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Nanotube electronics
and optoelectronics
Among the many materials that have been proposed to supplement and,
in the long run, possibly succeed Si as a basis for nanoelectronics,
carbon nanotubes (CNTs) have attracted the most attention. CNTs are
quasi-one-dimensional materials with unique properties ideally suited for
electronics. We briefly discuss the electrical and optical properties of
CNTs and how they can be employed in electronics and optoelectronics.
We focus on single CNT transistors, their fabrication, assembly, doping,
electrical characteristics, and integration. We also address the possible
use of CNTs in optoelectronic devices such as electroluminescent light
emitters and photodetectors.
Phaedon Avouris* and Jia Chen
IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598, USA
*E-mail: [email protected]
In the last few decades, we have witnessed exponential advances
associated leakage currents and passive power dissipation, short
in Si microelectronics. These advances in computing,
channel effects, variations in device structure and doping, etc.
communications, and automation are affecting just about every
46
The realization of the approaching limits of scaling has inspired a
aspect of our lives and are responsible for bringing in the
worldwide effort to develop alternative device technologies. Some
‘information age’. To a large extent, these advances have been the
involve radical departures from the existing technology and are
result of the continuous scaling, i.e. miniaturization, of electronic
considered for the long term. A shorter term approach discussed here
devices, particularly of the metal-oxide-semiconductor field-effect
maintains the field-effect transistor (FET) principle, as well as the
transistor (MOSFET), that has led to denser circuitry and faster
general current circuit architecture, but replaces the Si channel of the
switching1,2. For example, since 1970 the physical length of the
FET by a one-dimensional nanostructure with superior electrical
gate and thickness of the gate insulator of MOSFETs have been
transport properties. In addition to the efforts to develop new
scaled by factors of about 500 and 120, respectively. However, the
electronic devices, direct bandgap one-dimensional nanostructures are
scaling cannot continue forever; a number of fundamental
attracting attention because of the desire to base both electronic and
scientific as well as technological limitations place lower limits to
optoelectronic technologies on the same material. Among the different
the size of Si devices. These involve, among others, electron
one-dimensional materials, single-walled CNTs3,4 have many highly
tunneling through short channels and thin insulator films and the
desirable and distinctive device properties5-20.
OCTOBER 2006 | VOLUME 9 | NUMBER 10
ISSN:1369 7021 © Elsevier Ltd 2006
Nanotube electronics and optoelectronics
We briefly examine the unique electronic and optoelectronic
properties of CNTs. We discuss the structure and fabrication of single
CNT-FETs, their properties and electrical characteristics, doping and
device integration. We also describe simple CNT optoelectronic devices
such as electroluminescent light emitters and photodetectors.
Electronic structure of carbon nanotubes
Single-walled CNTs are extremely strongly bonded, hollow carbon
atomic structures with diameters typically in the range of 1-3 nm.
Their electronic structure is usually described in terms of the electronic
structure of a folded ‘graphene’ sheet (a layer of graphite)5-7. The CNT
circumference is then expressed by a chirality vector C connecting two
crystallographically equivalent sites of the two-dimensional graphene
sheet (Fig. 1): C = na1 + ma2, where a1 and a2 are the unit vectors of
the hexagonal honeycomb lattice. The structure of any CNT can
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an (n,m) CNT is metallic when n = m; it has a small gap when
n – m = 3i, where i is an integer; while CNTs with n – m ≠ 3i are truly
semiconducting5-7.
Semiconducting CNTs have a diameter-dependent bandgap Eg.
A single-particle, tight-binding description of the electronic structure
gives Eg = γ (2a/dCNT), where Eg is the bandgap energy, γ is the hopping
matrix element, a is the C-C bond distance, and dCNT is the diameter of
the nanotube. Inclusion of electron-electron interactions raises the size
of Eg significantly, but its 1/dCNT dependence appears to remain valid.
