Argonne National Laboratory Center for Nanoscale Materials U.S. Department of Energy

New Flexible Transparent Thin-Film Transistors

thin-film transistor

The thin-film transistor is flexible and transparent, and it performs as well as commercial versions. Displayed is an array of transistors — each of which is just 10 atomic layers thick.

SEM image of thin-film transistor

SEM image of the thin-film transistor fabricated by using atomic layers of graphene, hexagonal boron nitride, and tungsten diselenide

Argonne's thin-film transistor team

The TFT team (from left): Andreas Roelofs, Anirudha Sumant, and Richard Gulotty; in foreground, Saptarshi Das

Users from Argonne National Laboratory's High Energy Physics Division, working with Center for Nanoscale Materials researchers, report 10-atomic-layer-thick, high-mobility, transparent thin film transistors (TFTs) with ambipolar device characteristics fabricated on both a conventional silicon platform as well as on a flexible polyethylene terephthalate (PET) substrate. Monolayer graphene served as metal electrodes, 3-4 atomic layers of h-BN were used as the gate dielectric, and bilayers of tungsten diselenide (WSe2) comprised the semiconducting channel material. This result comprises the first report of a two-dimensional transparent TFT fabricated on a flexible substrate along with the highest reported mobility and current on-off ratio values thus far. The transistors, just a few atoms thick and therefore transparent, could lead to bright, high-resolution displays that are power-efficient and bendable.

The field effect carrier mobility was extracted to be 45 cm2/(V·s), which exceeds the mobility values of state-of-the-art amorphous silicon-based TFTs by 100 times. The active device stack of WSe2-hBN-graphene was >88% transparent over the entire visible spectrum, and the device characteristics were unaltered for in-plane mechanical strain of up to 2%. The device also demonstrated remarkable temperature stability over 77-400 K. A low contact resistance value of 1.4 kΩ-µm, sub-threshold slope of 90 mv/decade, current ON-OFF ratio of 107, and presence of both electron and hole conduction were observed in the all two-dimensional TFTs, which are extremely desirable but rarely reported characteristics of most of the organic and inorganic TFTs.

Transistors are electronic switches that turn current on and off in many types of electronics. In flat-panel displays, thin-film transistors made with amorphous silicon drive the lighting of individual pixels. There is interest in even thinner semiconducting materials such as WSe2 to make transistors because this two-dimensional material has better electronic properties than silicon. In particular, WSe2 has a larger band gap — the difference in energy between the conducting and nonconducting states — so transistors made from this material in theory needs less power to activate than silicon-based ones require. Electrons travel in the devices about 100 times faster than in amorphous-silicon devices. Such high electron mobility means faster switching transistors, which dictates a display's refresh rate and is necessary for high-quality video, especially three-dimensional video.

Fabrication and testing of the device was carried out in the Nanofabrication & Devices cleanroom facility of the Center for Nanoscale Materials. The group fabricated the devices on a plastic substrate following a standard protocol, depositing the materials layer by layer and using lithography and etching to pattern the layers.

S. Das, R. Gulotty, A. V. Sumant, and A. Roelofs, Nano Letters, 14, 2861 (2014)

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June 2014

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