Caption: A diagram showing the crystal structure of boron nitride, a key component of a new ferroelectric material that MIT researchers and colleagues have used to build a transistor with unique properties. The diagram shows how the structure can change when two ultrathin layers of boron nitride slide past each other under the application of an electric field. The P represents polarization, or negative/positive charge. Credit: Ashoori and Jarillo-Herrero Laboratories
In 2021, a team led by MIT physicists announced that they had created a new ultrathin ferroelectric material—one in which positive and negative charges separate into different layers. At the time, they noted the material’s potential for applications in computer memory and more. Now, the same team and colleagues, including two from the neighboring lab, have built a transistor with the material and shown that its properties are so useful that it could change the world of electronics.
Although the team’s results are based on a single transistor made in the lab, “in many respects its properties already meet or exceed industrial standards” for ferroelectric transistors produced today, says Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics, who led the work with Raymond Ashoori Professor of Physics. Both are also affiliated with the Materials Research Laboratory.
“In my lab, we do mostly fundamental physics. This is one of the first, and perhaps most spectacular, examples of how the most fundamental science has led to something that could have a major impact on applications,” Jarillo-Herrero says.
Ashoori says, “When I think about my entire career in physics, this is the work that I think could change the world in 10 to 20 years.”
Among the superlative properties of the new transistor:
It can switch between positive and negative charges (essentially the ones and zeros of digital information) at very high speeds, on the nanosecond scale (a nanosecond is one billionth of a second).
It is extremely durable. After 100 billion switches, it was still working without any signs of degradation.
The material behind this magic is just a few billionths of a meter thick, making it one of the thinnest of its kind in the world. That could enable higher storage density in computer memories. It could also lead to much more energy-efficient transistors because of the voltage required to scale with the material’s thickness. (Ultrathin equals ultralow voltages.)
The book is published in a recent issue of ScienceThe paper’s co-lead authors are Kenji Yasuda, now an assistant professor at Cornell University, and Evan Zalys-Geller, now at Atom Computing. Other authors are Xirui Wang, a graduate student in physics at MIT; Daniel Bennett and Efthimios Kaxiras of Harvard University; Suraj S. Cheema, an assistant professor in MIT’s Department of Electrical Engineering and Computer Science and affiliated with the Electronics Research Laboratory; and Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan.
What did they do
In a ferroelectric material, the positive and negative charges spontaneously move to different sides or poles. When an external electric field is applied, these charges switch sides, reversing the polarization. Polarization reversal can be used to encode digital information, and this information will be nonvolatile, or stable over time. It will not change unless an electric field is applied. For a ferroelectric to have wide application in electronics, all of this must occur at room temperature.
The new ferroelectric material presented in Science In 2021, the technology is based on atomically thin sheets of boron nitride stacked parallel to each other, a configuration that does not exist in nature. In bulk boron nitride, the individual layers of boron nitride are rotated 180 degrees.
It turns out that when an electric field is applied to this parallel-stacked configuration, one layer of the new boron nitride material slides over the other, slightly changing the position of the boron and nitrogen atoms. For example, imagine that each of your hands is made up of just one layer of cells. The new phenomenon is similar to pressing your hands together and then shifting them slightly on top of each other.
“The miracle is that if you slide the two layers by a few angstroms, you get radically different electronics,” Ashoori explains. The diameter of an atom is about 1 angstrom.
Another miracle: “Nothing wears out when you slide,” Ashoori says. That’s why the new transistor could be switched 100 billion times without degrading. Compare that to the memory in a USB stick made with conventional materials. “Every time you write and erase flash memory, you get some degradation,” Ashoori says. “Over time, it wears out, which means you have to use very sophisticated methods to distribute where you read and write on the chip.” The new material could make those steps obsolete.
A collaborative effort
Yasuda, the co-first author of the current Science paper, welcomes the collaborations involved in the work. Among them, “we [Jarillo-Herrero’s team] manufactured the equipment and, with Ray [Ashoori] And [co-first author] Evan [Zalys-Geller]“We measured its characteristics in detail. It was very exciting,” Ashoori says. “A lot of the techniques used in my lab naturally applied to the work done in the neighboring lab. It was a lot of fun.”
Ashoori notes that there is “a lot of interesting physics behind this” that could be explored. For example, “if you think about the two layers sliding past each other, where does that sliding start?” Additionally, Yasuda says, could ferroelectricity be triggered by something other than electricity, such as an optical pulse? And is there a fundamental limit to the number of switches the material can make?
There are still challenges. For example, the current method of producing the new ferroelectrics is difficult and not conducive to mass production. “We made a single transistor as a demonstration. If people could grow these materials on a wafer scale, we could make many, many more,” Yasuda says. He notes that different groups are already working on this.
Ashoori concludes: “There are some problems. But if we solve them, this material fits into many ways in the electronics of the future. It’s very exciting.”
More information:
Kenji Yasuda et al, High-endurance ultrafast memory based on sliding ferroelectrics, Science (2024). DOI: 10.1126/science.adp3575
Provided by the Massachusetts Institute of Technology
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