Superlattice transforms graphene into a semiconductor

September 22, 2014

Graphene placed on top of boron nitride, a step in forming a superlattice (credit: Berkeley Lab)

Graphene can be transformed into a new superlattice state that converts graphene — normally a metallic conductor — into a semiconductor, MIT and University of Manchester researchers have found.

In a research paper published in Science, the collaboration, led by MIT‘s theory professor Leonid Levitov and Manchester‘s Nobel laureate Sir Andre Geim, reports that they created a superlattice by placing graphene on top of boron nitride (BN) and then aligned the crystal lattices of the two materials.

The research suggests that transistors made from graphene superlattices should consume less energy than conventional semiconductor transistors because charge carriers drift perpendicular to the electric field, which results in little energy dissipation.

“It is quite a fascinating effect, and it hits a very soft spot in our understanding of complex, so-called topological materials,” said Geim. “It is extremely rare to come across a phenomenon that bridges materials science, particle physics, relativity and topology.”

Earlier research at Berkeley Lab showed that graphene supported on a boron nitride substrate had dramatically better electron mobility than graphene mounted on the most common semiconductor substrate, silicon dioxide.

“What first attracted investigators to boron nitride’s potential as a graphene substrate were its unusual structural properties,” according to a Berkeley Lab statement. “In its hexagonal structure (h-BN), alternating nitrogen and boron atoms closely mimic the way carbon atoms are arranged in graphene.”

Abstract of Science paper

Topological materials may exhibit Hall-like currents flowing transversely to the applied electric field even in the absence of a magnetic field. In graphene superlattices, which have broken inversion symmetry, topological currents originating from graphene’s two valleys are predicted to flow in opposite directions and combine to produce long-range charge neutral flow. We observe this effect as a nonlocal voltage at zero magnetic field in a narrow energy range near Dirac points at distances as large as several microns away from the nominal current path. Locally, topological currents are comparable in strength to the applied current, indicating large valley-Hall angles. The long-range character of topological currents and their transistor-like control by gate voltage can be exploited for information processing based on the valley degrees of freedom.