November 7, 2011 — Georgia Institute of Technology (Georgia Tech) researchers compared graphene doping techniques for device interconnects and found that edge passivation is 1000x more effective than surface treatment.
Chemical doping in 3D semiconductors involves substituting oxygen or nitrogen into the silicon lattice. Since graphene is two dimensional carbon, substituting atoms vastly degrades its electron mobility and other unique properties. In conventional three-dimensional semiconductors, doping controls the density of electron carriers, and it will be needed in graphene-based electronics as well.
While graphene cannot be chemically doped in the same way as 3D transistors, doping may be combined with other fabrication steps in making graphene devices and interconnects, says James Meindl, director of Georgia Tech’s Nanotechnology Research Center.
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| Image 1. This scanning electron microscope image shows contacts placed onto a graphene sheet. SOURCE: Kevin Brenner, GA Tech. |
“When we work with a three-dimensional semiconductor, we embed the dopant species in the bulk material and then fabricate it into a device,” said Kevin Brenner, a graduate research assistant in the Georgia Tech School of Electrical and Computer Engineering. “With graphene, we will dope the material as we process it and fabricate it into devices or interconnects. Doping may be done as part of other fabrication steps such as plasma etching, and that will require us to reinvent the whole process.”
Using sheets of exfoliated graphene, Brenner and collaborators Raghu Murali and Yinxiao Yang evaluated the effectiveness of edge passivation by coupling electron-beam lithography with a common resist material, and adsorption from coating the surface of the material. The edge treatment, which chemically reacts with defects created when the material is cut, was a thousand times more efficient at producing carriers in the graphene sheets than the surface treatment.
Edge treatment keeps the graphene’s center free of defects, maintaining very high mobility and other graphene-specific characteristics, said Brenner. Carrier densities were high.
Because of the two-dimensional nature of the graphene, controlling the edge chemistry can provide control over the bulk properties of the sheet. “At nanoscale dimensions, the edge atoms tend to dominate over surface adsorption techniques,” he added. “With a 7nm2 graphene device, passivating just 1 edge C-atom provides the doping equivalent of covering the entire surface.”
For doping the edge of a graphene structure, the team applied a thin film of hydrogen silsesquioxane (HSQ), a chemical normally used as a resist for etching, then used electron beam lithography to cross-link the material, which added oxygen atoms to the edges to create p-type doping. The resist and electron beam system combined to provide nanometer-scale control over where the chemical changes took place.
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| Image 2. Graduate research assistant Kevin Brenner holds a graphene sample under study in the Georgia Tech Nanotechnology Research Center. The cryogenic probe station shown behind him is used to study doping effects in the material. SOURCE: Gary Meek, GA Tech. |
Doping treatment could also be applied using plasma etching, Brenner said. Controlling the specific atoms used in the plasma, or conducting the etching process in an environment containing specific atoms, could drive those atoms into the edges where they would serve as dopants. Any edge is a location where you can passivate with a dopant, oxygen, nitrogen, hydrogen, etc.
Beyond fabricating electronic devices, Nanotechnology Research Center scientists are interested in using graphene for interconnects, potentially as a replacement for copper. As interconnect structures become smaller and smaller, the resistivity of copper increases. Edge-doped graphene sheets exhibit a trend of increasing doping with reduced dimensions, possibly becoming more conductive as their size shrinks below 50nm, making them attractive for nanoscale interconnects.
“The next step is to begin putting this into nanoscale devices,” Brenner said.
Details of the research were published online in the journal Carbon. Access it here: http://www.sciencedirect.com/science/article/pii/S0008622311007573.
The research was supported by the Semiconductor Research Corporation (SRC), the Defense Advanced Research Projects Agency (DARPA) through the Interconnect Focus Center, and the National Science Foundation (NSF).
Learn more about Georgia Institute of Technology at http://gatech.edu/