Reviewed by Lexie CornerSep 23 2024
In a study published in Nature Communications, a team led by Penn State researchers developed a synthesis process to produce a “rust-resistant” coating with extra features suitable for creating faster, more durable electronics.
These materials are made from molybdenum disulfide, a two-dimensional semiconductor, grown on a sapphire surface. The triangular shapes seen are aligned because of a special process called epitaxy, where the material follows the pattern of the surface it's grown on. Insulating layers, like amorphous boron nitride, are added during the process of making these ultra-thin materials, which are used to build next-generation electronic devices. Image Credit: J.A. Robinson Research Group/Penn State.
A long-standing issue for future technology is rust, which occurs when oxygen and moisture react with iron-containing metals. Rust significantly reduces the lifespan and utility of components in industries like automotive manufacturing.
In two-dimensional (2D) semiconductor materials, which control electrical flow in electronic devices, oxidation presents a similar challenge. While it may not be termed "rust" in the semiconductor industry, corrosion can render these atom-thin materials useless.
2D materials are extremely thin, only one or a few atoms thick. They show promise for advanced semiconductors because their thinness allows electrons to move faster and with less resistance through the material. This, in turn, enables faster and more efficient electronic performance.
Semiconductors are ideal for regulating electrical currents in electronic devices because they can conduct electricity under certain conditions while acting as insulators in others. This unique property makes them the foundational components of electronic devices, serving as the "brains" of computer chips.
One of the biggest issues that we see in 2D semiconductor research these days is the fact that the materials oxidize quickly. 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.
Joshua Robinson, Study Co-Corresponding Author and Professor, Materials Science and Engineering, Pennsylvania State University
Traditional methods to protect these materials from rust involve oxide-based coatings, but these processes often use water, which can ironically accelerate the oxidation they aim to prevent. The team sought a coating material and method that could eliminate the use of water entirely. This led them to amorphous boron nitride (a-BN).
Robinson added, “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.”
According to Robinson, a-BN, a non-crystalline form of boron nitride, is known for its strong thermal stability and electrical insulation properties. These properties make it ideal for use in semiconductors to insulate components, prevent unwanted electrical currents, and enhance device performance.
He emphasized that a-BN possesses high dielectric strength, a critical property for reliable electronic performance, which indicates the material’s ability to withstand high electric fields without degrading.
“The high dielectric strength demonstrated by a-BN is comparable to the best dielectrics available, and we don’t need water to make it. What we demonstrated in the paper was that including amorphous boron nitride yields improved device performance compared to conventional dielectrics alone,” stated Robinson.
Robinson noted that although the coating improved the performance of the 2D transistor, applying it to 2D materials was challenging. Two-dimensional materials lack dangling bonds—unpaired electrons on a material’s surface that interact or form bonds with other atoms—making the coating process difficult.
Using a standard one-step process at higher temperatures resulted in uneven and discontinuous coatings, far below the quality required for optimal electronic performance.
To address this, the team developed a new two-step atomic layer deposition method. They first deposited a thin low-temperature a-BN “seed layer” before heating the chamber to typical deposition temperatures of 250 to 300 °C.
This method allowed the researchers to coat the 2D semiconductors evenly with a-BN and improved transistor performance by 30 % to 100 %, depending on the transistor design.
Robinson stated, “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. The electrons can go faster through the 2D material than they could if they were between other dielectric materials.”
He also mentioned that despite a-BN's high dielectric strength, scientists are only beginning to explore its potential as a semiconductor device dielectric material.
Robinson noted, “We have room for improvement even though it’s already outperforming other dielectric materials. 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.”
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 are the other study authors.
The Semiconductor Research Corporation funded this research through a program sponsored by the 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 US National Science Foundation.
Journal Reference:
Chen, C. Y., et. al. (2024) Tailoring amorphous boron nitride for high-performance two-dimensional electronics. Nature Communications. doi.org/10.1038/s41467-024-48429-4