MIT engineers grow perfect atom-thin material

Image: By depositing atoms on a mask-coated wafer (top left), MIT engineers trap the atoms in individual pockets of the mask (middle middle), where they grow into a complete 2D single crystal layer (bottom). can encourage you to do so. correct).see more

Credits: Image credits: Jeehwan Kim, Ki Seok Kim, et al.

According to Moore’s Law, the number of transistors on a microchip has doubled every year since the 1960s. However, this trajectory is predicted to level off soon. This is because silicon, the backbone of modern transistors, loses its electrical properties when devices made of this material are below a certain size.

Enter 2D Materials — delicate two-dimensional sheets of perfect crystals as thin as a single atom. At the nanometer scale, 2D materials can conduct electrons much more efficiently than silicon. As such, the search for next-generation transistor materials has focused on his 2D materials as potential successors to silicon.

But before the electronics industry can move to 2D materials, scientists must first find ways to design them on industry-standard silicon wafers while preserving their perfect crystalline morphology. And MIT engineers may have a solution now.

The team developed a method that allows chip makers to make even smaller transistors from 2D materials by growing them on existing wafers of silicon and other materials. The new method is a form of “non-epitaxial single-crystal growth,” which the team has used for the first time to grow pure, defect-free 2D material on industrial silicon wafers.

With their method, the team fabricated simple functional transistors from a type of 2D material called transition metal dichalcogenides (TMDs). TMDs are known to conduct electricity better than silicon on the nanometer scale.

Jeehwan Kim, Associate Professor of Mechanical Engineering at MIT, said: “We unlocked ways to keep up with Moore’s Law using 2D materials.”

Kim and his colleagues describe the method in detail in a paper published in Nature. His MIT co-authors on this study include Ki Seok Kim, Doyoon Lee, Celesta Chang, Seunghwan Seo, Hyunseok Kim, Jiho Shin, Sangho Lee, Jun Min Suh, and Bo-In Park, and co-authors at the University of Texas at Dallas. Includes researchers. University of California, Riverside, Washington University in St. Louis, and institutions across Korea.

crystal patchwork

To generate 2D materials, researchers typically employ a manual process of carefully exfoliating atomically thin flakes from a bulk material, much like peeling the layers off an onion.

However, most bulk materials are polycrystalline, containing multiple crystals growing in random directions. Where one crystal meets another, “grain boundaries” act as electrical barriers. Electrons flowing in one crystal stop abruptly when they meet a crystal in the other direction, causing the material’s conductivity to decay. Even after exfoliating the 2D flakes, researchers still need to look for ‘monocrystalline’ regions from the flakes. This is a tedious and time-consuming process that is difficult to apply on an industrial scale.

Recently, researchers discovered another way to manufacture 2D materials. It is grown on wafers of sapphire, a material with a hexagonal pattern of atoms that encourages the 2D material to assemble in the same single crystal orientation.

“But no one in the memory or logic industry uses sapphire,” says Kim. “All infrastructure is based on silicon. Semiconductor processing requires the use of silicon wafers.”

However, silicon wafers lack the sapphire hexagonal support scaffold. When the researchers try to grow his 2D material on silicon, the crystals fuse together in random patchworks, forming numerous grain boundaries that inhibit conductivity.

“Growing a single crystal, 2D material on silicon is considered nearly impossible,” says Kim. “Now we show you what you can do. And our trick is to prevent the formation of grain boundaries.”

seed pocket

With the team’s new “non-epitaxial single crystal growth,” there’s no need to peel or hunt for flakes of 2D material. Instead, researchers use traditional vapor deposition methods to deliver atoms across a silicon wafer. Atoms eventually settle on the wafer, nucleate, and grow into two-dimensional crystal orientations. Left undisturbed, each “nucleus” or crystal seed grows in random directions across the silicon wafer. But Kim and his colleagues have found a way to align each growing crystal to create single-crystal regions across the wafer.

To do so, they first covered the silicon wafer with a “mask”. This was a coating of silicon dioxide, each patterned into tiny pockets designed to trap a crystal seed. Atomic gases were then flowed across the masked wafer, settling in each pocket to form a 2D material (TMD in this case). Pockets in the mask enclosed the atoms and encouraged them to assemble in the same monocrystalline direction on the silicon wafer.

“This is a very striking result. We see single-crystal growth everywhere, even if there is no epitaxial relationship between the 2D material and the silicon wafer,” says Kim.

The team used the masking method to fabricate a simple TMD transistor and showed that its electrical performance was as good as a pure flake of the same material.

They also applied this method to design multilayer devices. After covering the silicon wafer with a patterned mask, he grows one kind of 2D material to fill half of each square, then he grows a second kind of 2D material on top of the first layer I filled in the remaining squares. The result was an ultra-thin single-crystal bilayer structure within each square. Going forward, Kim says, multiple of his 2D materials can be grown and stacked in this way to create ultra-thin, flexible, multifunctional films.

“Until now, there has been no way to create monocrystalline 2D materials on silicon wafers, so the entire community has largely given up on pursuing 2D materials for next-generation processors,” said Kim. say. “Now we have completely solved this problem with a way to make devices smaller than a few nanometers. This will change the Moore’s Law paradigm.”

This work was supported in part by DARPA, Intel, the IARPA MicroE4AI program, MicroLink Devices, Inc., ROHM Co., and Samsung.

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MIT News Office, Written by Jennifer Chu

article title

Non-epitaxial wafer-scale single-domain 2D heterostructures obtained by confined layer-by-layer growth Y7B

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