Researchers safely integrate fragile 2D materials into devices, paving the way for unique electronic properties

Two-dimensional materials, which are just a few atoms thick, can exhibit incredible properties, such as the ability to carry electrical charges extremely efficiently, which could improve the performance of next-generation electronic devices.

However, integrating 2D materials into devices and systems such as computer chips is notoriously difficult. These ultrathin structures can be damaged by conventional manufacturing techniques, which often rely on the use of chemicals, high temperatures or destructive processes like etching.

To overcome this challenge, researchers at MIT and elsewhere have developed a new technique for integrating 2D materials into devices in a single step while keeping the material surfaces and resulting interfaces intact and free of defects.

Their method relies on surface engineering forces available at the nanoscale to allow 2D material to be physically stacked on top of other predefined device layers. As the 2D material remains intact, researchers can take full advantage of its unique optical and electrical properties.

They used this approach to fabricate 2D transistor arrays that offer new functionality compared to devices produced using conventional manufacturing techniques. Their method, versatile enough to be used with many materials, could have diverse applications in high-performance computing, sensing and flexible electronics.

At the heart of unlocking these new features is the ability to form clean interfaces, held together by special forces that exist between all matter, called Van der Waals forces.

However, such integration of materials by Van der Waals into fully functional devices is not always easy, explains Farnaz Niroui, assistant professor of electrical engineering and computer science (EECS), member of the Electronics Research Laboratory (RLE) and lead author of a new paper describing the work.

“Van der Waals integration has a fundamental limitation,” she explains. “Since these forces depend on the intrinsic properties of the materials, they cannot be easily tuned. As a result, some materials cannot be directly integrated with each other using only their Van der Waals interactions. We developed a platform to address this limitation to help make van der Waals integration more versatile, to promote the development of 2D material-based devices with new and improved functionalities.

The research will be published in Natural electronics.

Advantageous appeal

Fabricating complex systems such as a computer chip with conventional manufacturing techniques can be complicated. Typically, a rigid material such as silicon is chiseled to the nanoscale and then interfaced with other components such as metal electrodes and insulating layers to form an active device. Such treatment may damage materials.

Recently, researchers have focused on building devices and systems from the bottom up, using 2D materials and a process requiring sequential physical stacking. In this approach, rather than using chemical glues or high temperatures to bond a fragile 2D material to a conventional surface like silicon, researchers leverage Van der Waals forces to physically integrate a layer of 2D material onto a device. .

Van der Waals forces are natural forces of attraction that exist between all matter. For example, a gecko’s legs may temporarily adhere to the wall due to Van der Waals forces.

Although all materials exhibit a Van der Waals interaction, depending on the material, the forces are not always strong enough to hold them together. For example, a popular 2D semiconductor material known as molybdenum disulfide adheres to gold, a metal, but does not transfer directly to insulators like silicon dioxide by simply coming into physical contact with that surface.

However, heterostructures made by integrating semiconductor and insulating layers are essential elements of an electronic device. Previously, this integration was possible by bonding the 2D material to an interlayer like gold, then using that interlayer to transfer the 2D material onto the insulation before removing the interlayer using chemicals or high temperature.

Instead of using this sacrificial layer, MIT researchers embed the low-adhesion insulation into a high-adhesion matrix. This adhesive matrix allows the 2D material to adhere to the embedded low-adhesion surface, providing the forces necessary to create a van der Waals interface between the 2D material and the insulation.

Make the matrix

To make electronic devices, they form a hybrid surface of metals and insulators on a carrier substrate. This surface is then peeled off and flipped over to reveal a completely smooth top surface containing the building blocks of the desired device.

This smoothness is important because gaps between the surface and the 2D material can interfere with Van der Waals interactions. Next, researchers prepare the 2D material separately in a completely clean environment and put it in direct contact with the prepared device stack.

“Once the hybrid surface is brought into contact with the 2D layer, without the need for high temperatures, solvents or sacrificial layers, it can pick up the 2D layer and integrate it with the surface. In this way, we enable to Van der Waals an integration that would traditionally be prohibited but is now possible and allows the formation of fully functional devices in a single step,” explains Satterthwaite.

This single-step process keeps the 2D material interface completely clean, allowing the material to reach its fundamental performance limits without being held back by defects or contamination.

And because the surfaces also remain intact, researchers can engineer the surface of the 2D material to form features or connections with other components. For example, they used this technique to create p-type transistors, which are typically difficult to make with 2D materials. Their transistors have improved over previous studies and can provide a platform for studying and achieving the performance needed for practical electronics.

Their approach can be implemented at scale to create broader ranges of devices. The adhesive matrix technique can also be used with a range of materials and even other strengths to enhance the versatility of this platform. For example, researchers integrated graphene onto a device, forming the desired van der Waals interfaces using a matrix made of a polymer. In this case, adhesion relies on chemical interactions rather than van der Waals forces alone.

In the future, researchers aim to build on this platform to enable the integration of a diverse library of 2D materials to study their intrinsic properties without the influence of processing damage and develop new device platforms leveraging these superior capabilities.

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and education.