Optoelectronics with Carbon Nanotubes

Figure I-9. Schematic illustrations of band structures in ambipolar and unipolar
conduction. Note that the bandgap is smaller in the ambipolar case, allowing both
types of carriers to be injected from midgap-aligned contacts.
6. Electroluminescence from carbon nanotube devices
The semiconducting CNT is a direct bandgap material with almost an ideal one-
dimensional structure and high mobility; it allows us to study optoelectronics and photonics in
1D and is a promising material in developing an integrated nano-scale electronic and
optoelectronic technology. In EL, excitons are created by electrical excitation of carriers, so the
process is not restricted by dipole selection rules as in the case of photon absorption, and the
momentum conservation requirement is also different. This means that states that may not be
available to photo-excitation can be accessible by electrical excitation. From the technological
point of view, CNTFETs could function as an integrated transistor and an IR light emitter or
detector in an extremely compact package.
Emission from CNT devices involves (except for blackbody radiation) radiative
combination of electron and holes, either as free carriers or bound in the form of excitons. Two
basic mechanisms for creating electron-hole pairs in CNTs electrically are related to the transport
mechanisms discussed in the previous section. Ambipolar emission refers to the case in which
both positive and negative carriers are injected into the device from opposite contacts and
combine radiatively in the channel. In contrast, in unipolar emission, there is only one type of
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