PORTLAND, Ore. Pure layers of carbon atoms in a honeycomb lattice, also known as graphene, promises to become the successor to both silicon and gallium arsenide circuitry. Among its advantage are superior electron mobility, the ability to transport both electrons (like silicon) and holes (like gallium arsenide) and the prediction that graphene charge carriers move at a speed independent of their energy.
While some theoretical predictions for graphene have been confirmed, researchers at the National Institute of Standards and Technology (NIST) and Georgia Tech claim to have experimental proof using a new measurement device.
"We have used a new method to characterize the quantum electronic structure of graphene grown on silicon carbide, which is one of the most promising avenues to making new types of electronic devices," said NIST researcher Joseph Stroscio. "They were theoretical before, but now we have actually seen them."
Growing graphene on silicon carbide was pioneered at Georgia Tech by professor Walter de Heer. The technique uses high temperatures to boil off the silicon atoms from a wafer surface. The wafer then self-organizes into electronically independent layers of pure carbon atoms.
When left as a continuous layer covering the entire wafer, the material behaves like a conductor. To transform it into a semiconductor, other materials like silicon depend on doping with impurities that change its electronic behavior. With graphene, all that is needed is the ability to pattern it into shapes that confine electron mobility, thereby transforming the matieral from highly conductive to semiconducting.
"You can grow wafer-scale graphene material on silicon carbide, which can then be patterned into electronic devices," said Stroscio. "Graphene is a conductor, but you really want a semiconductor for device applications. And you can make graphene into a semiconductor by patterning it; for example, patterning it into a ribbon opens up a bandgap which you can use to make a transistor."
To measure its characteristics, NIST fabricated a "shuttle" instrument based on a scanning tunneling microscope capable of 1 billion times magnification while simultaneously subjecting the material to a high magnetic field in a vacuum at ultra-cold temperatures. By sweeping the strength of the magnetic field during its observations, the team was able to verify the non-uniform spacing among the discrete quantum energy states in the material.
Silicon and other materials all have uniformly distributed energy states when subjected to a magnetic field. The energy states for graphene are not only non-uniform, but include a hallmark zero-energy state.
The researchers concluded that the layers of graphene grown on silicon carbide wafers are electronically decoupled from one another, opening up the possibility of fabricating independent devices on adjacent layers.
The researchers also confirmed that electron speed is independent of energy, making electrons behave as if they were massless--like photons. The confirmation of this electron behavior in graphene should allow the eventual fabrication of novel electronic and photonic devices using the material.
"The possibilities for devices fabricated from graphene are just getting better and better," claimed Georgia Tech professor Phillip First.
The researchers are next attempting to grow single monolayers of graphene, not only to further characterize electronic behavior, but also to perfect methods of fabricating gate electrodes and other structures necessary for devices.