Cost-effective method produces semiconductor films from materials that perform better than silicon – ScienceDaily

Today, MIT engineers have developed a technique for making ultra-thin semiconductor films made from many exotic materials other than silicon. To demonstrate their technique, the researchers fabricated flexible films based on gallium arsenide, gallium nitride and lithium fluoride, materials that perform better than silicon but whose manufacture in functional devices was prohibitive.

According to the researchers, this new technique provides a cost-effective method of manufacturing flexible electronic components made from any combination of semiconductor elements, likely to provide better performance than current silicon-based devices.

"We have paved the way for the manufacture of flexible electronic components with as many different material systems, apart from silicon," says Jeehwan Kim, an associate professor of career development in the 1947 class at the mechanical engineering, science and technology departments. materials and civil engineering. Kim envisions that this technique can be used to make inexpensive and powerful devices such as flexible solar cells, laptops and sensors.

The details of the new technique are reported today in Nature Materials. In addition to Kim, the co-authors of MIT are: Wei Kong, Huashan Li, Kuan Qiao, Kim Yunjo, Kyusang Lee, Doyoon Lee, Osadchy Tom, Richard Molnar, Yang Yu, Bae Sang-hoon, Yang Shao-Horn and Jeffrey Grossman, along with researchers from Sun Yat-Sen University, the University of Virginia, the University of Texas at Dallas, the US Naval Research Laboratory, Ohio State University and Georgia Tech .

Now you see it, now you do not do it

In 2017, Kim and his colleagues developed a method for producing expensive "copies" of semiconductor materials using graphene – a thin layer of atomically shaped carbon atoms arranged in hexagonal shaped form. fence. They discovered that, when they stacked graphene on a wafer of pure and expensive semiconductor material such as gallium arsenide, gallium and arsenide atoms then flowed down the stack, and that these atoms interacted in some way with the underlying atomic layer, as if the intermediate graphene was invisible or transparent. As a result, the atoms assembled in the precise monocrystalline pattern of the underlying semiconductor wafer, forming an exact copy that could then be easily peeled off the graphene layer.

This technique, which they call "remote epitaxy", was an affordable way to make several gallium arsenide films using a single, expensive, underlying plate.

Shortly after reporting their first results, the team asked if their technique could be used to copy other semiconductor materials. They tried to apply remote epitaxy on silicon, as well as on germanium – two inexpensive semiconductors – but they found that when they passed these atoms on graphene, they did not interact with their respective underlying layers. It was as if graphene, previously transparent, suddenly became opaque, preventing the silicon and germanium atoms from "seeing" the atoms on the other side.

It turns out that silicon and germanium are two elements existing in the same group of the periodic table of the elements. Specifically, the two elements belong to group four, a class of ionically neutral materials, which means that they have no polarity.

"It gave us a clue," says Kim.

The team felt that atoms may only interact with each other by graphene if they have an ionic charge. For example, in the case of gallium arsenide, gallium has a negative charge at the interface compared to the positive charge of arsenic. This charge difference, or polarity, may have helped the atoms to interact through graphene as it was transparent and to copy the underlying atomic pattern.

"We found that the interaction via graphene is determined by the polarity of the atoms, and for the most powerful ion-binding materials, they interact even through three layers of graphene," Kim explains. "It looks like the way two magnets can attract even through a thin sheet of paper."

Opposites attract

The researchers tested their hypothesis using remote epitaxy to copy semiconductor materials with different degrees of polarity, from neutral silicon to germanium, through slightly polarized gallium arsenide and finally highly polarized lithium fluoride, a semiconductor better and more expensive than silicon.

They found that the higher the degree of polarity, the stronger the atomic interaction, even in some cases, through several sheets of graphene. Each film they were able to produce was flexible and only a few tens to several hundred nanometers thick.

The team discovered that the material through which the atoms interacted also counted. In addition to graphene, they experimented with an intermediate layer of hexagonal boron nitride (hBN), a material that resembles the atomic model of graphene and has a similar quality to that of Teflon, allowing superimposed materials to peel off easily. once copied.

However, hBN is composed of oppositely charged boron and nitrogen atoms, which generate a polarity in the material itself. In their experiments, the researchers found that the atoms circulating on hBN, even if they were very polarized themselves, were unable to fully interact with their underlying platelets, suggesting that the polarity of the Atoms of interest and intermediate material determines will interact and form a copy of the original semiconductor wafer.

"Now, we really understand that there are rules of atomic interaction via graphene," Kim said.

With this new understanding, he says, researchers can now view the periodic table and select two elements of opposite charge. Once they have acquired or manufactured a main wafer from the same elements, they can then apply the team's remote epitaxy techniques to make several exact copies of the wafer. ;origin.

"People have mostly used silicon wafers because they are cheap," says Kim. "Our method now paves the way for the use of higher performing non-silicon materials – you can simply buy an expensive wafer and copy it over and over again, without ceasing to reuse it." And now, the material library of this technique is totally extended. "

Kim thinks that remote epitaxy can now be used to make ultra-thin flexible films from a wide variety of previously exotic semiconductor materials, provided that materials are made from ## EQU1 ## Atoms having a degree of polarity. Such ultra-thin films could potentially be stacked on top of each other to produce tiny, flexible and multifunctional devices, such as portable sensors, flexible solar cells, and even, in the distant future, "cell phones that are going to work. attach to your skin ".

"In smart cities, where we could set up small computers around the world, we would need extremely sensitive, low-power, sensing and computing devices made from better materials," says Kim. "This [study] unlocks the way to these devices. "

This research was funded in part by the Advanced Defense Research Projects Agency, the Department of Energy, the Air Force Research Laboratory, LG Electronics, Amore Pacific, LAM Research and Analog Devices.