US researchers develop semimetal more conductive that copper

STANFORD researchers have developed a semimetal more conductive than copper, paving the way for an energy-efficient alternative for nanoscale electronics.

The team found the semimetal niobium phosphide conducted electricity better than copper in films that were only “a few atoms thick”.

These films are compatible with the low temperatures required for modern nanochips, which contain metal connective wires made from copper or aluminium.

Niobium phosphide is a topological semimetal which conducts electricity through its whole material, with the outer surface being more conductive than the centre. The material is already used in laser and high-power/high-frequency applications.

Eric Pop, a professor at Stanford’s School of Engineering, said: “Better materials could help us spend less energy in small wires and more energy actually doing computation.”

He added: “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.”

Copper problem

According to the researchers, copper becomes less effective at conducting electricity once it is thinner than around 50 nanometres. Copper wires also tend to struggle to keep up with rapid-fire electrical signals and lose their energy to heat.

The researchers found that niobium phosphide wires, at just 5 nanometres thick, were better conductors. The material’s non-crystalline structure improves conductivity as it becomes thinner.

Akash Ramdas, a doctoral student and part of the team, said: “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.”

He added: “Now we have another class of materials – these topological semimetals – that could potentially act as a way to reduce energy usage in electronics.”

Niobium phosphide films can be developed at low temperatures, with the researchers successfully depositing them at 400°C – a temperature low enough to avoid damaging existing silicon chips.

Future nanoelectronics

The Stanford team are now looking at ways to improve the performance of niobium phosphide, including testing alternative topological semimetals.

Pop said: “We’ve taken some really cool physics and ported it into the applied electronics world.”

He added: “This kind of breakthrough in non-crystalline materials could help address power and energy challenges in both current and future electronics.”