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Water vapour enables control of graphene bandgap

Posted: 03 Nov 2010     Print Version  Bookmark and Share

Keywords:graphene  water vapour  fabrication 

Rensselaer Polytechnic Institute claims to have discovered a simple method that uses water vapour to control the opening of the bandgap in graphene, pushing closer to the use of the carbon sheets for semiconductors.

Carbon sheets—graphene—are a strong candidate for future on-chip interconnection layers because they can conduct electricity up to a million times better than conventional silicon pathways on microchips. However, to use graphene as a semiconductor it is necessary to open up a bandgap across which electrons must jump, thus enabling the switching operations of a digital computer. This has now been done by researchers at RPI.

Also, by controlling the amount of humidity inside chip packages, the researchers showed that graphene's bandgap could be tuned for specific applications.

The bandgap of a material is measured from the bottom of the valence band (containing electrons bound in close orbits to the atom) to the top of the conduction band (where free electrons orbit). In pure conductors, like copper wires, the two bands have no gap as any electron is up for grabs when a voltage potential presents itself. For pure insulators, the bands are separated by a huge gap that can only be bridged by exceeding the breakdown voltage of the material. In between is the semiconducting region, where there is a reluctance for electrons to cross the band, allowing the switching mechanisms that make digital electronics work.

IBM reported earlier this year that its dual-gate bi-layer graphene architecture could effectively open a bandgap of .13eV. Now, Nikhil Koratkar, a professor at RPI, and his team have been able to open a bandgap of .2eV with simple water vapour. The RPI team also reports that they were able to tune the bandgap from zero (0—a pure metallic conductor) all the way to .2eV (a semiconductor suitable for IR detectors) by simply controlling the amount of humidity. The team claims its method of tuning the bandgap of graphene will allow optimisation for specific applications by trapping a precise amount of humidity inside a chip package.

Chip-packaging technologies, according to Koratkar, should allow chipmakers "to construct a small enclosure around certain parts or the entirety of a computer chip in which it would be quite easy to control the level of humidity."

A graphene film on a silicon dioxide substrate is being electrically tested using a four-point probe.

Other members of the team from RPI are Theodorian Borca-Tasciuc and Fazel Yavari. They are joined by researchers from Rice University, Pulickel Ajayan, Li Song and Hemtej Gulapalli.

Funding for the project was provided by the Advanced Energy Consortium, National Institute of Standards and Technology Nanoelectronics Research Initiative, and the US Department of Energy Office of Basic Energy Sciences.

- R. Colin Johnson
EE Times

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