Material with high electron mobility is like a highway without traffic. Electrons entering the material move without obstacles or delays that would slow them down or scatter them.

The higher the electron mobility, the more efficient the material's electrical conductivity, and less energy is lost as electrons pass through the material. Advanced materials with high electron mobility will be key to more efficient and sustainable electronic devices that can do more work with less energy consumption.

Now, physicists from MIT, the Army Research Laboratory, and other institutions have achieved a record level of electron mobility in a thin film of ternary tetradymite — a class of minerals naturally found in deep hydrothermal gold and quartz deposits.

For this study, scientists grew pure, ultra-thin films of the material in a way that minimized defects in its crystal structure. They discovered that this nearly perfect film — much thinner than a human hair — exhibits the highest electron mobility in its class.

The team was able to estimate the material's electron mobility by detecting quantum oscillations when an electric current passed through it. These oscillations are a sign of the quantum mechanical behavior of electrons in the material. Researchers found a specific rhythm of oscillations characteristic of high electron mobility — higher than any ternary thin film of this class so far.

Jagadeesh Moodera, a senior scientist in MIT's Department of Physics, says: "Before, what people achieved in terms of electron mobility in these systems was like traffic on a road under construction — you lag behind, you can't drive, it's dusty and messy. In this new optimized material, it's like driving on the Mass Pike without traffic."

The team's results, published today in the journal Materials Today Physics, suggest that ternary tetradymite thin films are a promising material for future electronics, such as wearable thermoelectric devices that efficiently convert waste heat into electrical energy. (Tetradymites are active materials causing a cooling effect in commercial thermoelectric coolers.) The material could also be the basis for spintronic devices, which process information using electron spin, using far less energy than conventional silicon-based devices.

The study also uses quantum oscillations as a highly efficient tool for measuring the material's electronic performance.

Hang Chi, the study's lead author and former scientist at MIT, now at the University of Ottawa, says: "We use this oscillation as a quick test kit. By studying this delicate quantum dance of electrons, scientists can begin to understand and identify new materials for the next generation of technologies that will power our world."
Chi and Moodera's co-authors include Patrick Taylor, a former member of MIT Lincoln Laboratory, along with Owen Vail and Harry Hier from the Army Research Laboratory, and Brandi Wooten and Joseph Heremans from Ohio State University.

Origin of Tetradymites
The name “tetradymite” comes from the Greek word “tetra” for “four” and “dymite,” meaning “twin.” Both terms describe the mineral's crystal structure, consisting of rhombohedral crystals that are “twins” in groups of four — that is, they have identical crystal structures sharing one side.

Tetradymites contain combinations of bismuth, antimony, tellurium, sulfur, and selenium. In the 1950s, scientists discovered that tetradymites exhibit semiconductor properties that could be ideal for thermoelectric applications: The mineral in its large crystal form could passively convert heat into electrical energy.

Then, in the 1990s, the late professor at the Institute Mildred Dresselhaus proposed that the thermoelectric properties of the mineral could be significantly enhanced, not in its large form, but within the microscopic, nanometric surface, where electron interactions are more pronounced. (Heremans was working in Dresselhaus's group at that time.)

Advances in Thin Film Growth
"It became clear that when you look at this material long enough and close enough, new things happen," says Chi. "This material was identified as a topological insulator, where scientists could see very interesting phenomena on their surface. But to continue discovering new things, we need to master the growth of the material."

To grow thin films of pure crystal, researchers used molecular beam epitaxy — a method where a beam of molecules is fired at a substrate, usually in a vacuum, with precisely controlled temperatures. As the molecules deposit on the substrate, they condense and slowly grow, one atomic layer at a time. By controlling the time and type of molecules that deposit, scientists can grow ultra-thin crystal films in exact configurations, with few or no defects.

Patrick Taylor, co-author, explains: "Typically, bismuth and tellurium can swap places, creating defects in the crystal. The system we used to grow these films I brought from MIT Lincoln Laboratory, where we use highly purified materials to reduce impurities to imperceptible limits. It's a perfect tool for this research."

Free Flow
The team grew thin films of ternary tetradymite, each about 100 nanometers thin. They then tested the film's electronic properties by looking for Shubnikov-de Haas quantum oscillations — a phenomenon discovered by physicists Lev Shubnikov and Wander de Haas, who found that the material's electrical conductivity can oscillate when exposed to a strong magnetic field at low temperatures. This effect occurs because the material's electrons fill specific energy levels that change as the magnetic field changes.

Such quantum oscillations can serve as a signature of the material's electronic structure and how electrons behave and interact. Most significant for the MIT team, the oscillations can determine the material's electron mobility: If the oscillations exist, it must mean the material's electrical resistance can change, and therefore, electrons can be mobile and flow easily.

The team looked for signs of quantum oscillations in their new films, first exposing them to ultra-cold temperatures and a strong magnetic field, then passing an electric current through the film and measuring the voltage along its path while adjusting the magnetic field up and down.

Hang Chi says: "It turned out, to our great delight and excitement, that the material's electrical resistance oscillates. That immediately tells you it has very high electron mobility."

The team estimates that the ternary tetradymite thin film exhibits electron mobility of 10,000 cm2/V-s — the highest mobility ever measured for a ternary tetradymite film. The team suspects that the film's record mobility has to do with its low number of defects and impurities, which they managed to minimize with their precise growth strategies. The fewer defects in the material, the fewer obstacles the electron encounters, and the more freely it can flow.

Jagadeesh Moodera says: "This shows that it is possible to make a big step forward when we properly control these complex systems. This tells us we are on the right path and have the right system for further progress, for further refining this material to even thinner films and close junctions for use in future spintronic and wearable thermoelectric devices."

This research was partially supported by the Army Research Office, the National Science Foundation, the Office of Naval Research, the Canada Research Chairs Program, and the Canadian Natural Sciences and Engineering Research Council.

Source: Massachusetts Institute of Technology