Optoelectronics with Carbon Nanotubes

In a split-gate scheme, a new level of control over electrical carrier-injection in CNTs is
achieved, which allows device operation in a low-power-density regime (~0.1 W/m compared to
~10 W/m) with zero threshold-current, negligible self-heating, and high carrier-to-photon
conversion efficiency (~10 -4 ). In this way, we have found that the EL spectra are significantly
narrower (observable in our set-up down to ~35 meV) than those in previous studies, allowing
the identification of emission from free and localized excitons. This phenomenon was discussed
briefly in Chapter III and we now expand upon it with more detailed analysis that is made
possible by the narrow linewidth of each peak.
2. Transport characteristics of the single CNT p-n diode
In Fig. IV-1 (b) we present the IV characteristics of a single SWNT diode under two
different biasing conditions. The dashed green curve shows the drain-source current IDS versus
drain-source voltage VDS when both gate biases are negative: VGS1 = VGS2 = -8 V. The tube then
behaves as a p-type resistor and a symmetric, almost ohmic conduction behavior is observed.
The deviation from a completely linear IV characteristic (dotted curve) at low VDS is attributed to
a voltage drop at the Schottky contacts between the metal electrodes and the CNT. This is
analogous to the non-linear I-VDS observed in Schottky-limited CNTFET devices at a low bias,
as in Figure III-5. By applying gate biases of opposite polarity a p-i-n diode is realized. The
solid red line in Fig. IV-1 (b) shows the IV characteristic recorded with VGS1 = -8 V and VGS2 =
+8 V. The device now clearly shows rectifying behavior. The corresponding bandstructure is
shown in Fig. IV-1 (c).
We found that in short devices (channel length < 4 μm), the diode behavior is often
imperfect, with a leakage current on the “off” side. This happens because the formation of the
doped regions is incomplete, i.e., Schottky barriers are most likely extending partially into the p-
and/or n-doped regions. The solution is to either increase the channel length under the gates, or
to improve the coupling between the split gates and the nanotube by employing a thinner and/or
a higher-κ dielectric.
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