Ultra-Clean Fabrication Platform Produces Nearly Ideal 2D Transistors 

Columbia engineers develop a clean, damage-free fabrication process that creates pristine transistors made from 2D material stacks 

Newswise — New York, NY—May 17, 2019—Semiconductors, which are the basic building blocks of transistors, microprocessors, lasers, and LEDs, have driven advances in computing, memory, communications, and lighting technologies since the mid-20th century. Recently discovered two-dimensional materials, which feature many superlative properties, have the potential to advance these technologies, but creating 2D devices with both good electrical contacts and stable performance has proved challenging. 

Researchers at Columbia Engineering report that they have demonstrated a nearly ideal transistor made from a two-dimensional (2D) material stack—with only a two-atom-thick semiconducting layer—by developing a completely clean and damage-free fabrication process. Their method shows vastly improved performance compared to 2D semiconductors fabricated with a conventional process, and could provide a scalable platform for creating ultra-clean devices in the future. The study was published today in Nature Electronics

VIDEO on the differences between 2D and 3D materials: https://youtu.be/4m-1vxXQHtY
VIDEO on the step-by-step nanofabrication of 2D material stacks: https://youtu.be/JgfkLhMo1fA

“Making devices out of 2D materials is a messy business,” says James Teherani, assistant professor of electrical engineering. “Devices vary wildly from run to run and often degrade so fast that you see performance diminish while you’re still measuring them.” 

Having grown tired of the inconsistent results, Teherani’s team set out to develop a better way to make stable devices. “So,” he explains, “we decided to separate the pristine device from the dirty fabrication processes that lead to variability.” 

As shown in this new study, Teherani and his colleagues developed a two-step, ultra-clean  nanofabrication process that separates the “messy” steps of fabrication – those that involve “dirty” metallization, chemicals, and polymers used to form electrical connections to the device—from the active semiconductor layer. Once they complete the messy fabrication, they could pick up the contacts and transfer them onto the clean active device layer, preserving the integrity of both layers. 

“The thinness of these semiconductors is a blessing and a curse,” says Teherani. “While the thinness allows them to be transparent and to be picked up and placed wherever you want them, the thinness also means there's nearly zero volume—the device is almost entirely surface. Because of this, any surface dirt or contamination will really degrade a device.” 

Currently, most devices are not encapsulated with a layer that protects the surface and contacts from contamination during fabrication. Teherani’s team showed that their method can now not only protect the semiconductor layer so that they don't see performance degradation over time, but it can also yield high performance devices. 

Teherani collaborated with Jim Hone, Wang Fong-Jen Professor of Mechanical Engineering, making use of the fabrication and analysis facilities of the Columbia Nano Initiative and the National Science Foundation-funded Materials Research Science and Engineering Center at Columbia. The team made the transferred contacts from metal embedded in insulating hexagonal boron nitride (h-BN) outside a glovebox and then dry-transferred the contact layer onto the 2D semiconductor, which was kept pristine inside a nitrogen glovebox. This process prevents direct-metallization-induced damage while simultaneously providing encapsulation to protect the device. 

Now that the researchers have developed a stable, repeatable process, they are using the platform to make devices that can move out of the lab into real-world engineering problems. 

“The development of high performance 2D devices requires advances in the semiconductor materials from which they are made,” Teherani adds. “More precise tools like ours will enable us to build more complex structures with potentially greater functionality and better performance.” 

About the Study 

The study is titled “Transferred via contacts as a platform for ideal two-dimensional transistors.” 

Authors are: Younghun Jung1, Min Sup Choi1;2, Ankur Nipane3, Abhinandan Borah3, Bumho Kim1, Amirali Zangiabadi4, Takashi Taniguchi5, Kenji Watanabe5, Won Jong Yoo2, James Hone1, and James T. Teherani3
1 Department of Mechanical Engineering, Columbia Engineering
2 SKKU Advanced Institute of Nano Technology, Sungkyunkwan University, Korea
3 Department of Electrical Engineering, Columbia Engineering
4 Department of Applied Physics and Applied Mathematics, Columbia Engineering
5 National Institute for Materials Science, Japan 

The study was supported by the National Science Foundation through CAREER Award (ECCS-1752401) and the Center for Precision Assembly of Superstratic and Superatomic Solids (DMR-1420634). This work is also supported by the National Research Foundation of Korea through the Global Research Laboratory (GRL) program (2016K1A1A2912707) and Research Fellow program (2018R1A6A3A11045864).

The authors declare no competing financial interests. 



Paper: http://dx.doi.org/10.1038/s41928-019-0245-y
DOI: 10.1038/s41928-019-0245-y
VIDEO on the differences between 2D and 3D materials: https://youtu.be/4m-1vxXQHtY
VIDEO on the step-by-step nanofabrication of 2D material stacks: https://youtu.be/JgfkLhMo1fA


Columbia Engineering
Columbia Engineering, based in New York City, is one of the top engineering schools in the U.S. and one of the oldest in the nation. Also known as The Fu Foundation School of Engineering and Applied Science, the School expands knowledge and advances technology through the pioneering research of its more than 220 faculty, while educating undergraduate and graduate students in a collaborative environment to become leaders informed by a firm foundation in engineering. The School’s faculty are at the center of the University’s cross-disciplinary research, contributing to the Data Science Institute, Earth Institute, Zuckerman Mind Brain Behavior Institute, Precision Medicine Initiative, and the Columbia Nano Initiative. Guided by its strategic vision, “Columbia Engineering for Humanity,” the School aims to translate ideas into innovations that foster a sustainable, healthy, secure, connected, and creative humanity.



Journal Link: Nature Electronics May 17 1029