Scientists have observed record electron mobility in new crystalline thin films

Researchers have bred thin films of ternary tetramagnesium (pictured) that exhibit record-high electron mobility. Image credits: Courtesy of researchers; Edited by MIT News

Materials with high electron mobility are like highways without traffic. Any electron flowing into the material will go through the commuter's dream, passing easily without any obstacles or congestion to slow them down or disperse them.

The higher the electron mobility of a material, the more efficiently it conducts electricity and the less energy is lost or wasted as electrons pass through. Advanced materials with high electron mobility are essential for more efficient and sustainable electronic devices that can do more with less power.

Now, physicists at MIT, the Army Research Laboratory, and elsewhere have achieved record levels of electron mobility in thin films of ternary tetrahydrosilicon, a class of minerals naturally occurring in deep hydrothermal deposits of gold and quartz.

In this study, scientists grew a pure ultrathin film of this material to minimize defects in its crystal structure. They found that the near-perfect film — much thinner than a human hair — exhibited the highest electron mobility in its class.

The team was able to estimate the electron mobility of the material by detecting the quantum oscillations as the current passes through. These oscillations are characteristic of the quantum mechanical behavior of electrons in materials. The researchers detected a peculiar oscillation rhythm, which is characteristic of high electron mobility, higher than any ternary film of its kind to date.

"Previously, what people achieved in terms of electron mobility in these systems was like traffic on a road under construction — you were backed up, you couldn't drive, dusty, and a mess," said Jagadeesh Moodera, a senior research scientist in MIT's physics department. "In this newly optimized material, it's like driving a Mass Pike without traffic."

The team's findings, published in the journal Materials Physics Today, point out that ternary tetrasylferous thin films are promising future electronic materials, such as wearable thermoelectric devices that can effectively convert waste heat into electricity. (Tetramites are the active substances that cause the cooling effect of commercial thermoelectric coolers.)

This material can also be used as a basis for spintronic devices, which use electron spins to process information and use much lower power than traditional silicon-based devices.

The study also uses quantum oscillations as an efficient tool for measuring the electronic properties of materials.

"We are using this oscillation as a rapid test kit," said Hang Chi, the study's author, a former research scientist at MIT who is now at the University of Ottawa. "By studying this subtle electronic quantum dance, scientists can begin to understand and identify new materials for the next generation of technologies that will power our world.

Chi 和 Moodera 的合著者包括前麻省理工学院林肯实验室的 Patrick Taylor,以及陆军研究实验室的 Owen Vail 和 Harry Hier,以及俄亥俄州立大学的 Brandi Wooten 和 Joseph Heremans。

The beam is downward

The name "tetradymite" is derived from the Greek word "tetra", which means "four", while "dymite" means "twin". Both terms describe the crystal structure of a mineral, which consists of rhombohedral crystals that are "twins" in groups of four, i.e. they have the same crystal structure and share one side.

Tetrahydromites are made up of a combination of bismuth, tellurium, antimony, sulfur, and selenium. In the 1950s, scientists discovered that tetramites exhibit semiconducting properties that may be ideal for thermoelectric applications: this bulk form of mineral is capable of passively converting heat into electricity.

Then, in the 1990s, the late Institute professor Mildred Dresselhaus proposed that the thermoelectric properties of minerals could be significantly enhanced, not in their monolithic form, but within their microscopic nanoscale surfaces, where the interaction of electrons is more pronounced. (Herrimans happened to be working on Drexelhouse's team at the time.)

"Obviously, when you look at these materials long enough, close enough, something new happens," Chi said. "This material was identified as a topological insulator, and scientists can see very interesting phenomena on its surface. But to keep discovering new things, we must master material growth.

To grow thin films of pure crystals, the researchers employed molecular beam epitaxy, a method of emitting molecular beams onto a substrate, usually in a vacuum, and at precisely controlled temperatures.

When molecules are deposited on the substrate, they condense and slowly build up, one atomic layer at a time. By controlling the timing and type of molecules being deposited, scientists can grow precisely configured ultra-thin crystal films with few defects.

"Often, bismuth and tellurium can swap their positions, which creates defects in the crystal," explains co-author Taylor. "The system we use to grow these films comes from MIT's 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 grew thin films of the ternary tetramites, each layer about 100 nanometers thin. They then tested the electronic properties of the film by looking for Shubnikov-de Haas quantum oscillations – a phenomenon discovered by physicists Lev Shubnikov and Wander de Haas, who found that the conductivity of the material oscillates when exposed to a strong magnetic field at low temperatures. This effect occurs because the electrons of the material are filled with a specific energy level that changes with the change in the magnetic field.

This quantum oscillation can be used as a feature of the electronic structure of the material, as well as the way the electrons behave and interact. Most notably, for the MIT team, oscillations can determine the electron mobility of a material: if there are oscillations, it must mean that the resistance of the material can change, and by inference, electrons can move and flow easily.

The team looked for signs of quantum oscillations in their new film, 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 as they adjusted the magnetic field up and down.

"It turns out that to our great joy and excitement, the resistance of this material is oscillating," Chi said. "Immediately, this tells you that it has a very high electron mobility.

Specifically, the team estimated that the ternary tetrahydrosilicon film exhibited an electron mobility of 10,000 cm2/V-s – the highest of any ternary tetrahydrogen film measured to date.

The team suspects that the film's record-breaking fluidity has something to do with its low defects and impurities, which they were able to minimize with a precise growth strategy. The fewer defects in the material, the fewer obstacles the electron encounters, and the freer it flows.

"This suggests that it is possible to go one step further when it comes to properly controlling these complex systems," Moodera said. "This tells us that we are moving in the right direction and that we have the right systems to move forward and continue to refine this material, even thinner films and proximity coupling, for future spintronics and wearable thermoelectric devices.

More information: Patrick J. Taylor et al., Magnetic Transport Properties of Ternary Tetramine Thin Films with High Mobility, Materials Physics Today (2024). DOI： 10.1016/j.mtphys.2024.101486