Key Takeaways
- Researchers developed a powerful new methodology that combines direct imaging of electrons and defects in 2D semiconductors with theory-driven simulations that predict electron behavior and explain why it occurs.
- Using this methodology, the team discovered that electron behavior changes dramatically depending on the number and type of defects in the semiconductor material.
- The findings improve understanding of how electrons behave in advanced 2D semiconductors, supporting development of increasingly miniaturized electronic devices with new capabilities.
A team of researchers led by the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) developed a powerful new approach that enabled them to directly observe how electrons interact with defects in advanced semiconductor devices with an unprecedented level of detail. Their methodology included an innovative simulation tool that enabled accurate theoretical interpretations of their experimental observations.
"Our methods open the door to the discovery of never-seen-before electron behaviors that can be used for new semiconductor capabilities. This will be important for the future development of semiconductor devices at the ultimate limits of miniaturization." – Mike Crommie
The study, published in Nature, focuses on cutting-edge, ultrathin devices known as two-dimensional (2D) semiconductors that have very unusual electron states.
"Our study yielded valuable insights into why electrons in 2D semiconductors behave the way they do," said Mike Crommie, a senior faculty scientist in Berkeley Lab's Materials Sciences Division and professor of physics at UC Berkeley, and one of the study's authors. "Our methods open the door to the discovery of never-seen-before electron behaviors that can be used for new semiconductor capabilities. This will be important for the future development of semiconductor devices at the ultimate limits of miniaturization."
Other research team members include Feng Wang, a faculty senior scientist in Berkeley Lab's Materials Sciences Division and a professor in UC Berkeley's Department of Physics, and scientists from the University of California, the Flatiron Institute, the University of New Mexico, Hofstra University, Arizona State University, and the National Institute for Materials Science (Japan).
An investigation of ultrathin semiconductors containing strongly interacting electrons
Conventional silicon-based semiconductor devices, such as transistors, computer chips, and sensors, are made of materials in which atoms are arranged in three-dimensional structures. In recent years, device developers have increasingly explored the possibility of using 2D semiconductors, which are materials made of just a single layer - or a few layers - of atoms. Their ultrathin structure changes how electrons behave, potentially enabling new capabilities in future semiconductor devices.
The Berkeley Lab-led research team investigated 2D materials in which the electrons are in an unusual state known as a Wigner solid. In ordinary semiconductor devices, electrons typically behave like independent particles moving through the material. Their interactions with one another have a relatively minor influence on their behavior - and on the overall operation of the device.
"Physicists call this conventional state a Fermi liquid because the electrons move around like waves in the ocean," said Crommie.
Under certain conditions, electrons can enter a Wigner solid state in which they separate from one another and become immobile. Physicists describe such a system as "strongly interacting" because the electrons' mutual repulsion becomes the dominant factor shaping their behavior. Rather than moving independently, electrons in Wigner solids organize themselves into a relatively orderly pattern and behave collectively as a group.
Defects have an outsized influence on electron behavior
Historically, it has not been possible for scientists to directly image how electrons and defects interact in strongly interacting 2D semiconductor systems. It is extremely difficult to design semiconductor devices so that they can be imaged with enough resolution to see features on the scale of individual atoms. As a result, researchers have often resorted to a less direct approach to understanding electron-defect interactions: connecting wires to semiconductor devices and measuring how easily electricity flows through them.
"Some of the conclusions in these previous experiments were ambiguous because the researchers could not actually see the electrons and the defects at the same time," said Crommie. "They were inferring the behavior based on electrical conductivity."
To address the imaging challenge, the team used a tool known as a scanning tunnelling microscope. This instrument hovers a tiny metal tip just above a material's surface and extracts electrical current as the tip scans across the surface. To achieve the highest possible structural and electronic resolution, the research team spent years carefully developing a device that sandwiches the 2D semiconductor material of interest - in this case, molybdenum diselenide - in between a graphite layer on top and a silicon wafer and boron nitride layer on the bottom. The metal tip scans the molybdenum diselenide through tiny holes cut into the graphite.
The team used this set-up to image semiconductor samples with different defect densities. After each image was taken, they changed the density of electrons in their 2D device to visualize the material's transition from Wigner solid to Fermi liquid.
The images the researchers obtained revealed that electron behavior in their devices varied dramatically based on the number of defects in the 2D semiconductor material. When many defects were present, the electrons became locked into an unexpectedly stable Wigner solid state exhibiting highly irregular, disordered patterns. With fewer defects present, the electrons arranged themselves into much more orderly, triangular, crystal-like patterns that transitioned more readily to the Fermi liquid state.
Crommie described the moment when he first viewed the images and realized he was seeing something fundamentally new. "It was exciting to see the Wigner solid melt into liquid-like waves splashing up against defects," he said. "We could see how electrons respond to defects in very different ways. Some defects acted like large potholes while others acted like tiny speed bumps."
Images agree with theory
To explain the defect-electron interactions observed in these images and confirm that the images were not showing artifacts due to the microscope's operation, the team needed to compare their images to a theoretical model. The challenge was that there was no existing theory in the scientific literature for accurately simulating images of 2D semiconductor systems with strongly interacting electrons and random defects. It is difficult to perform the complex calculations needed to predict the behavior of large collections of electrons under these conditions.
The team used a sophisticated computer simulation technique called Quantum Monte Carlo to calculate how strongly electrons in a 2D semiconductor material interact with defects based on the laws of quantum mechanics. The simulated electron behavior closely matched the patterns observed in the microscope images.
"This remarkable agreement confirmed the accuracy of both our theoretical simulation tool and our imaging technique," said Crommie. "It also helped us better understand the physics underlying the behaviors we observed in our images."
Crommie added that the results suggest that controlling both placement and types of defects will likely become a much more important consideration as device developers push digital electronics, sensors, and other semiconductor devices to ever smaller dimensions.
Other researchers and device developers can apply the team's methodology - an integrated combination of imaging and theoretical simulations - to understand the electronic behavior of other 2D semiconductors, particularly those with strongly interacting electrons.
"The semiconductors in today's smartphones and computers don't contain Wigner solids, but they may in the future as devices become more advanced," said Crommie. "Our methods can help researchers and manufacturers understand what electrons will do under these conditions, informing the design of increasingly miniaturized devices with advanced capabilities."
The team is extending its results by exploring how electron behavior is affected when electrons are confined to tiny channels in semiconductor devices. Another goal is to investigate how electrons behave in 2D semiconductors with fewer defects.
This research was supported by the Department of Energy's Office of Science, Basic Energy Sciences; the Department of Defense's Vannevar Bush Faculty Fellowship; the National Science Foundation; and the Simons Foundation's Flatiron Institute.
