Carbon semiconductors clear CMOS hurdle

PORTLAND, Ore. — Carbon semiconductors fashioned from pure crystalline sheets of graphene outperform silicon but have lacked a foolproof method for creating the p- and n-type devices required for complementary metal-oxide semiconductor (CMOS) transistors. Now the Georgia Institute of Technology claims to have a devised a one-step graphene doping process, paving the way for commercial fabrication.

Georgia Tech's technique uses a commonly available spin-on-glass (SOG) material applied to graphene sheets. The grayscale material can be patterned to provide either p-type or n-type doping by merely varying the dose of radiation.

To fabricate its test devices, Georgia Tech used "an electron beam with a diameter of 4 or 5 nanometers that allowed very precise doping patterns," said Raghunath Murali, a senior research engineer in Georgia Tech's Nanotechnology Research Center. For production, a traditional illumination source would be used.

The ability to dope with holes (p-type) or with electrons (n-type) from a single dopant material could enable carbon-based CMOS transistors to be fabricated more quickly than silicon transistors. The polymer material, hydrogen silsesquioxane (HSQ), can also be used to increase the conductivity of the graphene ribbons used for interconnections by exposing them to a plasma source.

The precise mechanism of doping is unknown, but it differs from silicon doping, which forces dopant atoms into silicon's crystalline lattice to add holes or electrons. HSQ patterning bonds oxygen or hydrogen molecules atop (instead of inside) the graphene for permanent conversion into a p- or n-type material, as is required by CMOS transistors.

The researchers are currently characterizing other polymer resists that may provide even better control or stronger doping levels.

Semiconductor Research Corp. and the Defense Advanced Research Projects Agency funded the project through Georgia Tech's Interconnect Focus Center. Georgia Tech doctoral candidate Kevin Brenner contributed to the work.