Mar 29, 2012
Electrons tunnel through ultrathin boron nitride
Ultrathin hexagonal boron nitride (h-BN) could make for the ideal dielectric layer in future electronic components. So says an international team of researchers, led by Nobel prize winners Andre Geim and Kostya Novoselov of the University of Manchester in the UK, that is looking at using the material as a barrier layer that is sandwiched between two conducting layers. The tunnel current through h-BN seems to increase as the material layer becomes thinner – a result that bodes well for applications such as tunnel devices and ultrafast field-effect transistors.
Like graphene, boron nitride can be exfoliated until you obtain a sample the thickness of a single atom, explains team member Liam Britnell. The advantage with BN is that it resists this process quite well, providing very thin and uniform samples with few defects. “As transistors become ever smaller, h-BN is probably the best material to be used as the dielectric layer in such miniature electronic components,” he told nanotechweb.org.
What is more, a new class of atomically thin multilayered heterostructures could be made by combining graphene and h-BN, he adds. This is because the two materials have very similar lattice constants but, unlike graphene (which is a semi-metal), h-BN is insulating with a large energy bandgap of 6 eV.
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 for 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,” said Britnell.
The team, which also includes scientists from the Netherlands, Portugal and Singapore, 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 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. “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 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.”
The work was reported in Nano Letters.
About the author
Belle Dume is contributing editor at nanotechweb.org.