Oct 22, 2010
Hydrogenated graphene makes good transistors
Hydrogenated or semi-hydrogenated graphene (called graphane and graphone respectively) could be better suited to making electronic devices than graphene itself. So says a team of researchers in Europe who have investigated the performance of these materials using powerful computer models. The simulations show that graphane/graphone-based transistors could comply with ITRS requirements for 2015–2016 technology because the transistors possess a large on-off current ratio – a parameter that shows how good the device is as behaving as a switch. The results can be explained by the large energy gaps in the materials (5.4 eV for graphane and 3.2 eV for graphone).
Graphene is extremely promising for use in fast and ultra-low-power devices thanks to the fact that electrons move through the material at extremely high speeds. Indeed, it is often touted to replace silicon as the material of choice in future electronics. However, the material does suffer from a major drawback in that it is a "zero-gap" semiconductor (it lacks an energy gap) and so cannot be used for digital applications. To open a gap of just 1 eV in the material would mean making graphene ribbons smaller than 2 nm across with single atom precision, something that is difficult using current fabrication technology.
Luckily, graphene can easily be hydrogenated by exposing it to a stream of hydrogen atoms to produce graphane. This process opens up a large energy gap in the material of a few electron-volts. Semi-hydrogenated graphene (graphone) has also been predicted to have ferromagnetic properties, which means that this material could be used in spintronics applications.
Reduced band-to-band tunnelling
Now, Gianluca Fiori and Giuseppe Iannaccone of the University of Pisa and colleagues in Nancy (France), Wuerzburg (Germany) and Uppsala (Sweden) have calculated that graphane and graphone could be used to make transistors with excellent properties. These come thanks to the direct bandgap of 5.4 eV for graphane and an indirect bandgap of 3.2 eV for graphone. The gaps strongly reduce band-to-band tunnelling in the material, which normally degrades the electronic properties of monolayer and bilayer graphene transistors.
The team accurately computed the energy bands using ab-inito (GW) calculations using supercomputers. In general, such calculations are demanding and cannot be used to compute transport in devices with more than around 1000 atoms. Fiori and co-workers adopted a multi-scale approach based on fitting the computed energy bands with a "three nearest-neighbour sp3 tight-binding Hamiltonian", which was then included in a semi-classical model that takes into account the ballistic movement of electrons in graphene. "We have thus been able to simulate current-voltage curves and the quantum capacitance for graphane/graphone-based transistors," Fiori told nanotechweb.org.
"We have shown that hydrogenated graphene has appealing electrical properties for digital applications," he added. "These could open the way for making graphene nanoelectronics a real alternative to silicon CMOS."
The simulations were performed for the best possible case scenarios. As well as assuming that there is ballistic transport of electrons in graphene, the researchers also assumed that the material was uniformly hydrogenated and had an ideal geometry. "Certainly, contact manufacture, carrier mobilities and the presence of non-ideal structures – like defects and impurities – will have to be considered in future calculations," insists Fiori.
The work was published in Physical Review B.
About the author
Belle Dumé is a contributing editor to nanotechweb.org