Optoelectronics with Carbon Nanotubes

would expect the width to be comparable to kBT if the lattice temperature had a significant effect
71, 73,
on the carriers. On the other hand, electroluminescence typically shows much larger widths
76, 85, 87, 97, 109, 111, 117 , usually a factor of few larger than PL. When FWHM vs. power is fit with a
linear equation and extrapolated to zero intensity (see Figure III-8), the minimum FWHM at zero
power is almost 7 times wider than 25 meV, i.e., 170 meV for this device. Other devices also
show a minimum width typically between 130 and 200 meV. Although Freitag et al. attribute
the fivefold EL broadening compared to PL in their data to high-temperature electrons and
phonons, they do not relate the broadening to the temperatures quantitatively. There are actually
several possible contributions to the width, such as tube heterogeneity, phonon scattering,
external electric field, exciton-exciton annihilation, electronic temperature, and blackbody
background, just to name a few. Some of these contributions are now discussed semi-
quantitatively.
The minimum width at zero power points to tube heterogeneity as one of the main
contributions to the large width. When only a single peak is used to fit the spectra, the sharpness
of the main peak is not reproduced by Equation III.1 (0.71 eV in Figure III-4 (a)), and this is
always the case with any of our CNTFET EL spectra. This suggests that the “main peak” is
actually a combination of multiple peaks that are broadened and combined to form one very
broad shape, obscuring the contribution by each peak. By examining Figure III-4 (a) more
closely, one can observe at least three peaks, at 0.66 eV, 0.71 eV and 0.88 eV. While the 0.66
eV is most easily seen at VDS = -7.0 eV, it gets obscured somewhat as |VDS| is increased. From
observations of double peaks in p-n junctions (see Chapter IV), this lower-energy peak is
considered to be a weakly localized excitonic peak, possibly bound to local defect sites. This
double-peak feature is discussed in greater detail in the section on single-tube p-n junction diodes
where the peaks are observed more distinctly, but suffice it to say that this alone adds 50 meV
(i.e., the difference between the peak positions) to the total width, provided that the two peaks
are of comparable intensity. The energy difference was even higher in some devices, up to 65
meV.
It also needs to be noted that these peaks are unlikely due to the aggregation of multiple
tubes in a bundle. As explained in the Methods section, bulk tubes (which are originally
bundled) are sonicated for at least 30 minutes at a high power in dichloroethane before deposited
onto the substrate, which is sufficient to separate them into individual tubes, as we have rarely
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