Research Better microelectronics with carbon?

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The University of Illinois Urbana-Champaign and its research partners demonstrated how carbon can play a significant role in the next generations of electronics.

Coal is a plentiful resource in the United States, which, unfortunately, has contributed to climate change due to its use as a fossil fuel. As the country transitions to other forms of energy production, it is important to reconsider and reassess the economic role of coal.

A joint research effort by the University of Illinois Urbana-Champaign, the National Energy Technology Laboratory (NETL), Oak Ridge National Laboratory and Taiwan Semiconductor Manufacturing Company has shown how coal can play an important role in next-generation electronics systems.

"Coal is usually seen as something bulky and dirty, but the manufacturing techniques we have developed can transform it into highly pure materials that are only a few atoms thick," said Qing Cao, Professor of Materials Science and Engineering at UoI and one of the leaders of the collaboration.

"Their unique atomic structures and properties are ideal for manufacturing the smallest possible electronics with performance superior to the current state of the art."

A process developed by NETL initially converts charcoal into nanoscale carbon flakes, referred to as "carbon dots." The university research group then demonstrated that these can be interconnected to form atomically thin membranes for use in two-dimensional transistors and memristors. These are technologies that will be crucial for the manufacturing of more advanced electronics. These research findings were published in the journal 'Communications Engineering.'  
 

Perfect for 2D electronics

In the constant quest for smaller, faster, and more efficient electronics, the final step will lead to devices made of materials that are only one or two atoms thick. It is impossible for these components to be any smaller. And due to their small size, they often operate much faster and consume much less energy.

While ultra-thin semiconductors have been extensively researched, atomically thin insulators are also necessary to construct functioning electronic components such as transistors and memristors.

Atomically thin layers of carbon with disordered atomic structure can serve as excellent insulators for constructing two-dimensional devices. The researchers involved in the collaboration have demonstrated that such carbon layers can be formed from carbon dots, which are in turn derived from charcoal. To showcase their capabilities, the group led by Cao at the UoI has developed two examples of two-dimensional devices.

"It's really quite exciting because it's the first time that coal, which we usually think of as low-tech, has been directly connected with the cutting edge of microelectronics," said Cao.

Transistor dielectric

Cao's group used carbon layers derived from coal as gate dielectric in two-dimensional transistors based on the semimetal graphene or the semiconductor molybdenum disulfide, enabling more than double the operational speed of devices with reduced energy consumption.

Like other atomically thin materials, the carbon layers derived from coal have no "dangling bonds" or electrons that are not associated with a chemical bond.

These sites, which are abundant on the surface of traditional three-dimensional insulators, alter their electrical properties by effectively acting as "traps" and slowing down the transport of mobile charges and thus the switching speed of the transistor.

In contrast to other atomically thin materials, the new carbon layers derived from coal are amorphous, meaning they lack a regular crystalline structure. Consequently, they do not exhibit boundaries between different crystalline regions that serve as conductive pathways and lead to "leakage," where undesirable electrical currents flow through the insulator, causing significant additional power consumption during device operation.

Memristor filament

Another application investigated by Cao's group is memristors - electronic components that can both store and process data, significantly enhancing the implementation of AI technologies. These devices store and represent data by modulating a conductive filament formed through electrochemical reactions between a pair of electrodes and an intervening insulator.

The researchers found that using ultrathin carbon layers derived from coal as insulators enables the rapid formation of such filaments with low energy consumption, achieving high device operating speeds with low power consumption. Additionally, atomic-sized rings in these carbon layers derived from coal limit the filament, improving the reproducibility of the devices and thus increasing the reliability of data storage.

From research to production

The new devices developed by the Cao group provide proof of concept for using carbon layers derived from coal in two-dimensional devices. It still needs to be demonstrated that such devices can be produced on a large scale.

"The semiconductor industry, including our team at Taiwan Semiconductor, is very interested in the potential of two-dimensional devices, and we are striving to meet this demand," said Cao. "In the coming years, UoI will continue to collaborate with NETL to develop a manufacturing process for coal-based carbon insulators that can be used in industrial settings."

The research report titled "Ultrathin Quasi-2D Amorphous Carbon Dielectric Prepared from Solution Precursor for Nanoelectronics" is available online. DOI: 10.1038/s44172-023-00141-9 (mbf)

* Henning Wriedt is a freelance specialist author.