Semiconductor Materials Offer Alternatives to Silicon

Silicon is a good semiconducting material because it’s abundant and cost-effective. Yet researchers have been looking for alternative materials that can perform even better for high-performance electronics. Researchers at MIT think they can identify some of those alternatives with a new technique for fabricating ultra-thin semiconducting films comprised of exotic materials other than silicon.

The scientists created flexible films from gallium arsenide, gallium nitride, and lithium fluoride. They have exhibited better semiconducting performance than silicon, but until now have been cost-prohibitive in terms of the production of functional devices, they said in an MIT news release.

“We’ve opened up a way to make flexible electronics with so many different material systems other than silicon,” said Jeehwan Kim, an MIT professor in the  departments of mechanical engineering and materials science and engineering, who worked on the research.

Kim and fellow researchers envision that the technique can be used to manufacture low-cost flexible solar cells and wearable computers and sensors that significantly outperform current devices that use silicon-based semiconductors—even “cellphones that attach to your skin,” he said in the MIT release.

Previous Work

The recent work builds on research the team conducted last year to use graphene to produce copies of expensive semiconducting materials. They stacked graphene on top of a pure, expensive wafer of semiconducting material such as gallium arsenide. They then flowed atoms of gallium and arsenide over the stack.

This produced a result in which the atoms appeared to interact in some way with the underlying atomic layer, as if the intermediate graphene were invisible or transparent, researchers found. As a result, the atoms assembled into the precise, single-crystalline pattern of the underlying semiconducting wafer, forming an exact copy that could easily be peeled away from the graphene layer.

Calling their technique “remote epitaxy,” researchers provided an affordable way to fabricate multiple films of gallium arsenide using just one expensive underlying wafer, they said.

New Hints

Once they accomplished this, the team set out to see if they could use remote epitaxy to copy other semiconducting materials using silicon and germanium. When the researchers flowed these atoms over graphene, however, the atoms failed to interact with their respective underlying layer as if the graphene—which had been transparent—became suddenly opaque, preventing atoms of silicon and germanium from “seeing” the atoms on the other side.   

Silicon and germanium exist within group four of the periodic table of elements—a class of materials that are ionically neutral. So researchers got a “hint” as to why they performed the way they did in the team’s method, Kim said.

“We found that the interaction through graphene is determined by the polarity of the atoms,” he said. “For the strongest ionically bonded materials, they interact even through three layers of graphene. It’s similar to the way two magnets can attract, even through a thin sheet of paper.” The researchers published a paper on their work in the journal Nature.

They tested their hypothesis using materials of various degrees of polarity, from neutral silicon and germanium to slightly polarized gallium arsenide and finally, highly polarized lithium fluoride—a better, more expensive semiconductor than silicon.

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Understanding the Rules

In producing flexible films at the nano-scale, researchers found that the greater the degree of polarity, the stronger the atomic interaction—even, in some cases, through multiple sheets of graphene. “Now we really understand there are rules of atomic interaction through graphene,” Kim said.

Researchers can look at the periodic table and pick two elements of opposite charge and—once they acquire or fabricate a main wafer made from the same elements—they can then apply remote-epitaxy techniques to fabricate multiple, exact copies of the original wafer, he said.

“People have mostly used silicon wafers because they’re cheap,” Kim said. “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.”

Elizabeth Montalbano is a freelance writer who has written about technology and culture for 20 years. She has lived and worked as a professional journalist in Phoenix, San Francisco, and New York City. In her free time, she enjoys surfing, traveling, music, yoga, and cooking. She currently resides in a village on the southwest coast of Portugal.