Waterless manufacturing approach could help advance integration of 2D electronics

The future of technology faces an age-old problem: rust. When ferrous metals react with oxygen and moisture, corrosion occurs, which severely limits the lifespan and use of parts in the automotive industry.

Though it’s not called “rust” in the semiconductor industry, oxidation is particularly problematic for two-dimensional (2D) semiconductor materials that control the flow of electricity in electronic devices, because corrosion can render the atomically thin materials useless.

Now, a team of academic and corporate researchers has developed a synthesis process that produces a “rust-resistant” coating with properties ideal for making faster, more durable electronics.

The team, co-led by researchers from Pennsylvania State University, published their findings in the journal Nature Communications.

2D materials are ultrathin, measuring just one or a few atoms in thickness. Their thinness provides short, direct paths for electrons to move quickly and without resistance through the material, making them promising advanced semiconductor materials, enabling faster and more efficient electronic performance.

Semiconductors are materials that conduct electricity under some conditions but not others, making them ideal for controlling electrical current in electronic devices. The electronic devices that are the “brains” of computer chips are made from these materials.

“One of the biggest problems seen in current research into 2D semiconductors is the rapid oxidation of the materials,” said Joshua Robinson, professor of materials science and engineering and co-corresponding author of the study.

“These materials are going to be built into transistors and sensors that are meant to last for years, so we need to ensure their long-term reliability. Right now, these materials don’t last more than a week outside.”

Traditional methods for protecting these materials from rust include oxide coatings, but these processes often use water, which can ironically accelerate the very oxidation they are trying to prevent. The team’s approach to this problem was to find a coating material and method that could avoid the use of water entirely. Enter amorphous boron nitride (a-BN).

“We wanted to get rid of water in our process, so we started thinking about what 2D materials we could make that didn’t require water to process, and amorphous boron nitride is one of them,” Robinson said.

A-BN, an amorphous form of boron nitride, is known for its high thermal stability and electrical insulating properties, making it ideal for use in insulating semiconductor components, preventing unwanted electrical currents and improving device performance, Robinson said.

He explained that a-BN a has high dielectric strength, a measurement of a material’s ability to withstand high electric fields without breaking down, a key factor for reliable electronic performance.

“The high dielectric strength demonstrated by a-BN rivals the best dielectrics currently available, and no water is required to create it,” Robinson said. “What we demonstrate in our paper is that the inclusion of amorphous boron nitride improves device performance compared to conventional dielectrics alone.”

Robinson said the coatings helped make better 2D transistors, but applying them to 2D materials was difficult because they don’t have dangling bonds, which are unpaired electrons on the surface of a material that can react with or bond with other atoms.

Standard one-step processes that coat materials at high temperatures result in uneven, discontinuous coatings that are far below the quality needed for electronic devices to function properly.

To uniformly coat the 2D materials with a-BN, the team developed a novel two-step atomic layer deposition method, in which they first deposit a thin, low-temperature a-BN “seed layer” and then heat the chamber to typical deposition temperatures of 250-300°C.

Not only did this enable the researchers to produce uniform a-BN coatings on 2D semiconductors, but it also improved the transistor performance by 30% to 100% compared to devices without a-BN, depending on the transistor design.

“Sandwiching a 2D semiconductor between amorphous boron nitride, even though it’s amorphous, creates a smoother road for electrons, so to speak, which allows for improved electronics,” Robinson said. “Electrons can travel faster through the 2D material than if it were sandwiched between other dielectric materials.”

Robinson noted that despite its high dielectric strength, researchers have only just scratched the surface of a-BN’s potential as a dielectric material in semiconductor devices.

“While it already outperforms other dielectric materials, there is still room for improvement,” Robinson said. “The main thing we’re working on now is improving the overall quality of the material and incorporating it into the complex structures we’ll find in future electronics.”