Optoelectronics with Carbon Nanotubes

inging the lowest energy peak(s) into the detection window. The unipolar emission still
requires a high gate and source-drain biases to create a high field within the channel; the
emission threshold is typically in the order of 1 μA, often approaching the current carrying limit
of CNTs at higher intensities required for spectral analysis. High fields and possible heating
effects can become problematic, which we shall examine as part of the high-bias emission.
2. Experimental Results and Discussion
i. Unipolar Electrical Transport Characteristics
For spectroscopic studies, p-type CNTFET devices were created from laser ablation and
arc-discharge tubes with a contact metal with a high work function (Pd). Figure III - 1 shows the
drain current (ID) versus the gate voltage (VGS) of a typical p-type CNTFET device as the drain-
source bias (VDS) is increased in equal steps from curve to curve. All curves show an “on”
behavior at a negative gate voltage, as the Schottky barrier becomes thinner for the valence band
and holes are injected into the channel (recall Figure I-8 in Introduction). The total device
resistance is in the order of ~10 MΩ at on-state, which is typical for a small-diameter
semiconducting CNT device with Pd contacts 56 . The large hysteresis in the DC measurement is
due to trap charges in silicon oxide, which is well known and was investigated recently in detail
by pulsed characterization 104 . Our EL measurements were taken at a large enough gate voltage
to be outside the hysteresis region. Note that for low VDS (-1 V), the ratio of Ion/Ioff is 4 to 5
orders of magnitude, but at a high VDS the on-current saturates while the off-current increases so
the Ion/Ioff decreases as |VDS| is increased from 1 V to 7 V, indicating that the Schottky barrier is
overcome somewhat even at the off-state at a higher |VDS|.
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