A Hall-bar device structure (see inset) is wire-bonded to a 16-pin chip-carrier. The chip-carrier allows for extensive electrical characterization of the device at both low temperatures and high magnetic fields. Image: Min Sup Choi/Columbia Engineering.Semiconductors are the basic building blocks of transistors, microprocessors, lasers, and LEDs, and they have driven advances in computing, memory, communications and lighting technologies since the mid-20th century. Recently discovered two-dimensional (2D) 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 now report their demonstration of 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. They report this work in a paper in Nature Electronics.
"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 described in the paper, 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 the scientists complete the messy fabrication, they can 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 and his colleagues showed that their method can now not only protect the semiconductor layer so that it doesn’t experience performance degradation over time, but it can also yield high performance devices.
By collaborating with Jim Hone, a professor of mechanical engineering, Teherani was able to make 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. Teherani’s team made the transferred contacts from metal embedded in insulating hexagonal boron nitride (h-BN), which they did outside a glovebox. They then dry-transferred this 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 says. "More precise tools like ours will enable us to build more complex structures with potentially greater functionality and better performance."
This story is adapted from material from Columbia Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.