Optoelectronics with Carbon Nanotubes

channel is created where electrons and holes coexist and can annihilate each other, some of
which emit light. Misewich, et al. 73 found that the light intensity is maximum when VGS =
VDS/2, that is, when the band structure is symmetrical with respect to the electrons and holes, so
that they are injected at equal rates. The light intensity is governed by the number of minority
carriers, since they limit the total number of electron-hole pairs that can be created per unit time.
Therefore, the greatest recombination rate occurs when the numbers of carriers are equal and
there is a region of strong concentration of both carriers.
Consequently, a later experiment using long-channel CNTFETs found that the ambipolar
radiative spot can be physically moved along the channel by changing the gate voltage 74 . The
size of the recombination spot was 1 μm (spatial resolution limit) or less, and a simple analytic
model with drift transport was found to describe the phenomenon quite well 75 . Because carriers
are injected directly, the quantum efficiency of the ambipolar emission can be calculated in a
straightforward manner in the case of equal-rate injection. The EL efficiency was found to be
10 -6 to 10 -7 per electron-hole pair compared to the observed PL efficiency of up to 10 -2 , and to
depend on the type of nanotube and the channel length 76 . However, these numbers are not
necessarily comparable since PL efficiency has been measured from tubes in solution or on
suspended CNTs 77-79 , unlike EL efficiency measured in on-substrate devices. PL from on-
substrate single tubes is very difficult to observe because of the severe quenching due to the
presence of the substrate. At this writing, the efficiency of PL on a substrate has not been
quantified, and there is yet to be a study of EL from suspended CNTFETs in the ambipolar
regime.
One way to improve efficiency is to electrostatically create p- and n- regions within the
channel so that the recombination can occur at a well-defined p-n junction. Such a device was
first demonstrated by Lee et al. in 2004 80 using an embedded split-gate design, which
remarkably showed a close to ideal p-n junction diode behavior. The device was consequently
improved so that the SWNT was suspended, which showed the ideal p-n diode characteristics
and enabled a photovoltaic investigation of the bandgap structure, excitonic transition states and
bandgap renormalization from the electronic self energy 81-83 . A natural extension of this work
was a single-tube carbon nanotube light-emitting diode, which was demonstrated by us in 2009
11 and was shown to give a narrow spectrum (compared to conventional EL) and a quantum
efficiency of about 10 -4 , an improvement of two to three orders of magnitude over the
23