical measurements at the Advanced Light Source."

Using infrared beamline 1.4 at the ALS, under the direction of ALS physicist Michael Martin and Zhao Hao of the Earth Sciences Division, Wang and his colleagues were able to send a tight beam of synchrotron light, focused on the graphene layers, right through the device. As the researchers tuned the electrical fields by precisely varying the voltage of the gate electrodes, they were able to measure variations in the light absorbed by the gated graphene layers. The absorption peak in each spectrum provided a direct measurement of the bandgap at each gate voltage.

"In principle we could have used a tunable laser to measure the optical transmission, but the 1.4 beamline is very bright and can be focused down to the diffraction limit – an important consideration when the graphene-flake target is so small," Wang says. "Also, compared to a laser, the beamline provides a wider range of frequencies all at once, so we don't have to painstakingly tune to each absorption frequency we're trying to measure."

The malleable electronic structure of bilayer graphene

The results from the ALS measurements were obtained with relative ease and efficiency, and showed that by independently manipulating the voltage of the two gates, the researchers could control two important parameters, the size of the bandgap and the degree of doping of the graphene bilayer. In essence, they created a virtual semiconductor from a material that is not inherently a semiconductor at all.

In ordinary semiconductors, the gap between the conduction band (unoccupied by electrons) the valence band (occupied by electrons) is finite, and fixed by the crystalline structure of the material. In bilayer graphene, however, as Wang's team demonstrated, the bandgap is variable and can be controlled by an electrical field. Although a pristine graphene bilayer has zero bandgap and conducts like a metal, a gated bilayer can have a bandgap as big as 250 milli-electron volts and behave like a semiconductor.

With precision control of its bandgap over a wide range, plus independent manipulation of its electronic states through electrical doping, dual-gated bilayer graphene becomes a remarkably flexible tool for nanoscale electronic devices.

Wang emphasizes that these first experiments are only the beginning. "The electrical performance of our demonstration device is still limited, and there are many routes to improvement, for example through extra measures to purify the substrate."

Nevertheless, he says, "We've demonstrated that we can arbitrarily change the bandgap in bilayer graphene from zero to 250 milli-electron volts at room temperature, which is remarkable in itself and shows the potential of bilayer graphene for nanoelectronics. This is a narrower bandgap than common semiconductors like silicon or gallium arsenide, and it could enable new kinds of optoelectronic devices for generating, amplifying, and detecting infrared light."

"Direct observation of a widely tunable bandgap in bilayer graphene," by Yuanbo Zhang, Tsung-Ta Tang, Caglar Girit, Zhao Hao, Michael C. Martin, Alex Zettl, Michael F. Crommie, Y. Ron Shen, and Feng Wang, appears in the June 11, 2009 issue of Nature. Zhang, Tang, and Girit are members of UC Berkeley's Department of Physics, in the groups of Professors Crommie, Shen, and Zettl respectively; Zettl, Crommie, and Shen are also members of Berkeley Lab's Materials Sciences Division.

This work was supported by the U.S. Department of Energy's Office of Science, Office of Basic Energy Sciences.

University of California - press release

Tunable semiconductors possible with hot new material called graphene

Tunable bandgap means tunable transistors, LEDs and lasers.

Berkeley - 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 material.

"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 reported 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.

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