Graphene could replace silicon in semiconductors, university says

Researchers at the University of Maryland have found that electrons travel more than 100 times faster in graphene than in silicon.

By Ann Steffora Mutschler, Senior Editor -- Electronic News, 3/26/2008

Physicists at the University of Maryland have found that graphene holds great promise for replacing conventional semiconductor materials, such as silicon, in applications ranging from high-speed computer chips to biochemical sensors thanks to the intrinsic limit to the mobility of the material.

The intrinsic limit of the mobility of a material measures how well a material conducts electricity, and graphene, which is a single-atom-thick sheet of graphite, has shown to be higher than any other known material at room temperature.

A team of researchers led by physics professor Michael S. Fuhrer of the university's Center for Nanophysics and Advanced Materials, and the Maryland NanoCenter explained that the findings are the first measurement of the effect of thermal vibrations on the conduction of electrons in graphene, which show that thermal vibrations have an extraordinarily small effect on the electrons in graphene.

Graphene has received significant interest as of late from industry leaders. Earlier this month, IBM researchers at the company's Yorktown Heights, NY-based lab detailed a discovery that could allow graphite to be used as a material for building nanoelectonic circuits smaller than those in today's silicon-based computer chips.

This work being disclosed today also reconfirms the findings by researchers at the University of Manchester in the United Kingdom, working in conjunction with teams at the Institute for Microelectronics Technology and High Purity Materials in Russia, the Radboud University Nijmegen in the Netherlands, and the department of physics at Michigan Technological University, that concluded that graphene offers the greatest intrinsic mobility of any known material.

The University of Maryland researchers noted that in any material, the energy associated with the temperature of the material causes the atoms of the material to vibrate in place, and as electrons travel through the material, they can bounce off these vibrating atoms, giving rise to electrical resistance. As such, this electrical resistance is 'intrinsic' to the material: it cannot be eliminated unless the material is cooled to absolute zero temperature, and hence sets the upper limit to how well a material can conduct electricity.

In the case of graphene, the vibrating atoms at room temperature produce a resistivity of about 1.0 microOhm-cm (resistivity is a specific measure of resistance; the resistance of a piece material is its resistivity times its length and divided by its cross-sectional area), which is approximately 35% less than the resistivity of copper, the lowest resistivity material known at room temperature.

Fuhrer explained in a statement, “Other extrinsic sources in today's fairly dirty graphene samples add some extra resistivity to graphene, so the overall resistivity isn't quite as low as copper's at room temperature - yet. However, graphene has far fewer electrons than copper, so in graphene the electrical current is carried by only a few electrons moving much faster than the electrons in copper.”

With semiconductors, a different measure is used to quantify how fast electrons move – mobility – with the limit to mobility of electrons in graphene set by thermal vibration of the atoms of approximately 200,000 square cm/Vs at room temperature, compared to about 1,400 square cm/Vs in silicon, and 77,000 square cm/Vs in indium antimonide, the highest mobility conventional semiconductor known.

“Interestingly, in semiconducting carbon nanotubes, which may be thought of as graphene rolled into a cylinder, we've shown that the mobility at room temperature is over 100,000 square cm/Vs,” Fuhrer said.

More specifically, mobility determines the speed at which an electronic device (such as a field-effect transistor, which forms the basis of modern computer chips) can turn on and off. The very high mobility makes graphene promising for applications in which transistors much switch extremely fast, such as in processing extremely high frequency signals, the researchers explained.

Also, mobility can be expressed as the conductivity of a material per electronic charge carrier, and so high mobility is also advantageous for chemical or bio-chemical sensing applications in which a charge signal from, for instance, a molecule adsorbed on the device, is translated into an electrical signal by changing the conductivity of the device.

Therefore, graphene is a promising material for chemical and bio-chemical sensing applications with the low resistivity and extremely thin nature of graphene also applicable in thin, mechanically tough, electrically conducting, transparent films, which are needed in a variety of electronics applications from touch screens to photovoltaic cells.

Fuhrer and fellow researchers demonstrated that although the room temperature limit of mobility in graphene is as high as 200,000 square cm/Vs, in present-day samples the actual mobility is lower, around 10,000 square cm/Vs, leaving significant room for improvement, and since graphene is only one atom thick, current samples must sit on a substrate, in this case silicon dioxide with trapped electrical charges in the silicon dioxide (a sort of atomic-scale dirt) can affect the electrons in graphene and reduce the mobility.

Finally, vibrations of the silicon dioxide atoms can also have an effect on the graphene which is stronger than the effect of graphene's atomic vibrations. This so-called 'remote interfacial phonon scattering' effect is only a small correction to the mobility in a silicon transistor, but because the phonons in graphene itself are so ineffective at scattering electrons, this effect becomes very important in graphene, they explained.

“We believe that this work points out the importance of these extrinsic effects, and creates a roadmap for finding better substrates for future graphene devices in order to reduce the effects of charged impurity scattering and remote interfacial phonon scattering,” Fuhrer concluded.