Electrons tunnel nicely through boron nitride
Apr 2, 2012 3 comments
Ultrathin sheets of hexagonal boron nitride (h-BN) could make for the ideal dielectric layer in future electronic components, thanks to a high degree of uniformity in how electrons tunnel through the material. That is the claim of an international team of researchers that is studying how the material performs as a barrier layer that is sandwiched between two conducting layers. The team found that the tunnel current depends exponentially on the number of h-BN atomic layers – down to just a single-layer thickness. The result bodes well for using h-BN in electronic components such as tunnel devices and ultrafast field-effect transistors, say the researchers.
Like graphene, sheets of boron nitride (BN) just one atom thick can be created by the exfoliation of much thicker samples. BN is interesting from a technological point of view because very thin and uniform samples with few defects can be made and – unlike graphene – the material is an insulator.
"As transistors become ever smaller, h-BN is probably the best material to use as the dielectric layer in such miniature electronic components," says team member Liam Britnell, who did the work at the University of Manchester in the UK along with Nobel-prize-winners Andre Geim and Konstantin Novoselov and others in the UK, the Netherlands, Singapore, Russia and the US. What is more, Britnell believes that a new class of extremely thin, multilayered structures could be made by combining graphene and h-BN. This is because the two materials have very similar lattice constants, but very different electronic properties.
Researchers have already shown that bulk BN could be ideal as a substrate for graphene electronics. BN can also be used as a barrier layer to stop electron tunnelling between two graphene layers and in graphene vertical-tunnelling transistors when it is more than six atomic layers thick. "Studying even thinner layers of BN is extremely interesting fundamentally because this material might find use in flexible-electronics applications, especially as the layer thickness can be controlled on the atomic scale," says Britnell.
In this latest study, the team looked at the electronic properties of tunnel diodes in which h-BN acts as a barrier layer between different conducting materials such as graphene, graphite and gold. Current–voltage measurements through the devices made over a range of temperatures revealed that a single atomic layer of h-BN indeed acts as an effective tunnel barrier and that the current through the material decreases as its thickness increases.
The researchers made their measurements on several types of device that they had fabricated as the following sandwich structures: gold/BN/gold, graphene/BN/graphene and graphite/BN/graphite. The BN layer was of varying thickness, ranging from one to four atomic layers.
Britnell and colleagues say that they would now like to find a suitable semiconducting layered material to complement the electronic properties of both BN and graphene – the latter being a semimetal. "It would be wonderful to find such a material, and our group is really concentrating on taking graphene, h-BN and other layered materials and then combining them to perhaps create new 3D structures. The hope is that we can find interesting new physics and discover other, fresh ways of making electronic devices," says Britnell
The work was reported in Nano Letters.
About the author
Belle Dumé is a contributing editor to nanotechweb.org