Optoelectronics with Carbon Nanotubes

iii. Film p-n diodes
The design of the film p-n diodes is the same as the single-tube p-n junction (Figure 2),
except that there are multiple tubes and most of the tubes do not traverse the entire channel
length so that carriers have to tunnel from tube to tube. Using the same CNT films as described
in Ref. 99, we redesigned the gate stack to construct a split-gate configuration to create separate
p- and n-doped regions along the transport channel. First, we identified film FET devices with
clear ambipolar characteristics and the on/off ratio of at least 10 3 . We used e-beam lithography
to open 100 μm × 100 μm windows over the critical device areas. The existing single top gate
(Al and Au) and the Al2O3 oxide layer were removed by wet-etching in H3PO4. The lithography
to wet-etch process was repeated once to completely remove the Al2O3 layer because the PMMA
layer tends to degrade after a time in the H3PO4 solution. We followed this process by a very
brief rinse in a KI/I2 gold etchant to remove residual gold particles, preserving only the CNT
films and the metal (Ti = 1 nm/Pd = 40 nm/Au = 20 nm) source and drain contacts.
A new layer of Al2O3 = 33 nm was deposited over the same device area to serve as the
top gate oxide. The top split-gate pattern was then defined by another round of e-beam
lithography, and Ti = 35 nm was deposited by e-beam evaporation. The distances between the
top gates were 1 to 2 μm, and the widths of top gates varied from 1.5 to 2 μm.
After the first round of measurements, many of the Al2O3 layers developed a significant
leakage when the voltage bias between a contact and a top gate exceeded 15 V. Subsequently,
the above process was repeated, but with a thicker Al2O3 layer (50 nm). In order to lift off the
PMMA covered with a hard shell of 50-nm Al2O3, the sample was exposed to UV light
(wavelength 252 nm) for 10 minutes in order to facilitate the breakdown of PMMA. The top
split-gate metal (Ti=15 nm) was patterned and deposited by e-beam lithography and e-beam
evaporation as before. The excessive processing seems to have degraded the film quality,
judging from the transport and luminescent properties after the second time.
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