As part of our IBM Research Q&A series we sat down with Shu-Jen Han to talk about carbon nanotubes and reducing resistance down to 9nm.
How are silicon and carbon similar when it comes to transistors?
Let’s start with carbon because it has so many different allotropes, from carbon nanotubes, graphene to diamonds. But diamonds, for example, are electrical insulators, not semiconductors – which are what we need for a transistor. Graphene is a two-dimensional sheet of pure carbon (yes, one-atom-thick) that can conduct current well, but it does not have a bandgap, therefore, transistors made with graphene cannot be switched off. Carbon nanotubes are a rolled-up form of graphene, which are somewhat similar to Silicon since they both have band gap and can be used as the center piece of the transistor – the channel.
Why are carbon nanotubes not in use like silicon?
Silicon has offered many advantages as a transistor material for the last half century. One biggest perhaps was that it forms a great gate dielectric – SiO2. It also comes with a very pure and high quality substrate, silicon wafers, to start with. And over time we’ve used other materials and device structures to improve its abilities, such as transitioning to high-k metal gate transistors and FinFETs.
On the other hand, for carbonnanotubes, many material issues have to be solved to obtain similar high-quality carbon nanotube wafers for device fabrication. We can’t switch to an entirely new material over night, but silicon is reaching its scaling limits.
How have you and your team solved this issue of contact resistance?
Carbon nanotubes conduct electricity much faster than silicon, and perhaps more importantly, they use less power than silicon. Plus, at just slightly over one nanometer in body thickness, they’re significantly thinner than today’s silicon, providing good electrostatic control. The challenge has, until now, been how to form high quality contacts between metal electrodes and carbon nanotubes.
In any transistor, two things scale: the channel and its two contacts. It’s at the contacts where carbon nanotube resistance, like silicon, has hindered performance. Especially when channel continues to shrink and channel resistance becomes less and less important. Essentially, current just cannot flow into the channel effectively when you hit atomic dimensions.
Dr. Qing Cao and my other teammates at the IBM Watson Research Center developed a way, at the atomic level, to weld – or bond – the metal molybdenum to the carbon nanotubes’ ends, forming carbide. Previously, we could only place a metal directly on top of the entire nanotube. The resistance was too great to use the transistor once we reached about 20 nm. But welding the metal at the nanotubes’ ends, or end-bonded contacts, is a unique feature for carbon nanotubes due to its 1-D structure, and reduced the resistance down to 9 nm contacts. Key to the breakthrough was shrinking the size of the contacts without increasing electrical resistance, which impedes performance. Until now, decreasing the size of device contacts caused a commensurate drop in performance.
What is necessary to scale this technology? And what is your next step in this work?
We must scale our carbon nanotube transistor onto a wafer. The challenge is twofold: it includes how to orient and place these 1 dimensional structures from the solution onto the wafer as well as how to purify them (initial solution has about 1/3 metallic tubes which are not useful for transistors and need be removed).
We’ve developed a way for carbon nanotubes to self-assemble and bind to specialized molecules on a wafer. The next step is to push the density of these placed nanotubes (to 10 nm apart) and reproducibility across an entire wafer.
What future nanotechnology are you looking forward to?
I can see the potential of our carbon nanotube chips to replace silicon for conventional computing uses. Better transistors can offer higher speed while consume less power. Plus, carbon nanotubes are flexible and transparent. They could be used in futuristic “more than Moore” applications, such as flexible and stretchable electronics or sensors embedded in wearables that actually attach to skin – and are not just bracelets, watches, or eyewear.
For more about this carbon nanotube transistor breakthrough, read “End-bondedcontacts for carbon nanotube transistors with low, size-independent resistance” (DOI: 10.1126/science.aac8006) by Qing Cao, Shu-Jen Han, Jerry Tersoff, Aaron D. Franklin, Yu Zhu, Zhen Zhang, George S. Tulevski, Jianshi Tang, and Wilfried Haensch in Science.