Growth of large-area semiconductor films opens up possibilities in flexible electronics.
Researchers dream of making miniature, flexible electronic circuits from films just a few atoms thick. But growing such 2D films at the scale needed to produce batches of reliable electronic devices has been a challenge.
Reporter Lizzie Gibney finds a class of materials with exotic properties – but can they one day rival silicon?
Now materials scientists have devised a way to grow single layers of a promising class of 2D semiconductor on silicon wafers that are 10 centimetres across — all the while maintaining the impressive electronic properties seen in smaller samples1. They used the films to make hundreds of transistors, which tests showed worked in 99% of cases.
“Lots of people are trying to grow single layers on this large scale, myself among them,” says Georg Duesberg, a materials scientist at Trinity College Dublin. “But it looks like these guys have really done it.”
The semiconductors in question are known as transition-metal dichalcogenides (TMDs). A single-layer TMD is three atoms thick: it comprises a sheet of atoms from a family of elements called transition metals (which include molybdenum and tungsten) sandwiched between two layers of chalcogen atoms (such as sulfur, selenium and tellurium).
Like their carbon-based cousin graphene, TMD sheets are strong, thin and flexible and conduct electrons. But unlike graphene, they are also semiconductors — meaning that the flow of electrons can be easily switched off and on. TMDs are unlikely ever to replace the most famous semiconductor, silicon, whose manufacture has been honed over decades. But they could form films more than a thousand times thinner than today's components made from silicon slabs, allowing for flexible transistors, displays, and light detectors.
TMD layers can be peeled from a multi-layered crystal, much as graphene can be pulled from graphite with sticky tape. But the results can be inconsistent and the process is time consuming. An alternative — growing the material atom by atom from a gas of precursor chemicals — has so far produced only small-area samples that are often more than one layer thick.
Publishing in Nature1 on 29 April, Jiwoong Park at Cornell University in Ithaca, New York, and his colleagues have adapted this technique to grow large single-layer films. Over 26 hours and at a temperature of 550 °C, they grew two types of TMD — molybdenum disulfide and tungsten disulfide — onto circular silicon wafers around 10 centimetres in diameter. They also grew successive layers separated by sheets of silicon dioxide, which they say opens up the possibility of making tiny, high-density 3D circuits in which components stack up vertically.
The ability to grow a layer just three atoms thick over a distance more than 100 million times that in length is a “dazzling engineering marvel”, says Philip Kim, an experimental condensed-matter physicist at Harvard University in Cambridge, Massachusetts.
The layers are not only uniform but also have electronic properties comparable to those of sheets peeled from a crystal, says Park. The trick, he says, was to source each ingredient from gases in which each molecule contained just one atom of the transition metal or chalcogen. By altering the pressure of the gas, the team could control the concentration of each ingredient and carefully engineer the film’s growth.
The technique is an exciting development, says Pulickel Ajayan, a material scientist at Rice University in Houston, Texas. But the films will need to be grown on other surfaces, including flexible substrates, to make commercial devices that harness the promise of 2D materials, he says.
The most exciting part of being able to make large-area films, says Duesberg, is that it should allow researchers to establish more realistically how TMDs might be used in electronics. TMDs have excited researchers not just because they are thin and flexible, but also because they have properties that might be exploited in experimental alternatives to electronics known as spintronics and valleytronics.
“A lot of people believe that TMD monolayers could revolutionize electronics, but until now people have only been making individual devices, where it's easier to produce fancy results," Duesberg says.
Kang, K. et al. Nature 520 656-660 (2015).
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Gibney, E. Batches of ultra-thin transistors made from 2D materials . Nature (2015). https://doi.org/10.1038/nature.2015.17435