Since its discovery in 2004, graphene has continued to amaze scientists thanks to its unique electronic and mechanical properties that make it useful for a host of device applications. The "wonder material", as it is called, might even replace silicon as the electronic material of choice in the future, according to some.

Graphene is the thinnest material known. It consists of a planar single sheet of carbon arranged in a honeycombed lattice and electrons travel through the material at extremely high speeds thanks to the fact that they behave like relativistic particles with no rest mass. This means that it could be used to make transistors faster than any that exist today.

However, as with any material, graphene has its fair share of problems to overcome. Phaedon Avouris' team at the IBM TJ Watson Research Center in New York has now shown that graphene heats up considerably when operated at saturated currents. This is difficult to avoid because high-performance graphene devices ideally need to be operated at these saturation current limits.

Remote scattering

The researchers have found that most of the heat flows directly into an underlying silica substrate. They suggest that the energy transfer involves a remote-scattering process in which the electrons in graphene can give up part of their energy directly to surface vibrations (phonons) of the SiO2 polar substrate.

Although this effect is known to occur in silicon transitors too, it is much smaller there because most of the conduction electrons in silicon reside a few nanometres away from the interface and the coupling falls off strongly with distance, explains team member Marcus Freitag. "Surface polar phonon scattering is much stronger in graphene because the graphene channel is only one atom thick and the SiO2 surface polar phonons can directly couple to the conduction electrons in graphene," he told nanotechweb.org.

The IBM team obtained its result by determining the temperature distribution in active graphene transitors using optical microscopy combined with electrical transport measurements. The researchers recorded the temperature-dependent Raman spectrum of devices at several different positions on a graphene flake to build up images of how temperature was distributed there. They also used heat-flow modelling to calculate how heat travels along and across a graphene flake. "The surface-polar phonon scattering mechanism explains both the low thermal resistance between graphene and the gate SiO2, and the saturated current-voltage characteristics we observe," said Freitag.

"Our work shows that substrate interactions become much more important in graphene electronics than in traditional MOSFETs and heterostructures," he added. "Engineers thus need to focus on non-polar substrates and substrates that do not trap charges."

The results were reported on ArXiv.