The lack of a bandgap, or energy range, on two-dimensional crystalline carbon, or graphene, has limited the outlook for the material with respect to electronic applications. Using a new approach, a team based at the U.S. Department of Energy, Lawrence Berkeley National Laboratory, has engineered a bilayer graphene bandgap that can be controlled within a range, enabling its use in transistors, LEDs and infrared-range optical devices [Zhang et al., doi:10.1038/nature08105].
A theoretical tunable bandgap in bilayer graphene was proposed some years ago and, using chemical doping, various attempts have been made to perfect the process. However, this has proved problematic, not least because the chemical dopants are not compatible with device applications.
Doping the bilayer substrate electrically has also been attempted, applying a continuously tunable electrical field perpendicularly with a single gate. This process did not conform to theory, the bilayer not behaving as an insulator until a temperature of less than one degree Kelvin was reached.
A two-gated, field-effect transistor device was built at Berkeley, controlling flow of electrons, using silicon dioxide as an insulator between graphene layers and a silicon substrate to form the bottom gate. The top, platinum gate was separated from the graphene by a layer of aluminium oxide. In order to effectively measure the voltage, the researchers observed the device's optical transmission – not the electrical resistance – so as to reduce the sensitivity to defects. An infrared Advanced Light Source provided a narrow beam through the device, absorption levels detected for different voltages offering a way of measuring the bandgap.
A virtual semiconductor was, in effect, created from a material that is not a semiconductor, with a controllable, variable bandgap. “This is the first time a widely tunable bandgap over 250 meV - 10 times the room temperature thermal energy - was observed and the Fermi energy is in the bandgap,” said Feng Wang, a member of Berkeley's Materials Sciences Division and assistant professor in the Department of Physics at the University of California.
With a narrower bandgap than silicon, bilayer graphene offers the possibility of new types of optoelectric devices, for applications such as generating and detecting infrared light. “We are trying to improve the bilayer devices for high performance tunable electronics and to understand the light emission behavior from such a tunable semiconductor,” continues Feng Wang.