The GaN thin film is then exfoliated by a flexible substrate, paving the way to flexible electronics and the reuse of the wafers. The technique can also be used with other materials such as gallium arsenide and lithium fluoride and the thin films stacked to create flexible systems.
“We’ve opened up a way to make flexible electronics with so many different material systems other than silicon,” said Jeehwan Kim, Associate Professor in the departments of Mechanical Engineering and Materials Science and Engineering.
The work is detailed in Nature Materials and backed by the the US Defense Advanced Research Projects Agency, Department of Energy and Air Force Research Laboratory, as well as LG Electronics, Amore Pacific, LAM Research and Analog Devices.
The technique uses a graphene layer on a silicon wafer as a substrate taht allows the thin film to be peeled off and the wafer re-used. “We found that the interaction through graphene is determined by the polarity of the atoms. For the strongest ionically bonded materials, they interact even through three layers of graphene,” said Kim. “It’s similar to the way two magnets can attract, even through a thin sheet of paper.”
The material through which the atoms interact also matters, the team found. In addition to graphene, they experimented with an intermediate layer of hexagonal boron nitride (hBN), a material that resembles graphene’s atomic pattern and has a similar Teflon-like quality, enabling overlying materials to easily peel off once they are copied.
However, hBN is made of oppositely charged boron and nitrogen atoms, which generate a polarity within the material itself. In their experiments, the researchers found that any atoms flowing over hBN, even if they were highly polarized themselves, were unable to interact with their underlying wafers completely, suggesting that the polarity of both the atoms of interest and the intermediate material determines whether the atoms will interact and form a copy of the original semiconducting wafer.
Next: Stacked films
“Now we really understand there are rules of atomic interaction through graphene,” said Kim. “People have mostly used silicon wafers because they’re cheap. Now our method opens up a way to use higher-performing, nonsilicon materials. You can just purchase one expensive wafer and copy it over and over again, and keep reusing the wafer. And now the material library for this technique is totally expanded.”
This could be used to fabricate ultrathin, flexible films from a wide variety of previously exotic, semiconducting materials as long as the materials are made from atoms with a degree of polarity, says Kim. Such ultrathin films could potentially be stacked, one on top of the other, to produce tiny, flexible, multifunctional devices, such as wearable sensors, flexible solar cells, and even, in the distant future, “cellphones that attach to your skin.”
“In smart cities, where we might want to put small computers everywhere, we would need low power, highly sensitive computing and sensing devices, made from better materials,” said Kim. “This unlocks the pathway to those devices.”
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