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Cornell chemists produce three atom-thick electronic sheets

05 May 2015

Researchers have demonstrated a way to create a new kind of semiconductor thin film that retains its electrical properties even when it is just atoms thick.

A molybdenum disulphide device array on a transparent silica wafer (photo: Kibum Kang/Cornell)

Making thin films out of semiconducting materials is analogous to how ice grows on a windowpane: When the conditions are just right, the semiconductor grows in flat crystals that slowly fuse together, eventually forming a continuous film.

This process of film deposition is common for traditional semiconductors like silicon or gallium arsenide – the basis of modern electronics – but Cornell scientists are pushing the limits for how thin they can go. They have demonstrated a way to create a new kind of semiconductor thin film that retains its electrical properties even when it is just atoms thick.

Three atom-thick layers of molybdenum disulphide were produced in the lab of Jiwoong Park, an associate professor of chemistry at Cornell. The films were designed and grown by post-doctoral associate Kibum Kang and graduate student Saien Xie. Their work was published April 30 in Nature, online edition.

“The electrical performance of our materials was comparable to that of reported results from single crystals of molybdenum disulphide, but instead of a tiny crystal, here we have a 4-inch wafer,” says Park.

Molybdenum disulphide, which is garnering worldwide interest for its excellent electrical properties, has previously been grown only in disjointed, 'archipelago-like' single crystal formations. But making smooth, flat, ultra-thin sheets, like paper, is the ultimate goal, and the bridge to actual devices.

The researchers tuned the growth conditions of their films using a technique called metal organic chemical vapour deposition (MOCVD). Already used widely in industry, but with different materials, it starts with a powdery precursor, forms a gas and deposits single atoms on a substrate, one layer at a time.

Park’s group optimised the technique by varying humidity and temperature. For example, they found that their crystals grew perfectly stitched together, but only with a little bit of hydrogen and in completely dry conditions.

The team also demonstrated their films’ efficacy when stacked layer-by-layer, alternating with silicon dioxide and employing standard photolithography, suggesting that these semiconducting films could be made into multi-level electronic devices of unsurpassed thinness.

It was also possible to change the MOCVD precursor to make other films; for example, the researchers grew a tungsten disulphide film with different electrical properties and colour. 

“These were only the first two materials, but we want to make a whole palette of materials,” Park says.

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