R&D
optical emission from a carbon nanotube ambipolar field effect transistor
(FET). Electrical measurements suggest that the emission arises from
radiative recombination of electrons and holes simultaneously injected into
the undoped nanotube.
The team believe their results are consistent with a model where thin
Schottky barriers are formed at the source and drain contacts of the
nanotube FET. The hope is that such a recombination source may form the
basis for ultra-small integrated photonic devices. Unipolar devices, with
only one carrier type (electron or hole), failed to emit light.
The semiconducting single-walled carbon nanotubes (s-SWNTs) measure 1.4nm in
diameter. The research team detected light with a wavelength of around
1500nm - in the range of wavelengths widely used in optical communications.
The device bandgap (about 750meV) corresponds to a 1650nm wavelength. Given
an inverse proportionality between tube diameter and bandgap, the
researchers expect to be able to control the emitted wavelength. Improved
injection efficiency and emission along with reduced operating voltage are
expected to result from aggressive scaling of the gate oxide thickness and
use of a high dielectric constant insulator.
The Schottky model suggests maximum emission with a gate voltage (Vg) being
half the drain (Vd of 4V and 8V). Emission should increase at higher Vd. The
model fits very well the observed intensity, but the researchers say this
could be "fortuitous". An alternative model based on the product of electron
and hole currents gave a poor intensity description but fitted the peak
width well.
