Optoelectronics with Carbon Nanotubes

In order to investigate whether the light generation efficiency is improved by the use of a
p-n junction, we plot in Figure V-8 (b) the integrated EL intensity as a function of the injected
electrical power, PEL = IDS · VDS for several voltage configurations. With the device biased in the
reverse direction, we obtain linear power dependency with almost identical slopes, independent
of the split gate voltages applied (Figure V-8 (b), solid blue and red circles). Again, we see little
effect of the split gates in the reverse mode. If the device is biased in the forward direction
(Figure V-8 (b), open squares), we observe that the slope of the intensity curve is steeper than the
curves in the reverse mode. This indicates a greater efficiency with which the device generates
light when biased in the forward direction.
More importantly and perhaps more interestingly, if the split gate voltages are tuned from
0V/0V to +5V/-5V and the device is swept in the forward direction, we observe a strong increase
of light generation efficiency (Figure V-8 (b)). The power dependence can be captured
adequately by fitting a quadratic form to the data points. The somewhat greater-than-linear
power dependence observed might be due to either higher order electrostatic field contributions
(across the junction) or higher order excitonic effects (in the CNTs); the investigation of both
effects is beyond the scope of this study. At PEL = 150 μW, the light generation efficiency has
increased by a factor of 4 as compared to the reverse mode.
We found that the “forward” bias is more efficient than in “reverse” bias, even without
split gate voltages applied. Here, “forward” and “reverse” simply refer to the polarity of VDS as
in Figure V-8 (a); since the top gates are not biased, there are no p- and n- segments created, at
least in theory (there may be some residual doping of the substrate, as is typically evidenced in
the hysteresis discussed in Chapter III). By switching the source and the drain, we find that the
efficiency changes significantly; compare the 0V/0V curves in Figure V-8 (b), for example. This
effect is associated with built-in non-uniformities of the CNT film and the potential profile of
individual devices. We found that a certain level of asymmetry almost always exists in the
electrical transport behavior (see for example the transport at 0V/0V, solid red circles vs. open
green squares, in Figure V-4 (b)), making devices operate more efficiently for a certain choice of
(source/drain) bias conditions. This is not surprising since the respective carrier injection rate
from each contact depends on the details of the tube-metal interfaces. Although the diode
behavior was also observable in the non-ideal source/drain choice, we consistently chose the
better source/drain assignment for diode in order to observe the split-gate effect more clearly.
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