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Carbon Nanotube Electronics

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Carbon Nanotube Electronics
Presented By: Yu-Jin Chen
Mentor: Professor Philip Collins
IM-SURE 2007
My Work:
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COMSOL model of a CNT device
Tested two methods of depositing SiO2:
пЃ® Electron beam evaporation (E-Beam)
пЃ® Plasma-enhanced chemical vapor
deposition (PECVD; BMR)
Also tested etching metal dots from the
surface of a chip (with and without oxide)
Carbon Nanotubes
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A carbon nanotube can
be modeled as a
hexagonal lattice
(graphene) “rolled up”
into a cylinder.
The nanotubes we work
with are single-walled
and typically 1 nm in
diameter.
Motivation
sensitive to gases
modified
for biosensing
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The Collins Group researches carbon nanotubes
because there are potential applications for
biosensors, sensing dangerous gases, studying
proteins, etc.
Nanotube Devices
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The carbon
nanotubes are
grown on silicon
wafers by chemical
vapor deposition
(CVD).
Lithographicallydeposited
electrodes allow us
to measure
nanotube wires
and transistors.
CNT Devices: Typical Device Sizes
~10 um
~150 um
~1 um
Nanotubes with Defects
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Defects can be added to a
nanotube and the defects can be
chemically modified.
Defects are important because
they are very sensitive to
changes in the environment.
The Big Picture
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First, we have a device with a
metal dot deposited on the
nanotube sidewall.
Next, we deposit oxide on this
device.
Finally, we etch the metal dot,
and are left with a nanotube
device that has an isolated
defect.
The ultimate goal is to develop a working recipe for
making oxide-covered nanotube devices that take
advantage of the sites of defects.
COMSOL
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The model consists of a silicon substrate, a silicon
oxide layer, two titanium electrodes and 105
cylindrical segments that represent the nanotube.
E-Beam Evaporator
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This uses an electron beam to evaporate materials
(silicon oxide in our case). The resulting gas then
expands from the crucible and hits the sample
mounted at the top of the machine.
This method is very controllable, capable of
depositing a film 1 Angstrom thick.
BMR PECVD
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The BMR PECVD system flows gases into a chamber
containing our sample. Then, a plasma is created,
which causes the other gases to react, forming oxide.
The oxide then sticks to the surface of the sample.
This method creates a very uniform and dense film.
Comparing Oxides
BMR
Leakage Current (A)
Leakage Current (A)
-10
E-Beam Oxide
3
2
1
0
-1
-2
-20
-30
-40
-50
-9
-3x10
-12
-60x10
-0.10
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-0.05
0.00
Voltage (RE) (V)
0.05
0.10
-100x10
-3
-50
0
Voltage (RE) (V)
50
Initial tests showed that the BMR film was much
better in quality compared to the E-Beam film.
100
Nanotube Test Results
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The results
show that the
nanotubes
survive the
electron beam
evaporator but
not the plasma
CVD.
Metal Etch
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We use samples in
which gold dots
have been placed on
the surface so that
we can test our
ability to etch metal
dots from under
oxide.
Metal Etch Results
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The sample on the left has oxide deposited on it. The
sample on the right has been etched in aqua regia.
Images
Problems with Etching
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The image on the left shows a sample before it was etched.
The right image shows the sample after it was etched. In both
cases, this was a bare surface with gold dots.
Breaking the Monolayer
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О»= 365 nm
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Au
We discovered that the
monolayer surrounding
the particles protected
them from etching.
After consulting the
literature and other
groups, we tried
exposing the samples to
UV as well as different
etching solutions to
solve this problem.
However, we have not
had success.
Summary
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What we can do:
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Deposition of metal dots on defects
Controlled deposit of very thin oxide films
Etch very large metal dots from under oxide
Future work:
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Adding more to the COMSOL model
More tests to etch away metal nano-dots
Characterizing the oxide films deposited by ebeam evaporation
Special Thanks
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Collins Group:
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Professor Phil Collins
Alex Kane
Bucky Khalap
Brett Goldsmith
John Coroneus*
Danny Wan
Steve Hunt
Tatyana Sheps
Phil Haralson
Images courtesy of the Collins
Group and the INRF website.
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INRF:
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IM-SURE:
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David Crosley
Vu Phan
Said Shokair
The NSF and UROP
Questions?
Electronic Properties
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Carbon nanotubes act
as one-dimensional
wires. They can be
either metallic or
semiconducting
depending on the
chirality of the tube, the
“twist” of the hexagonal
lattice.
Chirality
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The chiral
vector for a
(2,4) nanotube
(left) and an
unrolled (5,0)
nanotube with
one unit cell
highlighted in
red.
Gate Electrode
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The gate electrode is capacitively coupled to the
nanotube, so it is able to increase or decrease the
number of charge carriers in a semiconducting tube.
Goals
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The ultimate goal is to develop a
working recipe for making oxidecovered nanotube devices that
take advantage of the sites of
defects.
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