Optoelectronics with Carbon Nanotubes

is increased, contacts to smaller diameter tubes become transparent so they can carry current and
emit light as well, which adds to the spectral width. This explanation also agrees with the fact
that the Gaussian distribution fits the spectra better than the Lorentz function. Once all the
possible channels are participating in conduction, there is little temperature effect and other
mechanisms do not increase the width further, so the width saturates fairly quickly.
Perhaps more remarkably, we observe that the width saturates above a certain input
power, above ~ 50 W/m, regardless of the operating mode. We saw in single-tube unipolar
CNTFETs that there is significant broadening due to field-induced ionization in the
recombination region. Compared to the single-tube CNTFET operated as a unipolar emitter that
requires a high electric field (e.g. 25 to 40 V/μm) for exciton production, the film LED has a
much lower field in the emitting region, i.e., in the middle of the channel. This broadening effect
was estimated to be 60 meV at the low end of the field (Figure III-11). The first-order
approximation gives the maximum field of less than 7 V/μm due to the split gates; this of course
is zero if different voltages are not applied to the gates. Given the device geometry, the middle
of the channel is about two orders of magnitude farther away from the contacts than from the top
gates, so the field changes little when the drain bias is increased. This can explain why the
overall width is much smaller than in single unipolar devices, even when the film is operated as a
regular FET. Furthermore, bias-dependent broadening is practically non-existent because there
is no change in electric field between the gates, leading to the observed saturation behavior. The
width is slightly greater for split-gate mode, where there does exist a small field (~7 V/μm as
estimated above). Thus, the spectral width in the film device operated in different modes gives
strong evidence for the field-induced broadening explained in Chapter III.
Even when the film device is operated as a global-gate FET (the black triangles in Figure
V-10), the emission mechanism itself is still ambipolar recombination in the middle of the
channel, where the field is too small for exciton production by impact excitation. Figure V-11
shows the emission intensity as a function of current for the three different modes. It shows
clearly that much higher total current is required in the unipolar operating mode (black triangles),
because it is the number of the minority carriers that limits emission by recombination. Since
emission originates mid-channel, where the field is small, the overall width is much smaller than
in unipolar single-tube devices, where exciton production and recombination occur in high fields.
The width is comparable to those of the other two operating modes, since broadening
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