Optoelectronics with Carbon Nanotubes

4. Electroluminescence characteristics of the CNT film diode
Next, we discuss the EL properties of the CNT thin film devices. In general, the radiative
decay of excitons is responsible for light emission in CNTs. Excitons can be electrically
generated in CNTs via (i) simultaneous injection of electrons and holes that form excitons and
radiatively recombine (giving rise to threshold-less ambipolar electroluminescence), or (ii)
accelerating electrons or holes to energies sufficient to create excitons, i.e., the so-called impact
excitation (giving rise to unipolar electroluminescence with threshold characteristics) 111 . By
using a p-i-n design, we aim to improve the control of simultaneous injection of electrons and
holes in order to increase the efficiency of the threshold-less ambipolar electroluminescence,
making the CNT film a more efficient light emitter.
Figure V-6 (a) and (b) show images from optical reflection and an EL signal detected by
the infrared camera, and an overlay of the two images (c,d) to demonstrate that the signal is
coming from between the top gates. Given enough input power (power PEL = IDS · VDS), we find
that the devices emit infrared light in both non-diode mode (unipolar) and diode mode
(ambipolar). Since the top gates are covering the areas near contacts (Figure V-2 (b)), we can
only detect signals from between the top gates, as shown in Figure V-6 (d). Compared to single-
tube FETs, we find that even unipolar devices luminesce in the middle part of the channel as
opposed to near contacts. While Figure V-6 (d) may hint at a more localized emission character
for the ambipolar emission (red line and symbols), the limited spatial resolution does not allow
us to draw any conclusions on the differences between unipolar and ambipolar emission from the
spatial data. Given the distance between the top gates (~1 μm) and the EL peak wavelength (~2
μm), we are at the theoretical resolution limit.
Does a film LED produce more light than a single-tube LED? Figure V-7 compares the
total light output from the two different types of devices under the same source-drain bias.
Clearly, the overall light output of the film LED is far superior to that of the single-tube LED,
showing that we can indeed “scale up” carbon-nanotube devices in terms of light emission. At
VDS = 10V, the single tube and the film are carrying 350 nA and 18 μA, with the light output
yield of 4900 and 76000 in arbitrary units, respectively. On the other hand, this translates to
14,000 a.u. of emission per μA for the single-tube device versus 4,200 a.u. per μA for the film.
It is not surprising that the single tube diode is a more efficient emitter; after all, the CVD-grown
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