UNIVERSITY PARK, Pa. — The future of technology has an age-old problem: rust. When iron-containing metal reacts with oxygen and moisture, the resulting corrosion greatly impedes the longevity and use of parts in the automotive industry. While it’s not called “rust” in the semiconductor industry, oxidation is especially problematic in two-dimensional (2D) semiconductor materials, which control the flow of electricity in electronic devices, because any corrosion can render the atomic-thin material useless. Now, a team of academic and enterprise researchers has developed a synthesis process to produce a “rust-resistant” coating with additional properties ideal for creating faster, more durable electronics.   

The team, co-led by researchers at Penn State, published their work in Nature Communications. 

2D materials are ultra-thin, just one or a few atoms thick. They hold promise for advanced semiconductors because their thinness provides a shorter and more direct path for electrons to move quickly and with less resistance through the material. This in turn allows for faster and more efficient electronic performance.  

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

"One of the biggest issues that we see in 2D semiconductor research these days is the fact that the materials oxidize quickly,” said Joshua Robinson, professor of materials science and engineering and co-corresponding author of the work. “You need to ensure their long-term reliability because these are going into transistors or sensors that are supposed to last years. Right now, these materials don't last more than a week out in the open."  

Traditional methods to protect these materials from rusting involve oxide-based coatings, but these processes often use water, which ironically can accelerate the very oxidation they aim to prevent. The team’s approach to this problem was to seek a coating material and method that could avoid the use of water entirely. Enter amorphous boron nitride (a-BN).  

"We wanted to get away from using water in the process so we started thinking about what sort of 2D materials we can make that do not use water in its processing, and amorphous boron nitride is one of those," Robinson said.  

A non-crystalline form of boron nitride, a-BN is known for its high thermal stability and electrical insulation properties, making it ideal for use in semiconductors to insulate components, prevent unwanted electrical currents and improve device performance, Robinson said.  

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

“The high dielectric strength demonstrated by a-BN is comparable to the best dielectrics available, and we don’t need water to make it,” Robinson said. "What we demonstrated in the paper was that including amorphous boron nitride yields improved device performance compared to conventional dielectrics alone." 

While the coating helped produce a better 2D transistor, getting the coating on the 2D materials proved a challenge, according to Robinson. Two-dimensional materials lack dangling bonds, which are unpaired electrons on the surface of a material that react or bond with other atoms. A standard single-step process that uses higher temperatures to coat the materials resulted in uneven and discontinuous coatings, well below the quality electronics need to function properly.  

To evenly coat 2D materials with the a-BN, the team developed a new two-step atomic layer deposition method, which involves first depositing a thin low-temperature a-BN “seed layer” before heating up the chamber to typical deposition temperatures between 250 and 300 degrees Celsius. This not only allowed the researchers to produce an even a-BN coating over the 2D semiconductors but also led to a 30% to 100% improvement — depending on the transistor design — in transistor performance compared to devices not utilizing the a-BN. 

"When you sandwich 2D semiconductors between the amorphous boron nitride, even though it’s amorphous, you end up with a smoother electronic road, so to speak, that would enable improved electronics,” Robinson said. “The electrons can go faster through the 2D material than they could if they were between other dielectric materials.” 

Robinson noted that even with its high dielectric strength, researchers have only scratched the surface of a-BN's potential as a dielectric material for semiconductor devices.  

“We have room for improvement even though it’s already outperforming other dielectric materials,” Robinson said. "The primary thing that we're trying to do right now is improve the overall quality of the material and then integrate it into some complex structures you would see in future electronics.” 

Along with Robinson, other authors of the study from Penn State include Cindy Chen, graduate student in materials science and engineering; Riccardo Torsi, graduate research assistant in materials science and engineering; Ke Wang, associate research professor in the Materials Research Institute; Bangzhi Liu, associate research professor in the Materials Research Institute. Other co-authors include co-corresponding author Yu-Chuan Lin, National Yang Ming Chiao Tung University in Taiwan; Zhihong Chen and Joerg Appenzeller, Purdue University; Jessica Kachian, Intel Corporation; and Gilber B. Rayner Jr., The Kurt J. Lesker Company. 

The Semiconductor Research Corporation, through a program sponsored by National Institute of Standards and Technology, the Center for Emergent Functional Matter Science of National Yang Ming Chiao Tung University, the Ministry of Education of Taiwan and the U.S. National Science Foundation supported this research.