While the electrical and optical bandgaps of semiconducting CNTs
were initially considered to be identical, based on the single particle
model, we now know that the optical gap is smaller because of the
attractive electron-hole (e-h) interaction and that the optical
excitations of CNTs involve transitions to exciton states, not interband
transitions (Fig. 2)16-20.
therefore be described by a pair of integers (n,m) that define its chiral
Transport properties
vector.
The interesting electrical properties of CNTs are largely the result of
In general, scattering of carriers by defects or phonons determines the
the unusual electronic structure of graphene itself. In going from
electrical transport properties of materials and devices. Because of the
graphene to a CNT by folding, one has to account for the additional
lack of boundaries in the perfect, hollow cylinder structure of CNTs,
quantization arising from electron confinement around the CNT
there is no boundary scattering, which plagues other nanostructures
circumference. This circumferential component of the wave vector kC
such as thin body Si MOSFETs, nanowires, or graphene slices.
can take only values fulfilling the condition kC.C = 2n, where n is an
Furthermore, CNTs are quasi-one-dimensional materials in which only
integer5-7.
forward and backward scattering is allowed (low-angle scattering is
As a result, each graphene band splits into a number of one-
dimensional subbands labeled by n. These allowed energy states are
suppressed). Yet the fact that they are not truly one-dimensional
cuts of the graphene band structure. When these cuts pass through a
materials, that the cylinder has a finite size, allows the carriers in them
K point (Fermi point) of the graphene Brillouin zone, the tube is
metallic. Otherwise the tube is semiconducting. It can be shown that
to ‘bypass’ certain defects. Because of the large momentum change
involved in backscattering, only scatterers with a strong, short-ranged
Fig. 1 Representation of the CNT atomic structure through the folding of a graphene strip. The chirality vector C and the one-dimensional translational vector P of a
(5,2) CNT are shown as an example.
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Nanotube electronics and optoelectronics
Fig. 2 Computed carrier (electron, hole) energy dependence of the electron-phonon scattering rate of a (25,0) CNT at 300 K and 10 K. The phonon modes
corresponding to the peaks in the scattering rate are identified.
potential are effective. Thus, elastic scattering mean free paths in CNTs
are long, typically of the order of micrometers21-24.
At low energies (low applied voltages), the carriers in CNTs can
interact with the acoustic phonons of the lattice, but the small
electron-acoustic phonon (e-ph) coupling leads to long, temperaturedependent mean free paths of the order of a micrometer25. The radial
breathing acoustic mode, however, has a stronger e-ph coupling and
when resonantly excited does affect transport26,27. In long (many
micrometer) CNTs, acoustic phonon scattering leads to diffusive
transport, but the mobility can still be extremely high – of the order of
100 000 cm2/Vs28,29. At high bias, however, when the energy of the
carriers reaches those of the zone-boundary (~160 meV) and optical
phonons (~180 meV), strong inelastic scattering takes place and the
mean free path is reduced to about 10-20 nm27,30-33. Fig. 2 shows
theoretical results for the phonon scattering rate as a function of the
carrier energy27.
Carbon nanotube field-effect transistors
The first CNT transistors were fabricated back in 199834,35. In the early
efforts, an individual CNT was placed on top of two metal electrodes
on a thick (100-200 nm) SiO2 film and the heavily doped Si substrate
itself was used as a back gate. While functional, these devices had low
performance, primarily because of high series contact resistances
(≥1 MΩ) resulting from the weak van der Waals bonding between the
CNT channel and the source/drain metal electrodes. A more intimate
metal-CNT contact was achieved by depositing the metal electrodes on
top of the CNT and subsequently annealing the contacts36. This
approach increased the drive currents and transconductance by several
orders of magnitude and produced switching Ion/Ioff ratios of 106. Fig. 3
shows the output characteristics of such a back-gated CNT-FET37.
