Known as “strain engineering”, the use of localized strain to modify the properties of semiconductors is often used in the electronic industry. Already, based on earlier work at MIT, some degree of elastic strain has been incorporated in some silicon processor chips. Even a 1 percent change in the structure can in some cases improve the speed of a device by 50 percent, by allowing electrons to move through the material faster.
And unlike other ways of changing a material’s properties, like doping which produces a permanent, static change, strain engineering allows properties to be changed on the fly. “Strain is something you can turn on and off dynamically,” explains one of the authors Ju Li, MIT professor of nuclear science and engineering and of materials science and engineering. Very often, strain can be induced through localized heating.
But Strain can be applied in any of six different ways (in three different dimensions, each one of which can produce strain in-and-out or sideways), and with nearly infinite gradations of degree, so the full range of possibilities is impractical to explore simply by trial and error. “It quickly grows to 100 million calculations if we want to map out the entire elastic strain space,” Li says.
“Now we have this very high-accuracy method” that drastically reduces the complexity of the calculations needed, Li explains, thanks to a novel and systematic AI-powered exploration of materials’ property changes based on strain.
The new method, the researchers say, could open up possibilities for creating materials tuned precisely for electronic, optoelectronic, and photonic devices that could find uses for communications, information processing, and energy applications.
The team studied the effects of strain on the bandgap of both silicon and diamond. Using their neural network algorithm, they were able to predict with high accuracy how different amounts and orientations of strain would affect the bandgap.
“Tuning” of a bandgap can be a key tool for improving the efficiency of a device, such as a silicon solar cell, by getting it to match more precisely the kind of energy source that it is designed to harness.
Whereas this study focused specifically on the effects of strain on the materials’ bandgap, “the method is generalizable” to other aspects, which affect not only electronic properties but also other properties such as photonic and magnetic behavior, Li says.
From the 1 percent strain now being used in commercial chips, many new applications open up now that this team has shown that strains of nearly 10 percent are possible without fracturing. “When you get to more than 7 percent strain, you really change a lot in the material,” he says. The work was supported by the MIT-Skoltech program and Nanyang Technological University.
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