It’s inevitable, materials scientists say. Some day, in the endless chase for faster computers and smaller mobile devices, today’s silicon microchip technology is going to hit the wall. When that day comes, many believe that graphene will take up the cause.
Graphene, a form of carbon made up of a single layer of carbon atoms in a tight hexagonal arrangement, is sometimes called “atomic-scale chicken wire.” Since it was first isolated in the lab in 2004, the material has demonstrated remarkable electronic properties, including theoretical speeds 100 times greater than silicon. But putting graphene into a microchip that could outperform current silicon technology has proven difficult.
Joshua Robinson
The answer may lie in new nanoscale systems called two-dimensional layered materials. Based on ultrathin layers of materials with exotic properties, these systems could be important for microelectronics, sensors, catalysis, tissue engineering and energy storage. Researchers at Penn State have applied one such 2D layered material, a combination of graphene and hexagonal boron nitride, to produce improved transistor performance. Crucially, their process works at an industrially relevant scale.
“Other groups have shown that graphene on boron nitride can improve performance two to three times,” explains Joshua Robinson, assistant professor of materials science and engineering at Penn State, “but not in a way that could be scaled up. For the first time, we have been able to take this material and apply it to make transistors at wafer scale.”
To do so, the Penn State team devised a method for integrating a thin layer of graphene only one or two atoms thick with a second, slightly thicker, layer of hexagonal boron nitride (hBN), a synthetic mixture of boron and nitrogen that is used as an industrial lubricant and is found in many cosmetics.
Previous research by other groups has shown that hBN is a potential replacement for silicon dioxide and other high-performance dielectrics that have failed to integrate well with graphene. Because boron sits next to carbon on the periodic table, and hexagonal boron nitride has a similar arrangement of atoms as graphene, the two materials match up well electronically.
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Graphene is a two-dimensional crystal consisting of a single layer of carbon atoms arranged hexagonally.
To be of more than academic interest, however, the hBN-graphene bilayer had to be grown at wafer scale—between 75 and 300 millimeters (i.e., between 3 and 12 inches) in diameter. The Penn State team solved this problem by using a technique developed in their lab to produce “quasi-freestanding epitaxial graphene,” a uniform high-quality layer of the material suitable for high-frequency applications.
By attaching hydrogen atoms, the process essentially flattens and smooths the graphene film, Robinson explains. The hexagonal boron nitride was then grown on a transition metal substrate using a standard chemical vapor deposition technique. Finally the hBN was released from the substrate via a transfer process and layered on top of the graphene on a 75mm wafer.
In their earlier work with epitaxial graphene, the Penn State team had already increased transistor performance by two to three times. This research adds a further two-to-three-times improvement in performance and shows the strong potential for utilizing graphene in electronics, according to Robinson. In the near future, the Penn State team hopes to demonstrate graphene-based integrated circuits and high-performance devices suitable for industrial-scale manufacturing on 100mm wafers.
“We use all standard lithography, which is important for nanomanufacturing,” Robinson says, adding that in the highly competitive microchip industry, a new material system needs to be compatible with current processing technology as well as offering a significant performance boost.
Joshua Robinson, Ph.D., is assistant professor of materials science and engineering, jrobinson@psu.edu.
This research was published in the online edition of the journal ACS Nano. In addition to Robinson, the co-authors are Michael Bresnehan, Matthew Hollander, Maxwell Wetherington, Michael LaBella, Kathleen Trumbull, Randal Cavalero, and David Snyder, all of Penn State.
The work was supported by the Naval Surface Warfare Center Crane Division, and instrumentation support was provided by the National Nanotechnology Infrastructure Network at Penn State.