If you need to move electrons from here to there, you turn to copper. This common element is an excellent conductor and is easily fabricated into wires and circuit board traces. But the situation changes when you get small: really, really small on a nanometer scale. That same copper shows increasing resistance, which means that more of the electrical signal is lost to heat. It could take more energy to power a smaller and denser device, which is just the opposite of what you want for miniature electronics.

Researchers at Stanford led by Asir Intisar Khan in Eric Pop’s lab have been experimenting with a novel thin film scaled down to about 1.5 nanometers in thickness. They have found that as this film gets thinner, its conductivity increases, which is the opposite of how copper behaves.

They started with a sapphire substrate and then applied a seed layer of niobium (Nb). They experimented with various thicknesses of this Nb layer, from 4 nm to 1.4 nm. This layer helped the following layer of niobium phosphide (NbP) to form a polycrystalline film when deposited by a simple sputtering process. They made such NbP films from 1.5 nm to 80 nm thick and tested them. While the NbP layer was amorphous, it also held nanocrystals within that amorphous matrix. Importantly, these crystals formed regardless of the thickness of the underlying Nb seed layer.

The resulting NbP ultrathin films had very low electrical resistivity, which became lower as the film got thinner. At about 1.5 nm thick, the NbP layer had a resistivity of only about 34 microohm-centimeters at room temperature, which was about one-sixth the resistivity of thicker versions of the film. A conventional metal such as copper at a similar thickness has resistivity of about 100 microohm-centimeters.

The researchers found that the low resistivity of the thin film is due to its surfaces being more conductive than the bulk of the material. This behavior is what physicists refer to as a “topological semimetal” which is different from how metals like copper behave. As the NbP films gets thinner, there is less material in the middle and their surfaces conduct a larger percentage of the electricity.

Nanometers-thick films of niobium phosphide conduct better than copper on this chip.Asir Khan/Eric Pop

This development is important for the creation of smaller and smaller digital circuits. Reducing resistivity in the connections between transistors means that less energy is lost as heat, which in turn means that ICs will be more energy efficient.

Importantly, these films can be deposited at relatively low temperatures of 400 degrees Celsius, making them compatible with existing semiconductor fabrication processes. This contrasts with other experimental ultrathin conductors that rely on single-crystalline materials which must be synthesize at much higher temperatures.

Obstacles to commercialization remain, however. The tolerances for the film’s layers are critical for performance. For example, the thickness of the seed Nb layer was shown to impact the resistivity of the resulting films, because it can impact the quality of the NbP film.

What’s exciting is that “NbP may be just one type of new material that shows this kind of behavior,” says Eric Pop, the Stanford professor of electrical engineering who led the research. There are some other materials that are known to exhibit this same surface conduction, but it remains to be seen if they also show lower resistivity as the layer gets thinner. “They must be tested carefully,” he says. And “computational advances may discover even more materials with similar behaviors.”