48
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CNT-FETs fabricated using thick gate dielectrics (e.g. 100 nm SiO2)
and medium-to-high work function metals (e.g. Ti and Pd) are mostly
p-type in air. Annealing such a p-type transistor in vacuum can convert
the device to ambipolar and even further to n-type38,39. This was one
of the first experimental indications that Schottky barriers (SBs) at the
metal-CNT interfaces dominate the transistor switching.
Unlike conventional bulk-switched Si devices, whose source and
drain contacts are engineered to be ohmic1, switching in CNT-FETs
may arise in the bulk or at the contacts, depending on the CNT
diameter, nature and geometry of metal electrodes, gate dielectrics,
and device geometry. In general, charge transfer at the metal-CNT
interface leads to the generation of a SB. Experimental and theoretical
work has shown that SBs are much thinner in one dimension than
those in three dimensions, and carrier tunneling through the SBs
becomes important39-42. Adsorbed species on the electrodes change
the surface dipole, i.e. the local work function, thereby modifying the
energy level line-up at the metal-CNT contacts38,40. Other types of
experiments, such as the observation of large potential drops at the
source and drain contacts by scanning gate microscopy10,43, have
provided further evidence for the importance of SBs.
In a CNT-FET, Fermi-level pinning at the metal-nanotube interface
is weak40. To first order, the magnitudes of SBs at metal-CNT
interfaces are determined by the band gap of the CNT material (~1/d)
and the local work function of the contacting metal44. The larger the
CNT diameter, the smaller the SB will be. Because of the involvement
of tunneling and thermionic emission in the injection of carriers at the
contacts, the dependence of the on-current of the transistor on CNT
diameter (i.e. SB) is very strong (exponential), as shown in Fig. 444.
It is this strong dependence that necessitates the precise control of
the CNT diameter in electronics. So far, there have been no direct
Nanotube electronics and optoelectronics
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Fig. 3 Output characteristics (Ids against Vds) of a 600 nm long, 1.8 nm diameter, back-gated CNT-FET. Inset: schematic of the structure of the CNT-FET. (Adapted
from37 and reprinted with permission. © 2005 IEEE.)
experimental correlations between CNT-FET function and CNT
current for Pt CNT-FETs is significantly larger than that of Pd CNT-
chirality. However, for CNTs with diameters within the desirable range
FETs45.
for electronic applications, i.e. d ≥ 1.8 nm, Fig. 4 suggests that chirality
does not to play a significant role.
For hole transport, the higher the metal work function, the smaller
The width of the SB depends critically on the electrostatic
environment (screening). By reducing the gate oxide thickness and/or
increasing its dielectric constant k, the screening length (the distance
the SB (see Fig. 4). Thus, a small diameter CNT contacted with low
over which the metal-CNT contact fields penetrate into the CNT
work function electrodes, e.g. Al, has large SBs for p-type transport. On
channel) decreases, reducing the SB width and increasing its
the other hand, a large diameter CNT (d > 2 nm) contacted by certain
transparency41. Indeed, thin, high-k materials (HfO2, ZrO2) have been
high work function electrodes, e.g. Pd, can produce nearly ohmic
used as gate dielectrics to improve device performance42,46.
contacts in p-type transistors45.
In addition to the work function of the metal, other factors,
Unlike the surface of Si, the bonds on a CNT surface are satisfied
and there are no dangling bonds that could form interface traps. A
such as the adhesion (wetting) of the CNT by the metal, play
very efficient coupling of the gate to the channel can be accomplished
important roles in the transport properties of CNT-metal contacts. For
by using an electrolyte solution as the gate47. The high dielectric
example, the work function of Pt is similar to that of Pd, but the on-
constant of the electrolyte (~80) and its ultrathin (~0.5 nm) Helmholtz
Fig. 4 Experimental CNT-FET on-current (left axis) and computed SB height (right axis) as a function of the CNT diameter for three types of CNT-FETs involving Pd,
Ti, and Al source and drain contacts. (Adapted from44 and reprinted with permission. © 2005 American Chemical Society.)
