Semiconductor 'flipped' to insulator above room temperature

Researchers at the University of Michigan (U-M), supported by Cornell University, the U.S. Naval Research Laboratory and the Chinese Academy of Sciences, have developed a semiconducting material that performed a quantum ‘flip’ from a conductor to an insulator above room temperature — potentially bringing the world closer to a new generation of quantum devices and ultra-efficient electronics.

Observed in two-dimensional layers of tantalum sulfide only a single atom thick, the exotic electronic structure that supported this quantum flip was previously only stable at ultra-cold temperatures of -73°C. The new material remains stable at up to 77°C, according to a study published in the journal Nature Communications.

Today’s electronics use tiny electronic switches to store data; ‘on’ is one and ‘off’ is zero, and the data disappears when the power is turned off. Future devices could use other states, like ‘conductor’ or ‘insulator’, to store digital data, requiring only a quick blip of energy to switch between states rather than a steady stream of electricity. That could lead to far more powerful and more energy-efficient devices.

In the past, however, such exotic behaviour has only been observed in materials at super-cold temperatures. The ultimate goal is to develop materials that can quickly ‘flip’ from one state to another on demand and at room temperature. The U-M research could be an important step in that direction, according to corresponding author Robert Hovden.

“Previous research at ultra-cold temperatures has shown that it’s possible to make these kinds of flips happen on demand, over and over again,” Hovden said. “The fact that we were able to keep even one flip stable at room temperature opens a lot of exciting possibilities.”

The flip from conductor to insulator is supported by a phenomenon called a charge density wave — an ordered, crystal-like pattern of positive and negative electrical charge that occurs spontaneously in certain conditions. According to Hovden, “Charge density waves have been observed before in bulk samples of tantalum sulfide, but the material had to be at ultra-cold temperatures. By interleaving several two-dimensional layers together, we were able to make it much more stable.”

The team began by fabricating a sample of several single-atom-thick layers of tantalum sulfide sandwiched together. Each layer was a semiconductor in what’s called an octahedral state, which refers to a specific arrangement of tantalum and sulfur atoms. While some charge density waves were present, they were too unstable and disordered to give rise to exotic behaviour like a conductor-insulator flip.

Suk Hyun Sung, a graduate researcher in Hovden’s lab and the first author of the study, changed the sample’s properties by heating it in an oxygen-free environment while observing the process under an electron microscope. As the sample heated, layers began to switch, one by one, into a prismatic state — a different arrangement of the same atoms.

When most, but not all, layers had switched to prismatic state, Sung cooled the sample back to room temperature. He found that the layers that remained in the octahedral state were exhibiting charge density waves that were orderly and stable, and stayed that way at temperatures of up to 77°C. In addition, those layers had flipped from semiconductors to insulators.

“Most 2D materials are subject to all the defects of whatever they’re sitting on, whatever’s in the air, and that makes them very unstable,” Sung said. “But we discovered that when octahedral layers are nestled between several prismatic layers, they’re much more stable.”

The team is now examining the phenomenon further, tweaking more variables of the process and testing mechanisms to control the exotic behaviours spurred by the charge density waves. For now, the new discovery has given them an important glimpse into the workings of quantum states and two-dimensional materials.

“We’ve opened up a new playground for the future of electronic and quantum materials,” Hovden said. “It represents a whole new way to access exotic states.”

Image credit: ©stock.adobe.com/au/Brownfalcon

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