Nov 13, 2009
Graphene FETs get up to speed
Graphene could be ideal for making high-speed transistors, but there is a serious problem holding back the technology. Interactions between graphene and the dielectric material used as the top gate severely degrades the speed of charge carriers (electrons and holes) in graphene, which ultimately limits device performance. Now, researchers at IBM say that placing an organic polymer buffer layer between the graphene and conventional high-k dielectrics allows carrier mobilities to remain high.
As well as being extremely thin and a semiconductor, electrons move through "wonder material" graphene at extremely high speeds thanks to the fact that they behave like relativistic particles with no rest mass. This, and other unusual physical properties, means that graphene is often touted to replace silicon as the electronic material of choice and might be used to make faster transistors than any that exist today.
However, one problem holding back the progress of high-speed transitors based on graphene is the scattering of charge carriers in the material, which drastically limits the speed of electrons and holes. This scattering is due to interactions between graphene and the dielectric material used to make top-gated field-effect transistors (FETs).
Thin polymer film
Now, Phaedon Avouris' team at IBM's TJ Watson Research Center in New York may have come up with a solution to this problem. The researchers incorporated a thin polymer film between the graphene and high-κ dielectric materials and found that it does not significantly affect the field-effect mobility in graphene-based FETs. "This is important since previous attempts to passivate graphene with gate dielectrics always severely degraded the mobility of charge carriers in graphene, which limited the performance of finished transistors," Avouris told nanotechweb.org.
The main sources of carrier scattering in conventional graphene FETs are charged impurities and extrinsic surface phonons of the gate dielectric, he explained. The low-κ polymer buffer layer employed by the IBM team suppresses interactions between the surface phonons of the high-κ material and graphene. Moreover, the amount of charged impurities associated with the polymer are expected to be lower, also resulting in less scattering.
Although higher field-effect mobilities can be obtained in graphene FETs with a suspended channel region (that eliminates scattering effects from external surfaces), direct contact with dielectrics is a necessary evil in conventional FET architectures and cannot be avoided.
The researchers hope that their technique will enhance the performance of graphene-based devices and help advance graphene electronics technology.
They are now refining their process to make FETs for high-frequency operation. "Continuing to benchmark and push the operating speeds of these devices is one of our primary goals," said Avouris.
The work was published in Nano Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org