Seven years ago, silicene was little more than a theorist’s dream. Buoyed by a frenzy of interest in graphene — the famous material composed of a honeycomb of carbon just one atom thick — researchers speculated that silicon atoms might form similar sheets. And if they could be used to build electronic devices, these slivers of silicene could enable the semiconductor industry to achieve the ultimate in miniaturization.
This week, researchers took a significant step towards realizing that dream, by unveiling details of the first silicene transistor1.
Although the device’s performance is modest, and its lifetime measured in mere minutes, this proof of concept has already been causing a stir at conferences, says Deji Akinwande, a nanomaterials researcher at the University of Texas at Austin who helped to make the transistor. Guy Le Lay, a materials scientist at Aix-Marseille University in France, agrees.
“Nobody could have expected that in such a short time, something that didn’t exist could make a transistor,” he says.
Le Lay was one of the first scientists to create silicene in the lab2, in 2012 (see ‘The rise of silicene’). The debut coincided with a growing sense that graphene was unsuitable for making transistors. Graphene may be the world’s most conductive substance, but it is missing a crucial characteristic. Unlike the semiconductors used in computer chips, it lacks a band gap — the energy hurdle that electrons must vault before they can carry a current. Band gaps enable semiconductor devices to switch on and off and to perform ‘logic’ operations on bits.
The rise of silicene
Its carbon-based cousin graphene gets more attention, but silicene is catching up.
1994 First calculation of the structure of two-dimensional crystals of silicon (pictured) and of germanium.
2004 Andre Geim and Konstantin Novoselov report isolation of graphene.
2007 The name ‘silicene’ is coined.
2009 Fabrication of silicene nanoribbons; flurry of theoretical papers on silicene and germanene begins.
2010 Geim and Novoselov bag Nobel Prize in Physics for their experiments on graphene.
2012 Six independent reports of silicene sheets (formed on silver).
2015 First demonstration of silicene transistor.
“For logic applications, graphene is hopeless,” says Le Lay. By contrast, silicene can boast a band gap, because some of its atoms buckle upwards to form corrugated ridges, which puts some of its electrons in slightly different energy states. What is more, makers of electronic chips have been wary of ditching decades of silicon-manufacturing experience in favour of carbon, says Lok Lew Yan Voon, a theoretical physicist at the Citadel, the Military College of South Carolina in Charleston, who first named silicene and modelled its properties back in 2007 (ref. 3).
But handling silicene in the lab has been a huge challenge. The material cannot be peeled from a solid block using sticky tape, as graphene can from bulk graphite. Instead, researchers produce it by letting a hot vapour of silicon atoms condense onto a crystalline block of silver in a vacuum chamber, a much trickier process. And unlike robust graphene, naked silicene is extremely unstable in air, making it difficult to transfer the gossamer sheet to more useful substrates — such as the guts of a transistor. As recently as last year, some researchers were still questioning whether silicene even existed.
So Akinwande joined forces with Alessandro Molle at the Institute for Microelectronics and Microsystems in Agrate Brianza, Italy, to offer silicene some protection. They formed a silicene sheet on a thin layer of silver, and added a 5-nanometre-thick layer of alumina on top. Then they peeled this silicene sandwich off its mica base, flipped it silver-side-up, and laid it on an oxidized-silicon substrate. Finally, they gently etched away some of the silver to leave two islands of metal as electrodes, with a strip of exposed silicene between them.
“It’s a very clever trick,” says Le Lay, who is planning to try the process with germanene, a capricious, similarly structured ‘two-dimensional’ material made from germanium that his team created last year4.
Clever it may be, but the transistor will not be making an appearance in mobile phones any time soon: the exposed silicene degrades in about two minutes. Still, that is long enough to measure its properties. Although its electrons are sluggish in comparison to graphene’s, the device does indeed have a small band gap.
Laying an extra coating on top of the silicene transistor could also extend its life. Akinwande has used Teflon to help flakes of phosphorene — another air-sensitive, two-dimensional material, made of phosphorus — to survive for months5. Other researchers have shown that using multiple layers of silicene could allow the sacrificial top layers to protect those beneath for 24 hours6. Crucially, the technique used to make the silicene transistor could now help to test all of these ideas, and more, with various air-sensitive materials. “It’s definitely a game-changer,” says Lew Yan Voon. “This is the paper we’ve been waiting for in the field.”
Not everyone is so enthusiastic about silicene’s prospects. “There’s a lot of talk about silicene, germanene and phosphorene,” says Jari Kinaret of Chalmers University of Technology in Gothenburg, Sweden, who is the director of the European Union’s Graphene Flagship, a €1-billion (US$1.1-billion) research project to study two-dimensional materials and develop applications for them, “but the difficulties with them are still quite substantial.”
Le Lay, however, is convinced that researchers will flock to silicene. “Now that a device has been made,” he says, “other scientists will see it is not a dream material, it is a practical thing.”
- Journal name:
- Date published: