Standard metal wires worsen at conducting electricity as they get thinner, which limits the size, efficiency, and performance of nanoscale electronics.
Now, Stanford researchers have shown that niobium phosphide can conduct electricity better than copper in films that are a few atoms thick. Furthermore, the films can be created and deposited at low enough temperatures to be compatible with chip fabrication. Their work is detailed in Science.
“We are breaking a fundamental bottleneck of traditional materials like copper,” said Asir Intisar Khan, who received his doctorate from Stanford and is now a visiting postdoctoral scholar and first author on the paper. “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 centres that store and process information today.”
Niobium phosphide is a topological semimetal; 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 become worse at conducting electricity once they are thinner than about 50nm.
The researchers found that niobium phosphide became a better conductor than copper at film thicknesses below 5nm, 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 Eric Pop, the Pease-Ye Professor in the School of Engineering, a professor of electrical engineering, and senior author on the paper. “Better materials could help us spend less energy in small wires and more energy actually doing computation.”
Research into conductors for nanoscale electronics has yielded precise crystalline structures that need to be formed at very high temperatures. The niobium phosphide films made by Khan and his colleagues are said to be 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, a doctoral student at Stanford. “Now we have another class of materials – these topological semimetals – that could potentially act as a way to reduce energy usage in electronics.”
Because the niobium phosphide films don’t need to be single crystals, they can be created at lower temperatures. The researchers deposited the films at 400°C, a temperature low enough to avoid damaging or destroying existing silicon computer chips.
“If you have to make perfect crystalline wires, that’s not going to work for nanoelectronics,” said Yuri Suzuki, the Stanley G Wojcicki Professor in the School of Humanities and Sciences, a professor of applied physics and co-author on the paper. “But if you can make them amorphous or slightly disordered and they still give you the properties you need, that opens the door to potential real-world applications.”
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