Mismatched 2-D Layers Combine to Create New Optoelectronic Devices

For the first time, different lattice structures can be grown together to form a perfectly aligned bilayer

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Mismatched 2-D Layers Combine to Create New Optoelectronic Devices
Illustration: Xufan Li and Chris Rouleau/Oak Ridge National Laboratory/U.S. Dept. of Energy

Researchers have had success in using two-dimensional (2-D) materials to fabricate layered structures through epitaxy (the growth of crystals on a substrate), but only if they shared similar crystal lattices.

Now researchers at the U.S. Department of Energy’s Oak Ridge National Laboratory (ORNL) have shown that 2-D materials with dissimilar crystal lattices can still be grown together using epitaxy techniques. The ORNL team was able to grow a layer of gallium selenide—a p-type semiconductor—on top of molybdenum diselenide—an n-type semiconductor. This combination resulted in an atomically thin solar cell.

“Because the two layers had such a large lattice mismatch between them, it’s very unexpected that they would grow on each other in an orderly way,” said ORNL’s Xufan Li, lead author of the study, in a press release. “But it worked.”

In the research described in the journal Science Advances, the ORNL researchers first grew the molybdenum diselenide and then grew a layer of gallium selenide on top. The layers were held together by van der Waals forces. 

The resulting p–n junction generated a photovoltaic response to incoming light by separating electron–hole pairs.

This research holds out the promise that combining mismatched 2-D materials can create new families of materials and increase the number of atomically thin electronic devices that can be produced.

“These new 2-D mismatched layered heterostructures open the door to novel building blocks for optoelectronic applications,” said senior author Kai Xiao of ORNL in the press release. “They can allow us to study new physics properties which cannot be discovered with other 2-D heterostructures with matched lattices.”

Combined epitaxial growth of different 2-D materials has a bit of a history at ORNL. Two years ago, researchers there, along with collaborators at the University of Tennessee, demonstrated that it was possible to fabricate through epitaxy a combination of hexagonal boron nitride and graphene.

Just as in that previous work, this latest ORNL research demonstrated that the boundary where the two materials join can be made atomically precise. But what sets the latest device apart is that the boundary is the point at which light sets in motion the migration of charge carriers.

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Emily Cooper

Perhaps the most far-reaching technological achievement over the last 50 years has been the steady march toward ever smaller transistors, fitting them more tightly together, and reducing their power consumption. And yet, ever since the two of us started our careers at Intel more than 20 years ago, we’ve been hearing the alarms that the descent into the infinitesimal was about to end. Yet year after year, brilliant new innovations continue to propel the semiconductor industry further.

Along this journey, we engineers had to change the transistor’s architecture as we continued to scale down area and power consumption while boosting performance. The “planar” transistor designs that took us through the last half of the 20th century gave way to 3D fin-shaped devices by the first half of the 2010s. Now, these too have an end date in sight, with a new gate-all-around (GAA) structure rolling into production soon. But we have to look even further ahead because our ability to scale down even this new transistor architecture, which we call RibbonFET, has its limits.

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