CNT devices

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CNT devices
Since their first discovery and fabrication in
1991, CNTs have received considerable attention
because of the prospect of new fundamental
science and many potential applications.
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Avouris, IBM
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Stretching
And confined deformation
Strain of less than 1 results in the CNT
changing from metal to semiconductor.
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Twisting and bending
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Peapods
The encapsulated fullerenes can rotate freely in
the space of a (10, 10) tube at room temperature,
and the rotation of fullerenes will affect
C60_at_(10, 10) peapod electronic properties
significantly generally, orientational
disorderwill remove the sharp features of the
average density of states (DOS). However, the
rotation of fullerenes cannot induce a
metalinsulator transition. Unlike the
multicarrier metallic C60_at_(10, 10) peapod, the
C60_at_(17, 0) peapod is a semiconductor, and the
effects of the encapsulated fullerenes on tube
valence bands and conduction bands are
asymmetrical. The distances between the centres
of the fullerenes are 0.984 and 1.278 nm for the
C60_at_(10, 10) peapod and C60_at_(17, 0) peapod,
respectively.
J. Chen, and J. Dong, J. Phys. Condens. Matter,
16, 1401 (2004)
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Semiconducting behavior in nanotubes was first
reported by Tans et al. in 1998.
Fig. 5 shows a measurement of the conductance of
a semiconducting SWNT as the gate voltage applied
to the conducting substrate is varied. The tube
conducts at negative Vg and turns off with a
positive Vg. The resistance change between the on
and off state is many orders of magnitude. This
device behavior is analogous to a p-type
metaloxidesemiconductor field-effect transistor
(MOSFET), with the nanotube replacing Si as the
semiconductor. At large positive gate voltages,
n-type conductance is sometimes observed,
especially in larger-diameter tubes.
McEuen et al., IEEE Trans. Nanotechn., 1, 78
(2002)
It is shown that, by appropriate work function
engineering of the source, drain and gate
contacts to the device, the following desirable
properties should be realizable a sub-threshold
slope close to the thermionic limit a
conductance close to the interfacial limit an
ON/OFF ratio of around 1000 ON current and
transconductance close to the low-quantum-capacita
nce limit.
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Semiconducting nanotubes are typically p-type at
Vg0 because of the contacts and also because
chemical species, particularly oxygen, adsorb on
the tube and act as weak p-type dopants.
Experiments have shown that changing a tubes
chemical environment can change this doping
levelshifting the voltage at which the device
turns on by a significant amount. This has
spurred interest in nanotubes as chemical
sensors. Adsorbate doping can be a problem for
reproducible device behavior, however.
Controlled chemical doping of tubes, both p- and
n-type, has been accomplished in a number of
ways. N-type doping was first done using alkali
metals that donate electrons to the tube. This
has been used to create n-type transistors, p-n
junctions, and p-n-p devices. Alkali metals are
not air-stable, however, so other techniques are
under development, such as using polymers for
charge-transfer doping
Scattering sites in nanotubes
IV characteristics at different Vgs for a p-type
SWNT FET utilizing an electrolyte gate in order
to improve gate efficiency.
Implying a mean-free path of approx. 700 nm.
Maximum transconductance dI/dVg20uA/V at
Vg-0.9V. Normalizing this to the device width of
2nm 10mS/um.
McEuen et al., IEEE Trans. Nanotechn., 1, 78
(2002)
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Bottom - gated CNT FET
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Calculated conductance vs gate voltage at room
temperature, varying (a) the work function of the
metal electrode, and (b) doping of the NT. In
(a) the work function of the metal electrode is
changed by -0.2 eV (red dashed), -0.1 eV (orange
dashed), 0 eV (green), 0.1 eV (light blue), and
0.2 eV (blue), from left to right, respectively.
In (b) the doping atomic fraction is n-type 0.001
(red), 0.0005 (orange), and 0.0001 (green), and
p-type 0.0001 (blue dashed), from left to right,
respectively.
Thus the gate field induces switching by
modulating the contact resistance (the junction
barriers). Oxygen adsorption at the junctions
modifies the barriers (i.e. the local
band-bending of the CNT) and affects the
injection of carriers (holes or electrons).
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The inverse subthreshold slope, which is a
measure of the efficiency of the gate field in
turning on the device, decreases with a decrease
in gate oxide thickness. This behavior cannot be
explained by conventional field-effect transistor
models, and has in fact been shown to be a result
of the presence of Schottky barriers at the
metal/nanotube interface at the source and drain.
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There is a clear difference in the inverse
subthreshold slope for the case of sweeping all
gate segments together (S400 mV/dec) versus
sweeping only the inner segments (S180 mV/dec).
