Creating 'rust-free' semiconductor materials

These materials, which regulate the flow of electricity in electronic devices, can become entirely useless if corrosion occurs. However, a collaboration between academic and industry researchers has now devised a new synthesis process to create a 'rust-resistant' coating with additional properties suited for faster, more robust electronics. This work, co-led by Penn State researchers, has been published in Nature Communications.

2D materials, which are only a few atoms thick, hold great potential for advanced semiconductor technology. Their thin structure allows electrons to travel faster and with less resistance, improving the efficiency and speed of electronic devices.

"One of the biggest issues that we see in 2D semiconductor research these days is the fact that the materials oxidise 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."  

Conventional methods to protect these materials from oxidation often involve oxide-based coatings, which ironically rely on water—a substance that can exacerbate the very problem they are meant to prevent. To counter this, the research team sought a coating material and process that entirely avoids water. Their solution? 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.  

Amorphous boron nitride, a non-crystalline form of the compound, is known for its excellent thermal stability and electrical insulation. These qualities make it ideal for use in semiconductors, where it can insulate components, prevent unwanted electrical currents, and enhance device performance. Robinson noted that a-BN also possesses a high dielectric strength, which refers to its ability to endure strong electric fields without breaking down—a critical property for reliable electronics.

“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." 

Although the coating significantly enhanced 2D transistors, applying the a-BN coating to the 2D materials posed a challenge. Due to the absence of dangling bonds—unpaired electrons on a material’s surface that can react or bond with other atoms—traditional methods of applying the coating produced uneven and insufficient coverage, unsuitable for high-performance electronics.

To address this, the team developed a novel two-step atomic layer deposition method. This process involves first depositing a thin 'seed layer' of a-BN at a low temperature, followed by heating the chamber to typical deposition temperatures of 250 to 300 degrees Celsius. This technique allowed them to achieve a uniform a-BN coating on the 2D materials and resulted in a 30% to 100% improvement in transistor performance, depending on the design of the device.

"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,” said Robinson. “The electrons can go faster through the 2D material than they could if they were between other dielectric materials.” 

Robinson emphasised that, despite the high dielectric strength of a-BN, much more remains to be explored in terms of its potential as a dielectric 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.” 

Alongside Robinson, the study’s authors include Cindy Chen, Riccardo Torsi, Ke Wang, and Bangzhi Liu from Penn State’s Materials Research Institute, as well as co-lead author Yu-Chuan Lin from Taiwan’s National Yang Ming Chiao Tung University. Other contributors are Zhihong Chen and Joerg Appenzeller from Purdue University, Jessica Kachian from Intel, and Gilber B. Rayner Jr. from The Kurt J. Lesker Company.

The research was supported by the Semiconductor Research Corporation, the National Institute of Standards and Technology, Taiwan’s Ministry of Education, the Center for Emergent Functional Matter Science at National Yang Ming Chiao Tung University, and the U.S. National Science Foundation.