"This research is teaching us a lot on fundamental mechanisms of graphene growth," said TI Fellow Luigi Colombo. "We believe that the results of this work will lead scientists and engineers to further increase the size of the single crystal graphene and also improve the electrical characteristics of the material."
TI recently demonstrated growing large-grain graphene crystals—up to half a millimeter in diameter—in collaboration with IBM’s T.J. Watson Research Center, the Nanoelectronic Research Initiative (NRI), and the University of Texas. Using low-pressure chemical vapor deposition inside copper-foil enclosures, with methane as a precursor, the resulting graphene films were then transferred to doped single-crystal silicon and silicon-on-insulator substrates where their electrical characteristics were tested. The team is also pursuing ways of growing graphene directly on dielectric substrates.
"Growth on other substrates, preferably dielectrics or metals more stable than copper, are also being investigated," said Colombo.
Large-grain crystals of graphene start as perfect hexagons but grow more quickly at their tips (rather than the sides of the hexagon) resulting in snowflake-like fractal islands.
For several years now, TI has been quietly pursuing graphene growth methods, previously reporting surface nucleation followed by a two-dimensional growth process on copper substrates with CVD. However, the domains were only 10-to-20 microns, 30 times smaller than the half-millimeter (500 micron) domains on which TI is now reporting.
Using low-energy electron microscopy the research team was able to confirm that the films were relatively uniform single-crystal graphene monolayers. Using Raman spectroscopy, the researchers were able to confirm an electron mobility of 4000 square centimeters per volt second–compared to 1,400 cm2/Vs for silicon and 8500 cm2/Vs for gallium arsenide. Theoretically, graphene can achieve electron mobilities of 10,000-to-100,000 cm2/Vs.
"4000 [cm2/Vs] is reasonably high, but not as high as the highest value possible for exfoliated films, so we still have some improvements to make in our process," said Colombo. "But we have high hopes for these large-domain films."
The large-domain graphene growth was promoted inside the copper-foil cage at a relatively high temperature—over 1,035 degrees Celsius (1,895 Fahrenheit). After processing the films, field-effect transistors were fabricated by transferring the films to highly doped silicon substrates, which served as the back-gate contact, with source and drain electrodes made from nickel. Electron mobility was inferred from measuring resistance was as a function of back-gate voltage.
Funding was provided by the National Science Foundation, the Office of Naval Research and Semiconductor Research Corp.’s NRI at the SouthWest Academy of Nanoelectronics.
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