University of Washington makes 2D semiconductor junctions
Scalable technique would suit mass-production
University of Washington (UW) researchers have demonstrated connecting two single-layer semiconductor materials to form a heterojunction using monolayers of molybdenum diselenide and tungsten diselenide. The research was published online this week in Nature Materials.
"Heterojunctions are fundamental elements of electronic and photonic devices," said senior author Xiaodong Xu, a UW assistant professor of materials science and engineering and of physics. "Our experimental demonstration of such junctions between 2D materials should enable new kinds of transistors, LEDs, nanolasers, and solar cells to be developed for highly integrated electronic and optical circuits within a single atomic plane."
Collaborators from the electron microscopy centre at the University of Warwick, UK found that all the atoms in both materials formed a single honeycomb lattice structure, without any distortions or discontinuities. This provides the strongest possible link between two single-layer materials, necessary for flexible devices. Within the same family of materials it is feasible that researchers could bond other pairs together in the same way.
As seen under an optical microscope, the heterostructures have a triangular shape. The two different monolayer semiconductors can be recognised through their different colours.
The researchers created the junctions in a small furnace at the UW. First, they inserted a powder mixture of the two materials into a chamber heated to 900degC. Hydrogen gas was then passed through the chamber and the evaporated atoms from one of the materials were carried toward a cooler region of the tube and deposited as single-layer crystals in the shape of triangles.
After a while, evaporated atoms from the second material then attached to the edges of the triangle to create a seamless semiconducting heterojunction.
"This is a scalable technique," said Sanfeng Wu, a UW doctoral student in physics and one of the lead authors. "Because the materials have different properties, they evaporate and separate at different times automatically. The second material forms around the first triangle that just previously formed. That's why these lattices are so beautifully connected."
With a larger furnace, it would be possible to mass-produce sheets of these semiconductor heterostructures, the researchers said. On a small scale, it takes about five minutes to grow the crystals, with up to two hours of heating and cooling time.
"We are very excited about the new science and engineering opportunities provided by these novel structures," said senior author David Cobden, a UW professor of physics. "In the future, combinations of two-dimensional materials may be integrated together in this way to form all kinds of interesting electronic structures such as in-plane quantum wells and quantum wires, superlattices, fully functioning transistors, and even complete electronic circuits."
The researchers have already demonstrated that the junction interacts with light much more strongly than the rest of the monolayer, which is encouraging for optoelectric and photonic applications like solar cells.
