Optoelectronics with Carbon Nanotubes

Carbon nanotube’s superior conduction due to limited scattering and hence the high
mobility in SWNTs is a result of their unique one-dimensional structure. Because small-angle
scattering is not allowed in a 1D material (i.e., carrier motion is limited to forward and
backward), and because contemporary manufacturing methods produce SWNTs relatively free of
defects and impurities, the elastic mean free path (mfp) can be very long, in the order of microns
62 . This means that the transport properties are largely determined by inelastic scattering, mostly
of phonons. At low bias and/or low temperature, only low-energy acoustic phonons can
participate in inelastic scattering. However, the coupling between acoustic phonons and
electrons is weak because of the aforementioned scattering-angle restriction, and also due to the
energy-momentum conservation requirement, resulting in only a small fraction of acoustic
phonons that can contribute to this process. The optical phonon energy is very high (~200 meV),
so they do not come into play at low-bias/temperature. Thus, the carrier mobility is very high
(upward of ~ 100,000 cm 2 /Vs) in SWNTs 63 , even at room temperature. At high bias, carriers
can produce high-energy optical phonons, which modify the carbon-carbon bonds in SWNTs and
thereby change the electronic structure; such phonons naturally couple strongly to carriers. As
discussed in the current saturation section above, optical phonons have a very short mfp which
leads to the maximum current limit, especially in suspended devices, where phonons cannot be
easily dissipated through the substrate and even lead to negative differential conductance 53 .
The third important mechanism that affects the transport, the Schottky barrier, is best
understood in the context of carbon nanotube field-effect transistors (CNTFETs). The
conventional CNTFET design is shown in Figure I-7 (a) and has been used in many experiments
because of the relative ease of fabrication and its effectiveness and reliability. The carbon
nanotube itself provides the transport channel when contacted on each end by bulk metal, and the
heavily doped silicon substrate acts as the back gate, with SiO2 (or another dielectric material) as
the gate dielectric. The switching behavior of a CNTFET is such that one can obtain an Ion/Ioff
ratio of 10 5 to 10 7 and an on-current of ~1 μA at the operating drain-source voltage VDS of 1V.
The details of the conduction characteristics depend on the tube diameter, the choice of the
contact metal, etc. (Figure I-7 (b)).
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