Researchers at Columbia Engineering say that their method shows vastly improved performance compared to 2D semiconductors fabricated with a conventional process. Morever, it could provide a scalable platform for creating ultra-clean devices in the future.
"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."
The team created a two-step, ultra-clean nanofabrication process that separates the "messy" steps of fabrication - those that involve "dirty" metallisation, 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 Assit Prof Teherani. "While the thinness allows them to be transparent and to be picked up and placed wherever you want them, it 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. The 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.
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," Assit Prof Teherani adds. "More precise tools like ours will enable us to build more complex structures with potentially greater functionality and better performance."
