Optoelectronics with Carbon Nanotubes

the optical phonon population is not saturated as in suspended tubes, with the temperature <
1200 K.
As the applied power is increased, the electronic temperature should rise, unless the
kinetic energy is dissipated via some other decay channel, which we claimed is the impact
excitation mechanism. We have already cited temperature saturation as its supporting evidence.
If the impact excitation mechanism is responsible for the lack of electronic temperature increase,
that should result in an increase in emission per injected carrier. Figure III-12 shows that
emission intensity per injected carrier (i.e., current) does increase almost to the third power as the
applied power is increased, indicating that the EL efficiency is much greater at a higher power.
If the 60 meV broadening is indeed due to electronic heating at a constant temperature, the figure
is much smaller than reported by Freitag et al. where a five-fold broadening has been attributed
to electronic heating. The difference, compared to our small electronic heating, is most likely
due to the partially suspended nature of their device. Optical phonon saturation has been known
to occur in suspended structures 87 , leading to a higher electronic temperature in their device.
Figure III-12. Integrated Lorentz peak intensity as a function of current. The fit
(solid line) shows that there is almost a cubic dependence on the current.
While the EL efficiency increase supports a constant in electronic temperature, it does not
guarantee it (electron temperature can still increase in the presence of a greater EL efficiency),
58