One-step graphene doping could enable complementary metal oxide graphene transistors
By Suzanne Deffree, Managing Editor, News -- EDN, February 17, 2010
Researchers at the Georgia Institute of Technology have claimed a one-step process that produces both n-type and p-type doping of large-area graphene surfaces and that could facilitate use of the material for future electronic devices.
The doping technique -- produced by applying a commercially available SOG (spin-on-glass) material to graphene and then exposing it to electron-beam radiation -- can also be used to increase conductivity in graphene nanoribbons used for interconnects, according to the university.
Both types of doping were created by varying the exposure time to the to e-beam radiation, the university said, explaining that higher levels of e-beam energy produced p-type areas, while lower levels produced n-type areas.
The technique was used to fabricate high-resolution p-n junctions. When properly passivated, the doping created by the SOG is expected to remain indefinitely in the graphene sheets studied by the researchers, Georgia Tech said.
"This is an enabling step toward making possible complementary metal oxide graphene transistors," said Raghunath Murali, a senior research engineer in Georgia Tech's Nanotechnology Research Center, in a statement.
In the doping process, Murali and graduate student Kevin Brenner began by removing flakes of graphene one to four layers thick from a block of graphite. Next, they placed the material onto a surface of oxidized silicon, then fabricated a four-point contact device. They then spun on films of HSQ (hydrogen silsesquoxane) and cured certain portions of the resulting thin film using e-beam radiation. According to Georgia Tech, the technique provides precise control over the amount of radiation and where it is applied to the graphene, with higher levels of energy corresponding to more cross-linking of the HSQ.
"We gave varying doses of electron-beam radiation and then studied how it influenced the properties of carriers in the graphene lattice," Murali said. "The e-beam gave us a fine range of control that could be valuable for fabricating nanoscale devices. We can use an electron beam with a diameter of four or five nanometers that allows very precise doping patterns."
Electronic measurements showed that a graphene p-n junction created by the new technique had large energy separations, indicating strong doping effects, he added.
Researchers elsewhere have demonstrated graphene doping using a variety of processes including soaking the material in various solutions and exposing it to a variety of gases. Georgia Tech said it believes its process is the first to provide both electron (n-type) and hole (p-type) doping from a single dopant material.
In the process, the doping is believed to introduce atoms of hydrogen and oxygen in the vicinity of the carbon lattice. The oxygen and hydrogen do not replace carbon atoms, but instead occupy locations atop the lattice structure, the university said.
In volume manufacturing, the e-beam radiation would likely be replaced by a conventional lithography process, Murali said. Varying the reflectance or transmission of the mask set would control the amount of radiation reaching the SOG, and that would determine whether n-type or p-type areas are created.
"Making everything in a single step would avoid some of the expensive lithography steps," he said. "Gray-scale lithography would allow fine control of doping across the entire surface of the wafer."
For doping bulk areas such as interconnects that do not require patterning, the researchers coat the area with HSQ and expose it to a plasma source. The technique can make the nanoribbons up to 10 times more conductive than untreated grapheme, Georgia Tech claimed.
However, the researchers noted that a better understanding of how the process works and whether other polymers might provide better results is needed.
"We need to have a better understanding of how to control this process because variability is one of the issues that must be controlled to make manufacturing feasible," Murali said. "We are trying to identify other polymers that may provide better control or stronger doping levels."
A paper describing the technique appeared February 10, in the journal Applied Physics Letters. The research was supported by the Semiconductor Research Corp and the Defense Advanced Research Projects Agency through the Interconnect Focus Center.
