Optoelectronics with Carbon Nanotubes

We also note that the onset of the forward-biased emission coincides with the current onset,
while the onset of the reverse-biased emission lags behind the current onset. This is discussed in
more detail for V-8 (b) inset below, and additionally in the investigation of electroluminescence
width.
It should be noted that we do not observe a significant influence of the split gate in the
intensity characteristics of the emitted light in the reverse direction; on the negative VDS side, the
intensity is almost identical for all the split-gate settings at any given VDS value. Although there
is a potential drop between the top gates, the field created by the VTGS biasing does not appear to
be greater than the impact excitation threshold, since there is little dependence on that parameter.
In contrast, a single-tube device operated as a unipolar emitter shows a clear dependence on
bottom global gate voltage (not shown), which is reasonable because the light generation
depends of the degree of band bending at contacts that controls the carrier injection rate (see
Figure IV-5 inset for schematics). From such split-gate effects and the differences between the
forward and reverse biases, we can conclude that the CNT film device functions as a light-
emitting diode.
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