OCTOBER 2006 | VOLUME 9 | NUMBER 10
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Nanotube electronics and optoelectronics
layer lead to a very high transconductance, but also to a slow switching
a reasonable Ion/Ioff ratio (≥104 is typically desirable in logic
rate.
applications). However, the physics that leads to efficient gate
SBs in a CNT-FET impact the characteristics of both the on and off
states of the transistor. When the gate voltage Vgs is much higher than
the threshold voltage Vth, the CNT-FET is turned on and its drive
switching (e.g. thinner gate insulators) and high on-currents in small
bandgap CNTs also tends to increase the off-current.
In a SB-CNT-FET, the injection of carriers is governed by the
current increases with increasing CNT diameter: log10Id ∝ -Eg ∝ -1/dCNT
transparencies of the contacts (dependent on the local electrical fields).
(Fig. 4)44. In addition to the bandgap dependence on dCNT, electron and
Thus, the injection of holes is controlled by the gate field and electrons
hole effective masses m*, decrease with increasing dCNT. Tunneling
by the drain field. When one of the SBs is much smaller than the other
through the SBs will be more efficient as a result of the lower m*,
(e.g. the source contact), transport through the device is dominated by
leading to higher drive currents. This is consistent with reports of
the carriers (holes) injected from that contact. Transport under this
higher on-currents for CNT-FETs based on large diameter (2-4 nm)
condition involves only one type of carrier: it is unipolar. When the
CNTs, usually produced by chemical vapor deposition
techniques45,
gate and drain fields become comparable (the two SBs are
rather than smaller diameter (0.8-1.5 nm) CNTs, produced by high-
comparable), it is then possible to inject both electrons and holes
pressure CO conversion (HipCO) or laser ablation techniques36.
through the two opposite contacts of the transistor. Transport in this
When Vgs < Vth, the transistor is in its off state. An important
parameter in this regime is the inverse subthreshold slope S that
regime involves two types of carriers: it is ambipolar. CNT-FETs with
thin gate dielectrics and/or small bandgap CNTs under high drain bias
measures the how efficiently the gate controls conduction though the
tend to become ambipolar. I-V characteristics of such an ambipolar
channel. It is defined as S = (dlog10Id /dVgs)-1. In a transistor with
device are shown in Fig. 548. The drain current is a leakage current that
ohmic source and drain contacts (as in a conventional Si MOSFET), S is
increases exponentially with increasing Vds , thus reducing the Ion/Ioff
limited by thermionic emission over the channel and is ~kBT/q. Thus, at
ratio49. This problem can be solved by electrostatically controlling the
room temperature, its limiting value is 60 mV/dec. In a transistor with
source and drain contacts separately with multiple gates or introducing
SBs dominating the transport, S is generally higher and depends on the
charge-transfer doping to obtain different injection rates for electrons
electrostatics of the device, including the thickness and dielectric
and holes48,50.
constant of the gate dielectric, geometry of the electrodes, and trapped
Most experiments on CNT-FETs have been on back-gated devices
charge around the contacts. The early back-gated, bottom-contact
because of the simplicity of their fabrication. To build integrated
CNT-FETs with thick gate oxides had S as high as 1 V/dec. Currently,
circuits, however, it is vital to be able to gate each CNT-FET
devices with thin oxides (<10 nm) have S in the range of
independently. Wind et al.51 fabricated the first independently top-
100-150 mV/dec. Theoretical modeling has also predicted
gated, thin gate oxide, high-performance CNT-FETs. These CNT-FETs
improvements in transistor switching by using thinner and higher
provide the first evidence that CNT-FETs could compete with state-of-
dielectric constant gate insulators and smaller contacts40-42.
the-art Si MOSFETs. Later, Lin et al.48 demonstrated a double-gate
It is not only the on-current of the transistor that is important, the
CNT-FET structure that allows the independent control of the
current in the off state is equally important. It is critical to maintain a
switching of the SBs and of the CNT channel, and successfully converts
low leakage current to keep both the passive power at a minimum and
ambipolar CNT-FETs to unipolar ones. These double-gate CNT-FETs
Fig. 5 Transfer characteristics of a large diameter CNT-FET (~1.8 nm) at different drain voltages.
50
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Nanotube electronics and optoelectronics
REVIEW FEATURE
Fig. 6 (a) Device transfer characteristics of a CNT-FET before and after triethyloxonium hexachloroantimonate (C2H5)3O+SbCl6- (OA) doping at Vds = -0.5 V.
(b) Transfer characteristics of a CNT-FET before and after n-doping with hydrazine at Vds = 0.5 V. (Part (b) adapted from59 and reprinted with permission. © 2005
American Chemical Society.)
produces S values very close to the thermal limit48. Subsequently, by
More recently, a more complex and sophisticated circuit, a five-
using the same CNT-FETs and filtering the electron energy distribution
stage ring-oscillator (RO), was built on a single, long CNT63. ROs are
through band-to-band tunneling, S values below the thermal limit can
essential for the characterization of the ac properties of CNT-FETs. In
be achieved (~40 meV/dec at 300
K)52.
CNT-FETs with very long
this RO circuit, five pairs of p- and n-type CNT-FETs in CMOS
channels (many micrometers) operating in the diffusive regime are
configurations are wired along the length of an ambipolar CNT. Pd
dominated by the bulk barrier and are, therefore, bulk-switched like
metal is used for all source and drain electrodes. Since the threshold
MOSFETs29.
voltages for the p- and n-branches of the ambipolar CNT are quite
different, two different metals are used to construct the top gates: Pd
Doping of nanotubes
for the p-type and Al for the n-type transistors. The work function
For complementary metal-oxide-semiconductor (CMOS)-type
difference of the two metals effectively shifts the FET thresholds and
applications, one would like to have both p- and n-type CNT-FETs.
allows the formation of the CMOS structure. This novel approach
However, semiconducting CNTs cannot be doped using the traditional
eliminates the need for doping altogether. As gate insulator, a high-k
approach for bulk semiconductors of ion implantation. Furthermore,
material, AlOx , was used so as to improve the coupling of the CNT and
especially in small-diameter CNTs, carbon substitution by a dopant
the gate. In addition to the five-stages, two more inverters are added,
such as B or N induces high strain and leads to a defective CNT
one at the beginning of the chain to determine the ideal operating
lattice53,54.
conditions and one at the end to avoid interference with the
For this reason, charge-transfer doping has been employed.
Charge-transfer dopants are used primarily for two reasons in CNTs:
measurement setup (spectrum analyzer). The complete circuit is shown
(a) to convert ambipolar devices to p-type50, and (b) to convert p-type
in Fig. 7.
devices to n-type by, for example, depositing electron donors on them
such as K
atoms39,55
or amine-containing
molecules56-59.
Figs. 6a and
6b show examples of both p- and n-type CNT doping. Apart from
With this first circuit, an oscillation frequency of up to ~70 MHz
can be achieved (at VDD = 1 V), which corresponds to a delay of
1.4 ns per stage. Although this frequency is much higher (~105 times)
chemical doping, electrostatic doping through multiple gates has been
than those achieved by oscillators based on different CNTs61,62, it is
used successfully to suppress SBs and produce bulk-switched p-i-p and
still relatively low. However, this frequency does not represent an
n-i-n structures48.
inherent limitation of the CNTs, which is expected to be in the
terahertz range64, but is determined by the parasitics of the circuit that
Integrated nanotube electronic circuits
can be eliminated by improvements in fabrication.
Following the optimization of individual transistors, the obvious next
step is to attempt to integrate them into logic circuits. The first CNT
Self-assembly of nanotube devices
logic gate, a NOT gate, was demonstrated by Derycke et al.60. In this
In addition to their superior electrical properties, CNTs may offer the
case, a single CNT was patterned into a p-type and an n-type (through
possibility of simpler, less expensive device fabrication. As a molecular
K-doping) FET pair to form a voltage inverter. This work was followed
entity, CNTs should be able to self-assemble into desirable structures.
by demonstrations of a number of different logic gates, usually built
For the most part, two different strategies have been pursued:
with FETs involving separate CNTs61,62.
(a) prepatterning of the substrate with chemical species to which CNTs
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Nanotube electronics and optoelectronics
(a)
(b)
Fig. 7 Scanning electron microscope image of a five-stage CNT ring oscillator circuit. A single, long CNT molecule is patterned to form the oscillator (see upper right
insert). p- and n-type FETs are formed by using different gate metals, Pd and Al, respectively. A 10 nm thick AlOx film is used as the high-k gate insulator (bottom
left insert). (Adapted from63 and reprinted with permission. © 2006 American Association for the Advancement of Science.)
tend to adhere65,66 or be repulsed by; and (b) functionalizing the CNTs
released as heat (phonons), but a fraction of the recombination events
themselves with groups that selectively adsorb to a particular structure
may involve the emission of a photon. This process is termed
on the
substrate67,68.
Assembly of CNTs on insulators, particularly
high-k materials such as the basic oxides HfO2 and Al2O3, is of
electroluminescence and is widely used to produce solid-state light
sources such as light-emitting diodes (LEDs).
particular interest in electronics. Acidic groups, such as phosphonic
In order to fabricate LEDs or any other electroluminescent device,
(-PO2OH) or hydroxamic (-NHOH) groups, attach rather strongly to
one must recombine significant populations of electrons and holes.
these oxides66. Thus, selective adsorption can be achieved by
Conventionally, this is achieved at an interface between a hole-doped
patterning such an oxide surface, for example by imprint lithography,
and an electron-doped material (e.g. a p-n junction). In ambipolar CNT-
with molecules of the type L-R-PO2OH, where L is a group with affinity
FETs at an appropriate bias, however, electrons and holes can be
for CNTs, e.g. a -NH2 group, or a nonadhering group, e.g. -CH3.
simultaneously injected at the opposite ends of the CNT channel. This
A typical example of CNT assembly through functionalization is
provided by DNA
wrapping69-72.
Functionalizing the CNTs themselves
could provide even better control of their placement. However, the
functionalization must be reversible so that it does not degrade the
allows electroluminescence to occur73. While the emission mechanism
is the same as that in p-n junctions, ambipolar CNT-FETs do not
require chemical doping.
CNT electroluminescence exhibits a variety of interesting properties.
excellent intrinsic electrical properties of the CNT in the final device.
The emitted light is strongly polarized along the tube axis. The
Covalent functionalization tends to destroy these properties by
radiation also has a characteristic energy that depends on the diameter
converting the sp2 hybridization of the C atoms to sp3. Recently, an
and chirality of the excited CNT, just as the optical bandgap does, and
approach was proposed that bypasses this problem. The CNTs are
the length of the electroluminescent region is lrec ≤ 1 µm74. In short
reacted with derivatized diazonium salts of the type
X - N2+-R -L ,
to give
devices (L < lrec , where L is the channel length), the light emission
covalently bonded derivatives of the type CNT-R-L, which as discussed
encompasses the entire CNT. In long devices (L >> lrec), on the other
above, adhere to the appropriate oxide structures through the group L.
hand, the emission will be localized wherever the concentrations of
In this case, however, the CNT-R bond can be cleaved cleanly after
electrons and holes overlap most strongly. This overlap region can be
deposition by thermal annealing, and the electrical properties of the
physically moved using a gate electrode, since the relative
pristine CNT can be recovered68.
contributions of electrons and holes to the total current is strongly
gate dependent75. Therefore, a CNT-LED is a movable light source. The
52
Optoelectronic devices
gate bias Vg can smoothly and continuously position the site of
Electron and hole carriers in semiconductors can recombine by a
emission75. In Fig. 8, we demonstrate the translation of the emission
variety of different mechanisms. In most cases, the energy will be
spot between two electrodes by applying different gate voltages.
