1

Abstract

With the introduction of nanotechnology and the discovery of carbon nanotubes, new

promises in the field of semiconductors have been made. Traditional silicon is being replaced

by carbon nanotubes and transistors are being replaced by Field Effect Transistors (FETs) but

a number of hurdles are faced from the synthesis of CNTs to the fabrication of nano FETs.

Thus paper focuses on discussing about synthesis of CNTs, their properties, CNT structure,

their uses in semiconductor devices, fabrication on FETs and various advantages/promises

made over conventional semiconducting devises based upon various published research

papers.

2

Introduction

The structure of the nanotubes (Figure 1) can be described in the following way: take

the graphite plane, cut a strip out of it and "glue" it into a cylinder (in fact, of course, the

nanotubes grow quite differently). But before the experimental discovery of nanotubes,

theoreticians and scientists could not predict the structure of nanotubes. They could only

study them and wonder until methods of studying them had evolved would seem that it may

be easier - you take the graphite plane and you turn into a cylinder [Schmidt M.W., and all,

1993].

Fig. 1. Schematic of a Multi-Walled Carbon Nanotube.

But what was surprising was that all these amazing nanotubes of 100 thousand times

thinner than a human hair proved to be extremely durable material. The nanotubes are 50-100

times stronger than steel and are up to six times less dense. Young's modulus - the level of the

material strain resistance - of nanotubes is twice higher than that of conventional carbon

fibers. That is, the tube is not only durable, but also flexible. It resembles behavior unlike

brittle straw, but rather that of a hard rubber tube. Under the influence of mechanical stresses

above the critical, the nanotubes behave quite extravagantly; they are neither get "torn", nor

"brake" but simply get rearranged [Poole C. P.. Owens F.J., 2003].

Currently, the maximum length of the nanotubes is in tens or hundreds of microns -

which of course is very large on the atomic scale, but not enough for everyday use. However,

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the length of the resulting nanotubes has gradually increased and now scientists have come

close to the centimeter boundary. Multi-walled nanotubes have obtained a length of 4 mm

[Schmidt M.W., and all, 1993].

Nanotubes come in different forms: single-layer or multi-layer, straight or spiral.

Furthermore, they exhibit a range of unexpected electrical, magnetic and optical properties.

4

Producing Carbon Nanotubes

Currently, the most common method is thermal spraying of graphite electrodes in an

arc discharge plasma. The synthesis is performed in a chamber filled with helium at a

pressure of about 500 torr. When combustion is intense plasma thermal evaporation anode,

wherein the cathode surface at the end, a precipitate forms in which the carbon nanotubes are

formed. Most nanotubes form when the plasma current is minimal, and its density is about

100 A/cm2. The experimental setup voltage between the electrodes typically is about 15-25

V, the discharge current several tens of amperes, the distance between the ends of the

graphite electrodes 1-2 mm. During the synthesis of about 90% by weight of the anode is

deposited on the cathode [Poole C. P.. Owens F.J., 2003].

The resultant nanotubes have a length multiple of about 40 microns. It builds up on

the cathode is perpendicular to the flat surface of its end face and arranged in tufts cylindrical

diameter of about 50 microns. Nanotube bundles regularly cover the surface of the cathode to

form a honeycomb structure. It can be found by considering the precipitate on the cathode

with the naked eye. It is the space between the bundles of nanotubes filled with a mixture of

disordered nanoparticles and single nanotubes. The content of the carbon nanotubes in the

sediment (deposit) may be close to 60% [P. NI. Ajayan, L. S. Schadler. P. V. Braun., 2003].

To separate the components of the precipitate obtained using ultrasonic dispersion, the

cathode deposit is placed in methanol and solicited. The result is a suspension, which (after

addition of water) is subjected to separation in a centrifuge [Stewart J.J.P, 1989]. Larger soot

particles adhere to the walls of the centrifuge, and the nanotubes remain floating in

suspension. Then, nanotubes in nitric acid are washed and dried in a stream of gaseous

oxygen and hydrogen in a ratio of 1:4 at a temperature of 750 °C for 5 min. This treatment

turns out quite easily resulting in a porous material consisting of a multilayer nanotubes with

an average diameter of 20 nm and a length of about 10 microns. Technology for production

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of nanotubes is quite complicated, so as of now the nanotubes are an expensive material (one

gram costs a few hundred dollars).

Fig. 2. Micrographs of nanotubes produced by different methods.

6

Carbon Nanotube Structure

An ideal nanotube - is the cylinder which results in rolling a planar hexagonal mesh

graphite without sutures. The relative orientation of the hexagonal grid of graphite and the

longitudinal axis of the nanotube determines an important structural characteristic of the

nanotube, which is called chirality [Jacobsen, R. L., and all, 1995]. Chirality is characterized

by two integers (m,n), which indicate the location of the hexagon mesh, which is a result of

drilling to coincide with a hexagon, located at the origin [Schmidt M.W., and all, 1993].

