Most computing devices today are made of silicon, which is second only to oxygen and is the second largest oxygen-containing element on earth, in various forms in rocks, clay, sand and soil. On Earth, although silicon is not the best semiconductor material, it is the easiest to obtain. Therefore, silicon is the dominant material in most electronic devices such as sensors, solar cells, computers, and smart phones.

Now, MIT engineers have developed an ultra-thin semiconductor film made of special materials. They made flexible films made of gallium arsenide, gallium nitride and lithium fluoride, which exhibited better performance than silicon. However, these materials have so far been too costly to produce in functional device applications.

The researchers say the new technology provides a more cost-effective method for fabricating flexible electronic components made from a combination of semiconductor components, which performs better than current silicon-based devices.

“We have opened up a new way to make flexible electronic devices from many materials other than silicon,” said Jeehwan Kim, associate professor of mechanical engineering, materials science and engineering. He hopes the technology can be used to make low-cost, high-performance devices such as flexible solar cells, wearable computers and sensors.

On October 8, the paper on this new technology was published in the journal Nature Materials. In addition to Kim, the co-authors of the paper include Wei Kong, Huashan Li, Kuan Qiao, Yunjo Kim, Kyusang Lee, Doyoon Lee, Tom Osadchy, Richard Molnar, Yang Yu, Sang-hoon Bae, Yang Shao of the Massachusetts Institute of Technology. -Horn and Jeffrey Grossman, as well as researchers from Sun Yat-Sen University, the University of Virginia, the University of Texas at Dallas, the US Naval Research Laboratory, the Ohio State University, and the Georgia Institute of Technology, and the US Defense Advanced Research Projects Agency, USA Partial support from the Department of Energy, the US Air Force Laboratory, LG Electronics, Amore Pacific Group, Fanlin Group, and Analog Devices.

In 2017, Kim and colleagues used graphene to design a solution for making "copy" of expensive semiconductor materials. They found that graphene is stacked on pure, expensive semiconductor wafer materials such as gallium arsenide. When gallium and arsenic atoms flow through the graphene stack, the atoms appear to interact with the underlying atomic layer in some way. The middle graphene seems to be invisible or transparent. As a result, these atoms are grouped into a precise single crystal pattern of the underlying semiconductor wafer to form a precise "copy" that can be easily peeled off from the graphene layer.

They refer to this technology as "remote epitaxy," providing a low-cost solution for fabricating multilayer gallium arsenide films using only one expensive underlying wafer.

Shortly after the first results were reported, the team wondered if the technology could be used to replicate other semiconductor materials. They tried to apply "remote epitaxy" to two inexpensive semiconductors, silicon and germanium, but they found that when these atoms flowed over graphene, they could not interact with their lower layers. The "transparent" graphene changed again. It is "opaque", preventing silicon and germanium atoms from "seeing" the atoms on the other side.

In fact, silicon and germanium are two elements that exist within the same group of the periodic table. Specifically, these two elements belong to the fourth group, and such materials are ion neutral and have no polarity.

“This gives us a hint,” Kim said. The team concluded that perhaps atoms can pass through graphene interactions only through certain ionic charges. For example, in the case of gallium arsenide, at the interface, arsenic has a positive charge and gallium has a negative charge. This difference in charge or polarity may help the atoms interact through the graphene as it is transparent and replicate the atomic pattern below.

"We found that the interaction through graphene depends on the polarity of the atoms. For the strongest ionic bond materials, they can even interact through three layers of graphene," Kim said. “It's similar to the way two magnets are attracted, even a thin piece of paper.”

To test their hypothesis, the researchers used remote epitaxy to replicate semiconductor materials with different polarities, from neutral silicon and germanium to slightly polarized gallium arsenide, and finally highly polarized lithium fluoride (a type of A semiconductor that is better and more expensive than silicon).

They found that the deeper the degree of polarization, the stronger the atomic interaction and, in some cases, the ability to pass multiple sheets of graphene. Each film they can produce is flexible, only a few tens to hundreds of nanometers thick.

The team found that substances that interact with atoms are also important. In addition to graphene, they experimented with a hexagonal boron nitride (HBN) intermediate layer, a material similar to the graphene atom pattern, and having a Teflon-like quality. When replicating, the material stacked on top of it can be very Easily peeled off.

However, hexagonal boron nitride is composed of electrically opposite boron and nitrogen atoms, which create polarity within the material itself. In their experiments, the researchers found that any atoms flowing through hexagonal boron nitride, even if they are highly polar, do not fully interact with the wafers underneath them. It also shows that the polarity of the atoms and intermediate materials determines whether the atoms will interact and form a copy of the original semiconductor wafer.

"Now, we really understand the rules for the interaction of atoms through graphene," Kim said.

He said that with this new rule, researchers can now simply look at the periodic table and select two oppositely charged elements. Once they acquire or manufacture the main wafer through the same elements, they can use the team's remote epitax technology to make multiple layers of accurate copies of the original wafer.

“People mostly use silicon because they are cheap,” Kim said. “Now, our technology opens up a way to use higher performance non-silicon materials. You can buy an expensive wafer and copy it over and over again, and reuse it. Now, the material library for this technology has Fully expanded."

Kim envisions that remote epitaxy can now be made into ultra-thin flexible films using previously considered semiconductor materials, as long as these materials are made of atoms of a certain polarity. These ultra-thin films can be stacked together layer by layer to produce tiny, flexible and versatile devices such as wearable sensors, flexible solar cells, and even in the distant future, "mobile phones can be attached to your skin." ."

“In smart cities, we want to place small computers anywhere, which requires low-power, high-sensitivity computing and sensing devices made from better materials,” says Kim. “This research has opened the way for these devices.”

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