Optoelectronics with Carbon Nanotubes

Chapter III Unipolar, High-Bias Emission
1. Introduction
Chapter I showed that both ambipolar and unipolar emission has been demonstrated with
CNTFET devices 10, 71-74, 76, 85, 97 . In a sense, ambipolar emission gives greater experimental
control; one can manage the relative electron and hole injection rates by setting the drain (VDS)
and the gate biases (VGS) appropriately 74 . In contrast, in the unipolar scheme we do not know
the electric field strength of an emission spot, or the distribution of carrier kinetic energy.
Nevertheless, certain features of ambipolar emission pose practical experimental
problems that make it less appropriate for our purposes; to study the electrically-induced
emission by investigating intensities and spectra from CNT devices and to build more efficient
and robust devices. In order to have a symmetric device with respect to both carriers, the Fermi
level needs to align mid-gap, and the device needs to be operated in the “off” state (i.e., at the
bottom of the “V” shape in the gate sweep; see Figure III-3 for example) with a large source-
drain bias in order to inject enough carriers. In addition, the radiative efficiency of electrical
excitation has been shown to be about 10 -6 at best 76 .
Because of the high blackbody background noise from the optical components at ambient
temperature (see Method), the long-wavelength limit of the detection window is around 2100
nm, or 0.590 eV. Ambipolar emission requires larger diameter tubes of at least 2 nm or larger
for sufficiently thin Schottky barriers, and the corresponding E11 energy is around 0.55 to 0.56
eV or lower 15 which lies outside the detection window. In addition, a large source-drain bias
means a large stress on the device, leading to heating and/or oxidation/destruction of the device.
Since it needs to operate in the “off” state where VGS ~ VDS/2, it is difficult to pass large enough
currents to obtain intensity-dependent spectra.
For these reasons, for this study the unipolar scheme was chosen where multiple electron-
hole pairs can be created from a single injected carrier type. Since only one type of carriers
needs to be injected, to facilitate transport the Fermi level of the metal can be aligned to either
the conduction or valence band edge. In order to obtain emission intensity comparable to that
from an ambipolar device, a larger bandgap tube and therefore a larger E11 energy can be used,
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