PORTLAND, Ore.—Carbon sheets—graphene—can conduct electricity up to a million times better than conventional silicon pathways on microchips, making them a strong candidate for future on-chip interconnection layers. To use graphene as a semiconductor, however, requires opening a bandgap across which electrons must jump, thereby enabling the switching operations of a digital computer. Now Rensselaer Polytechnic Institute (RPI) claims to have discovered a simple method of opening a bandgap in graphene with water.
What's more, by controlling the amount of humidity inside chip packages, RPI 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 .13 electron volts (eV), but RPI professor Nikhil Koratkar's team was able to open a bandgap of .2 eV with simple water vapor. In addition, the RPI team reports tuning the bandgap of graphene anywhere from 0 (a pure metallic conductor) all the way to .2 eV (a semiconductor suitable for infrared detectors) by simply controlling the amount of humidity. The team claims its method of tuning the bandgap of graphene will allow optimization for specific applications by trapping a precise amount of humidity inside a chip package.
Chip-packaging technologies, according to Koratkar, should allow chip makers "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 Koratkar's team included professor Theodorian Borca-Tasciuc and doctoral candidate Fazel Yavari from RPI, and from Rice University professor Pulickel Ajayan, postdoctoral research fellow Li Song and doctoral candidate Hemtej Gulapalli.
Funding for the project was provided by the Advanced Energy Consortium (AEC), National Institute of Standards and Technology (NIST) Nanoelectronics Research Initiative, and the U.S. Department of Energy Office of Basic Energy Sciences (BES).