We attribute the observed change in S to a change
from Schottky barrier modulation to bulk
switching. (b) shows linear plots of the
subthreshold portion (where the current
is dominated by carrier density) of the transfer
characteristics when the inner gate segments are
swept together or separately. The current nearly
identical, despite the fact that the effective
gate lengths differ by a factor of 1.6 . This is
in contrast to the expected behavior of diffusive
transport, where the current varies inversely
with the gate length.
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Calculated output characteristics of the
symmetric (dashed lines) and the asymmetric
(solid lines) CNFET.
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We have introduced nanotemplate to control
selective growth, length and diameter of CNT.
Ohmic contact of the CNT/metal interface was
formed by rapid thermal annealing (RTA). Diameter
control and surface modification of CNT open the
possibility to energy band gap modulation.
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Diode-like rectifying behavior was observed in a
CNx /C multiwalled nanotube due to its being one
half doped with nitrogen. FETs based on an
individual CNx /C nanotube were fabricated by
focused ion-beam technology. The nanotube
transistors exhibited n-type semiconductor
characteristics, and the conductance of nanotube
FETs can be modulated more than four orders of
magnitude at room temperature. The electron
mobility of a CNx /C NT FET estimated from its
transconductance was as high as 3840 cm2/Vs. The
n-type gate modulation could be explained as due
the effect of bending of the valence band in the
Schottky-barrier junction.
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CNTs doped with fullerenes inside nanotubes
(so-called peapods) are interesting materials for
novel CNT FET channels. Transport properties of
various peapods such as C60-, Gd_at_C82-, and
Ti2_at_C92-peapods have been studied by measuring
FET I-V characteristics. Metallofulleren peapod
FETs exhibited ambipolar behavior both p- and
n-type characteristics by changing the gate
voltage, whereas C60-peapod FETs showed unipolar
p-type characteristics similar to the FETs of
intact single-walled nanotubes. This difference
can be explained in terms of a bandgap narrowing
of the single-walled nanotube due to the
incorporation of metallofullerenes. The bandgap
narrowing was large in the peapods of
metallofullerene, where more electrons are
transferred from encapsulated metal atoms to the
fullerene cages.
The entrapped fullerene molecules are capable of
modifying the electronic structure of the host
tube. It is, therefore, anticipated that the
encapsulation of fullerene molecules can play a
role in band gap engineering in nanotubes and
hence that peapods may generate conceptually
novel molecular devices.
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Schematic illustration of elastic strain
distributed around the site of metallofullerenes
in a small-diameter nanotube peapod and the
corresponding changes in conduction and valence
band edges.
Charge transport in a partially filled peapod FET
in metal-on-top setup. (a) Transfer
characteristics at various temperatures. Data
were taken at Vds 0.3 V.
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CNT junction
Current vs. voltage characteristics of an
all-carbon transistor with semiconducting
nanotube as channel, with different voltages at
the carbon gate. The back gate is kept at 0 V.
The measurements were carried out at 4 K.
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Ambipolar conduction leads to a large leakage
current that exponentially increases with the
power supply voltage, especially when the tube
diameter is large. An asymmetric gate oxide SB
CNTFET has been proposed as a means of
suppressing ambipolar conduction. SB CNTFETs of
any type, however, will likely suffer from the
need to place the gate electrode close to the
source (which increases parasitic capacitance)
and metal-induced gap states, which increase
source to drain tunneling and limit the minimum
channel length.
The band profile of the SB CNTFET at the minimal
leakage bias (VG0V) for VD0.6V. The band
profile of the MOS CNTFET when the source-drain
current is low. (VD0.6V and VG-0.3V). The
channel is a (13,0) nanotube.
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Id vs. Vd characteristics at VG 0.4V for the
MOS CNTFET (the solid line) and the SB CNTFETs
(the dashed lines). The off-current of all
transistors (defined at Vd0.4V and Vg0) was set
at 0.01µA by adjusting the flat band voltage for
each transistor. For the SB CNTFETs, three
barrier heights we simulated. The channel is a
(13,0) nanotube, which results in a diameter of
d 1 nm, and a bandgap of Eg 0.83 eV.
Id vs. Vg characteristics at Vd 0.4V for the
zero barrier SBFET and the MOS CNTFET. The gated
channel of both transistors is a 5nm-long,
intrinsic (13, 0) CNT.
By eliminating the Schottky barrier between the
source and channel, the transistor will be
capable of delivering more on-current. The
leakage current of such devices will be
controlled by the full bandgap of CNTs (instead
of half of the bandgap for SB CNTFETs) and
band-to-band tunneling. These factors will depend
on the diameter of nanotubes and the power supply
voltage.