As computer chips continue to get smaller and more complex, the ultrathin metallic wires that carry electrical signals within these chips have become a weak link. Standard metal wires get worse at conducting electricity as they get thinner, ultimately limiting the size, efficiency, and performance of nanoscale electronics.
In a paper published in Science, Stanford researchers show that niobium phosphide can conduct electricity better than copper in films that are only a few atoms thick. Moreover, these films can be created and deposited at sufficiently low temperatures to be compatible with modern computer chip fabrication. Their work could help make future electronics more powerful and more energy efficient.
“We are breaking a fundamental bottleneck of traditional materials like copper,” said First Author Asir Intisar Khan, who received his doctorate from Stanford and is now Assistant Professor, Electrical Engineering and Computer Sciences (EECS), University of California, Berkeley. “Our niobium phosphide conductors show that it’s possible to send faster, more efficient signals through ultrathin wires. This could improve the energy efficiency of future chips, and even small gains add up when many chips are used, such as in the massive data centers that store and process information today.”
Niobium phosphide is what researchers call a topological semimetal, which means that the whole material can conduct electricity, but its outer surfaces are more conductive than the middle. As a film of niobium phosphide gets thinner, the middle region shrinks but its surfaces stay the same, allowing the surfaces to contribute a greater share to the flow of electricity and the material as a whole to become a better conductor. Traditional metals like copper, on the other hand, become worse at conducting electricity once they are thinner than about 50 nanometers.
The researchers found that niobium phosphide became a better conductor than copper at film thicknesses below 5 nanometers, even when operating at room temperature. At this size, copper wires struggle to keep up with rapid-fire electrical signals and lose a lot more energy to heat.
“Really high-density electronics need very thin metal connections, and if those metals are not conducting well, they are losing a lot of power and energy,” said Senior Author Eric Pop, the Pease-Ye Professor in the School of Engineering, Professor, Electrical Engineering. “Better materials could help us spend less energy in small wires and more energy actually doing computation.”
Many researchers have been working to find better conductors for nanoscale electronics, but so far the best candidates have had extremely precise crystalline structures, which need to be formed at very high temperatures. The niobium phosphide films made by Khan and his colleagues are the first examples of non-crystalline materials that become better conductors as they get thinner.
“It has been thought that if we want to leverage these topological surfaces, we need nice single-crystalline films that are really hard to deposit,” said Co-Author Akash Ramdas, Stanford doctoral student. “Now we have another class of materials — these topological semimetals — that could potentially act as a way to reduce energy usage in electronics.”
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