Jun 11, 2009
Tuning a bandgap in graphene
Researchers in the US have made a semiconductor from bilayer graphene, whose bandgap can be electrically controlled for the first time. The result is a big improvement on conventional semiconductors in which the electronic bandgap is finite and fixed by the material's crystal structure. It also confirms that graphene is a promising material for future nano-electronics and photonics.
Electrons move through graphene at extremely high speeds thanks to the fact that they behave like relativistic particles that have no rest mass. This, and other unusual physical properties, means that graphene is often touted to replace silicon as the electronic material of choice in the future.
The electronic bandgap is a basic property of all semiconductors and gives them their electrical transport and optical properties. It is therefore very important in modern physics and technology because it is responsible for how devices like p-n junctions, transistors, photodiodes and lasers work. Scientists would like to be able to control the size of a bandgap in semiconductors so that they can optimize such devices. And it would be especially good if the bandgap could be controlled by an external electric field.
The bandgap in conventional semiconductors is fixed by their crystalline structure, so this rules out any such bandgap control in these materials. However, researchers at the University of California at Berkeley, led by Feng Wang, and colleagues have shown that the bandgap in bilayer graphene can be gate controlled from 0 to 250 meV (0 to infrared) at room temperature.
The narrow bandgap range means that it could now be possible to make new types of nanotransistors, nano-LEDs and other nano-optical devices in the infrared range from graphene.
Normally, graphene does not have a bandgap – it is a "zero bandgap" semiconductor. However, a gap can be introduced in bilayer graphene, which consists of two graphene layers lying on top of each other, by disturbing the symmetry of the two layers. Although this can be achieved by chemically doping one of the layers – for example, by using metal atoms – this technique is difficult to control and incompatible with device applications.
Wang and colleagues overcame this problem by making a two-gated bilayer device from graphene. The device, which is a field-effect transistor, is built on a silicon substrate (the bottom gate) and contains a thin insulating layer of silicon dioxide between the substrate and the graphene layers. There is a transparent layer of sapphire (aluminium oxide) over the graphene layers and on top of this, the top gate, made of platinum.
The researchers varied the applied voltage to the gate electrodes and measured how the bandgap changed. They did this by sending an infrared beam (from the Advanced Light Source facility at the Lawrence Berkeley National Lab) into the device and measuring variations in the amount of light absorbed by the graphene layers. The size of the absorption peak in each spectrum gives the exact size of the bandgap at each gate voltage.
The team is now trying to improve the bilayer device for use in high-performance tunable electronics. "We will also try to understand the light emission behaviour from such a tunable semiconductor," Wang told nanotechweb.org.
The work was published in Nature.
About the author
Belle Dumé is contributing editor at nanotechweb.org