Scientists observe recording of electron mobility in a new crystal film

A material with high electron mobility is like a highway without traffic. Any electrons flowing into matter experience the dream of a traveler, passing without any obstacles or jams to slow or scatter them from their path.

The higher the electron mobility of a material, the more efficient its electrical conductivity, and the less energy is lost or wasted as electrons zip around. Advanced materials that exhibit high electron mobility are essential for highly efficient and stable electronic devices that can do more work with less power.

Now, physicists at MIT, the Army Research Laboratory, and elsewhere have achieved record levels of electron mobility in a thin film of ternary tetradymite—a type of mineral found naturally in deep hydrothermal deposits of gold and quartz.

For this study, the scientists created pure, ultrathin films that minimized defects in its crystal structure. They found 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 motion by detecting quantum oscillations when electricity was passed through it. These oscillations are a signature of the quantum mechanical behavior of electrons in a material. The researchers detected a specific oscillation rhythm characteristic of high electron mobility – more than any triple thin films of this class to date.

„Before, what people achieved in terms of electron mobility in these systems was like traffic on a road under construction — you’re backed up, you can’t drive, it’s dusty, and it’s a mess,” says Jagadish Mudera, a senior research scientist in MIT’s Department of Physics. „With this newly improved material, it’s like riding a mass bike without traffic.”

The panel’s findings appear in today’s issue of the Journal Materials today are physics, identify ternary tetradiimide thin films as promising materials for future electronics, such as wearable thermoelectric devices that convert waste heat into electricity. (Tetradymites are the active ingredients that produce the cooling effect in commercial thermoelectric coolers.) The material could also be the basis for spintronic devices that process information using the spin of an electron using far less power than conventional silicon-based devices.

This study uses quantum oscillations as a very useful tool to measure the electronic performance of a material.

„We use this swing as a quick testing tool.” says study author Hong Xi, a former research scientist at MIT who is now at the University of Ottawa. „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 improve our world.”

Si and Mootera’s co-authors include Patrick Taylor, formerly of MIT Lincoln Laboratory, Owen Weill and Harry Hier of the Army Research Laboratory, and Brandi Wooten and Joseph Hermans of Ohio State University.

Beam down

The name „tetradymite” is derived from the Greek „tetra” meaning „four” and „dymite” meaning „double”. Both terms describe the mineral’s crystal structure, which consists of rhombohedral crystals that „double” into groups of four—that is, they have identical crystal structures that share a face.

Tetradymites contain a combination of bismuth, antimony tellurium, sulfur and selenium. In the 1950s, scientists discovered that tetradymites exhibited semiconducting properties that made them suitable for thermoelectric applications: the mineral in its bulk crystalline form was able to passively convert heat into electricity.

Later, in the 1990s, Institute Professor Mildred Dresselhaus proposed that the thermoelectric properties of the mineral could be significantly improved, not in its bulk form, but on its microscopic, nanometer-scale surface, where the interactions of electrons are most pronounced. (Hermans worked for the Dresselhaus Group at the time.)

„When you look at this material longer and closer, it becomes clear that new things happen.” C says. „This material has been identified as a topological insulator, where scientists can see very interesting phenomena on their surface. But to uncover new things, we need to master material growth.

To grow thin films of the pure crystal, the researchers used molecular beam epitaxy – a beam of molecules is fired onto a substrate, usually in a vacuum, and with a precisely controlled temperature. As the molecules deposit on the substrate, they condense, slowly forming one atomic layer at a time. By controlling the timing and type of deposited molecules, scientists can grow ultrathin crystalline films in perfect configurations, with few defects.

„In general, bismuth and tellurium can change their position, creating defects in the crystal.” Co-author Taylor explains. „The system we used to grow these images came from MIT Lincoln Laboratory, where we use high-purity materials to reduce impurities to undetectable limits. It’s the perfect tool to explore this research.

Free flow

The team created thin films of tetradymide, each about 100 nanometers thin. They then probed the film’s electronic properties by looking for Shubnikov-de Haas quantum oscillations—discovered by physicists Lev Shubnikov and Vander de Haas, who discovered that the electrical conductivity of a material oscillates when exposed to a strong magnetic field at low temperatures. . This effect occurs because the material’s electrons fill specific energy levels when the magnetic field changes.

Such quantum oscillations act as a signature of a material’s electronic structure and the ways in which electrons behave and interact. Most importantly for the MIT team, the oscillations can determine the electron movement of a material: if the oscillations are present, the material’s electrical resistance can change, and by inference the electrons will be mobile and able to flow more easily.

The team looked for signs of quantum oscillations in their new films by first exposing them to ultracold temperatures and a strong magnetic field, then running a current through the film, measuring the voltage across its path, and enhancing the magnetic field. under.

„To our great delight and excitement, the electrical resistance of matter oscillates” C says. „Instantly, that tells you it has very high electron mobility.”

Specifically, the team estimates that a ternary tetradiimide thin film exhibits an electron mobility of 10,000 cm.²/Vs — the highest mobility of any triple tetradiimide film ever measured. The team suspects that the film’s record-breaking run is related to its minimal flaws and impurities, which they were able to minimize through their precise development strategies. If a material has fewer defects, the electron encounters fewer obstacles and can flow more freely.

„This shows that when these complex systems are properly controlled, a big step forward can be made,” says Mudera. „This tells us that we are in the right direction and that we have the right system to continue further and synthesize this material into even thinner films and near future applications in spintronics and wearable thermoelectric devices.

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