PhD,
Materials Science & Engineering, Stanford
PhD
Minor, Electrical Engineering, Stanford
Area
of focus: Nanotechnology, including nanomaterials and nanoelectronics
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.
Labels: carbon_nanotubes, nanotechnology, semiconductor, silicon