OCTOBER 2006 | VOLUME 9 | NUMBER 10
Nanotube electronics and optoelectronics
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Fig. 8 Movement of the emission spot between source and drain electrodes produced by e-h recombination in an ambipolar CNT-FET by varying the applied gate
voltage. (Adapted from75 and reprinted with permission. © 2004 American Physical Society.)
(a)
(b)
(c)
Fig. 9 (a) Schematic of the structure used to generate localized light emission under unipolar transport conditions in a CNT. A trench is etched in the SiO2 along the
channel of a back-gated CNT-FET. (b) Optical microscope image of the trench and infrared emission near its edge. (c) Variation of the emission intensity with gate
voltage. (Parts (b) and (c) adapted from77 and reprinted with permission. © 2005 American Association for the Advancement of Science.)
In addition to this translatable emission, localized
excitation of a CNT generates an electric current, which can be used as
electroluminescence is also observed from particular spots on a CNT
a nanosized photodetector79 or as a spectroscopic tool80. Alternatively,
under unipolar transport conditions75,76. In this case, the current is
in the open-circuit configuration, the device generates a
carried by only one type of carrier (electrons or holes). Since both types
photovoltage79. Thus, a CNT-FET device can be used as a transistor,
of carriers are necessary to produce light, these sites must actively
generate e-h pairs. This process occurs near defects, trapped charges in
the insulator, or any other inhomogeneities that produce voltage drops
along the CNT and generate large, local electric fields. The resulting
‘hot’ carriers produce e-h pairs through a most efficient intra-nanotube
impact excitation process (Fig. 9a)77,78. The efficiency of this process
can be traced in the quasi-one-dimensional confinement and the weak
screening of the Coulombic interactions77,78. The monitoring of
localized electroluminescence provides a new tool for detecting defects
in CNT devices. Artificial structures can also be fabricated that create
the conditions locally, i.e. sudden change in the potential, that generate
e-h pairs and light emission77. Such a device is shown in Fig. 9b. Unlike
the ambipolar device emission, the light intensity of the unipolar
devices depends exponentially on the current, a fact that supports the
impact mechanism (Fig. 9c)77.
Photoconductivity is the reverse of electroluminescence, with
optical radiation producing electron and hole carriers. An example of a
photoconductivity measurement is shown in Fig. 10. The resonant
Fig. 10 Photoconduction current against light energy in the region of the
second allowed exciton state, E22, of a CNT. Inset: polarization dependence of
the photocurrent.
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Nanotube electronics and optoelectronics
light emitter, or light detector. Choosing among these different modes
technology will be replaced by CNTs or, for that matter, by any other
of operation only requires the bias conditions to be changed.
type of technology anytime soon. The speed at which CNT technology
evolves will depend strongly on breakthroughs, such as the
Conclusions
development of novel integration processes. CNTs also provide the
It is clear that the unique properties of CNTs make them excellent
opportunity for a wealth of new types of applications, e.g. in
candidates for nanoelectronics and photonics, and the devices already
bioelectronics, and most importantly for new, low-cost methods of
demonstrated prove this point. However, as is true with any new
fabrication. Guided self-assembly techniques could be used to fabricate
technology, there are numerous technical and other types of problems
circuits at a low level of integration, but with very high performance
that need to be resolved before a competitive CNT-based technology
characteristics, for example in telecommunications, without the
can be developed. It is very unlikely that highly developed Si
multibillion dollar costs of current facilities.
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OCTOBER 2006 | VOLUME 9 | NUMBER 10