Scientists Look to TMDs for Future of Transistors

05 December 2016

Graphene, with its exceptional conductive properties, has been touted for its potential to replace conventional semiconductors in transistors, shrinking them even further. But even though the potential of this remarkable one-atom-thick, 2-D material has yet to be fully realized, researchers already are looking beyond graphene to perhaps an even better material for future transistors.

UTD researchers Dr. Fan Zhang, right, and Armin Khamoshi. Credit: University of Texas at Dallas UTD researchers Dr. Fan Zhang, right, and Armin Khamoshi. Credit: University of Texas at Dallas Physicists at the University of Texas at Dallas (UTD) have published new findings examining the electrical properties of so-called transition metal dichalcogenides, or TMDs, that could be harnessed for next-generation transistors and electronics.

Transition metal dichalcogenide monolayers are atomically thin semiconductors of the type MX2, with M as a transition metal atom and X as a chalcogen atom, where one layer of M atoms is sandwiched between two layers of X atoms. In recent years, scientists and engineers have become interested in TMDs in part because they are superior in some ways to graphene.

"It was thought that graphene could be used in transistors, but in transistors, you need to be able to switch the electric current on and off. With graphene, however, the current cannot be easily switched off,” said Dr. Fan Zhang, UTD assistant professor of physics who, along with senior physics student Armin Khamoshi, recently published their research on transition metal dichalcogenides.

Like graphene, TMDs can be made into thin, 2-D sheets, or monolayers, a mere few molecules thick. However, “TMDs have something graphene does not have—an energy gap that allows the flow of electrons to be controlled, for the current to be switched on and off," said researcher Khamoshi. "This gap makes TMDs ideal for use in transistors. TMDs are also very good absorbers of circularly polarized light, so they could be used in detectors. For these reasons, these materials have become a very popular topic of research."

But one of the challenges is to optimize and increase electron mobility in TMD materials—a key factor if they are to be developed for use in transistors, researchers said.

In their most recent project, Zhang and Khamoshi provided the theoretical work to guide their collaborating scientists at the Hong Kong University of Science and Technology on the layer-by-layer construction of a TMD device, as well as on the use of magnetic fields to learn how the electrons move through the device. For the project, each monolayer of TMD was three molecules thick, and the layers were sandwiched between two sheets of boron nitride molecules.

The group discovered that how electrons behave in the TMDs depends on whether an even or odd number of TMD layers were used.

Results of the study not only help decipher the intrinsic properties of TMD materials, “but also demonstrate that we achieved high electron mobility in the devices,” said Zhang. “This gives us hope that we can one day use TMDs for transistors.”

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