Optoelectronics with Carbon Nanotubes

power for a single-tube unipolar FET emitter (Figure III-8). In the film device, there is a brief
rise in width to ~ 50 W/m, followed by a more or less constant width, even up to a power input
of 500 W/m. This trend does not depend on the operating mode; top gate biasing has little effect
on the way the width increases as a function of power.
Given the fabrication and operation of the film device, we can safely assume that some of
the mechanisms discussed in Chapter III are also at work here. In Chapter III, we estimated that
about 70 meV was due to tube heterogeneity and phonon scattering at a power as low as 40 W/m,
of which at least 20 meV is due to temperature-dependent acoustic phonons. We do not have
enough data points to extrapolate the width reliably to zero power, but the minimum width is at
least 100 meV (see Figure V-10). We assume that the degree of tube heterogeneity should be
comparable to that of single tubes (i.e., the same raw material and similar fabrication processes).
The effect of high-energy optical phonon scattering, which result in multiple-peak effect that
contribute to overall width, should be less film devices, since current per tube is much lower.
Since exciton-exciton annihilation does not seem to play a significant role in single tube
devices, it is unlikely that it plays a role here where we have even smaller exciton density per
channel. While we do not have information on the current load per channel, it is undoubtedly
much smaller than on a single-tube device carrying the same magnitude of current. Therefore,
other effects such as temperature-based broadening and exciton-exciton annihilation effects, both
of which do not make a significant contribution to the width in a single-tube device, should also
be quite limited.
101