One of the many challenges is that electronic components generate increasingly more heat as they are miniaturised. A significant issue lies in making the wires which connect the transistors on the chip thinner while ensuring that the minimum amount of heat is released.

These interconnects are typically made from copper, and as we start to scale them down to nano-scale thicknesses, their electrical resistance increases rapidly because the electrons moving along the wires have a higher probability of colliding into the surface of the wire. Known as scattering, this leads to energy being released in the form of waste heat, meaning you need more power to maintain the same level of performance.

A group at Stanford University has published a new paper showing that thin films of a material known as niobium phosphide (NbP) exhibit much higher conductivity than copper below a thickness of 5 nanometres (nm) (the typical thickness of the wiring in today’s chips is about 10nm-30nm). This improvement is because NbP is a material with unique quantum properties.

NbP is in a class of materials known as topological materials, which have unique electronic properties due to their atomic structure and were the subject of the 2016 Nobel prize in physics. Their conductivity along the surface is extremely high and stays the same regardless of changes to the shape or size of the material (physicists refer to this as a “topologically protected surface state”).

In other words, it is robust and not easily destroyed by reshaping it. Even when the material is made thinner, bent or given ridges, the special conductive properties that can be observed on the surface remain present.

This is important, as the ability to decrease the thickness of the electrical interconnects while still allowing electricity to flow easily is key to balancing the fabrication cost and complexity against energy efficiency. This gives NbP a significant advantage over other materials.

Disorder vs order

What is particularly remarkable about the discovery by the Stanford team is that these quantum properties were observed in disordered films, meaning the niobium phosphide wasn’t fabricated in the most controlled way to maximise its conductive properties.

There’s a good analogy in making bars of chocolate: it’s vital to control the cooling process of melted chocolate to create the glossy end-product that you’re used to. This process is known as tempering. Anyone who has tried this likely knows how dull and soft the untempered version can be – and how it affects the taste.

The difference between tempered and untempered chocolate is the size of the individual crystals within the chocolate’s structure (or to put it another way, the amount of ordering). Untempered chocolate contains multiple small crystals that are disordered with respect to one another. To make tempered chocolate, a manufacturer will cool it in a controlled way and also add a piece of tempered chocolate to promote the growth of a uniform crystal structure.

Now imagine how useful it would be if you could simply skip this tempering. Not only would it make the production process more efficient, it could also decrease the energy required. This is essentially what the Stanford team have shown in relation to their material.

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For the full article by Dr Mark Greenaway, Dr Fasil Dejene, and Professor Kelly Morrison, visit the Conversation.