Chirality nanotubes can also be uniquely determined angle a, formed by the folding

direction of the nanotube and the direction in which adjacent hexagons have a common side.

There are so many options clotting nanotubes, but among them are the ones as a result of

which there is no distortion of the structure of the hexagonal grid. These directions

correspond to the angles a = 0 and a = 30° that corresponds chirality (m,0) and (2n,n).

Chirality indices monolayer tube define its diameter D:

D = (1)

Where d0 = 0,142 nm - the distance between the carbon atoms in a hexagonal grid of

graphite.

The above expression allows us to determine the diameter of the nanotube chirality.

[Saito, R., and all, 1992].

Among the single-walled nanotubes are of particular interest to the nanotube chirality

(10, 10). Calculations showed that the nanotubes with such a structure should have metallic

conductivity, and have improved stability and resistance as compared to other tubes chirality.

The validity of these claims has been experimentally confirmed in 1996, when the first was

the synthesis of nanotubes with D = 1, 36 nm, which corresponds to the chirality of (10, 10).

7

Multilayer carbon nanotubes are different from wider variety of shapes and

configurations. Possible variations of the transverse structure of multi-walled nanotubes. The

structure shown in Figure 4, and was called the Russian nesting dolls. It is coaxially nested

single-walled nanotubes are cylindrical.

By increasing the number of layers in greater deviation from the ideal cylindrical

shape. In some cases, the outer shell takes the form of a polyhedron. Sometimes, the surface

layer structure is disordered arrangement of carbon atoms. In other cases, the ideal hexagonal

mesh outer layer nanotube defects in the form of pentagons and heptagons, leading to

disruption of cylindrical shape. The presence of a pentagon is convex, and Heptagon - a

concave curve of the cylindrical surface of the nanotube. Such defects lead to the appearance

of curved and spiral nanotubes, which are in the process of growth squirm, twist together,

forming loops and other complex shape elongated structure [Schmidt M.W., and all, 1993].

8

Properties of Carbon Nanotubes

1. Capillary effect

In order to observe the capillary effect, open the nanotube by taking the upper caps

off. There are various easy ways for doing it. One way of doing it is by annealing the

nanotube at 850°C for several hours in a stream of carbon dioxide. By the oxidation method,

about 10% of the nanotubes are opened. Another way of destroying the closed ends of the

nanotubes is by pouring CNTs in a solution of concentrated nitric acid for 4.5 hours at a

temperature of 240°C as a result of which 80% of the nanotubes are open. Figure 3 shows a

schematic of an open end CNT.

Fig. 3. The restructuring of the nanotube.

The first studies of capillary phenomena have shown that there is a relationship

between the magnitude of the surface tension of the liquid and the possibility of it being

drawn into the channel of the nanotube. It was found that the liquid penetrates the nanotube

channel, if the surface tension is not higher than 200 mN/m. Therefore, the input of any

9

substance inside nanotubes using solvents has a low surface tension [Jacobsen, R. L., and all,

1995]. For example, to enter into a channel of some metals used nanotubes concentrated

nitric acid, wherein the surface tension is small (43 mN/m). Then, annealing is carried out at

400 ° C for 4 hours under hydrogen atmosphere, which leads to a metal recovery. Thus,

obtained is nanotube containing nickel, cobalt and iron [Dillon, A. C. and all, 1995].

Along with metal carbon nanotubes may be filled with gaseous substances, such as

hydrogen in the molecular form. This ability is of great practical importance, since opens the

possibility of safe storage of hydrogen, which can be used as a clean fuel in internal

combustion engines [Hutchison K., and all, 1999].

2. Specific electrical resistance of carbon nanotubes

Because of the small size of carbon nanotubes in 1996 no one could directly measure their

electrical resistivity. Hence a specialized method was developed in which silicon oxide strips

were deposited on gold surface in vacuum. In the interval between them is sprayed nanotube

length of 2-3 microns. Then, one of the selected measuring nanotubes coated tungsten wire

four 80 nm thick. Each of the tungsten wire was in contact with one of the gold bars. The

distance between the contacts on the nanotube is from about 0.3 to 1 micron. The results of

direct measurements showed that the resistivity of nanotubes can vary significantly - from 5.1

- 6 to 0.8 ohm / cm. The minimum value of r is lower than that of graphite. Most of the

nanotubes have metallic conductivity and the smaller exhibits the properties of a

semiconductor with a band gap of 0.1 to 0.3 eV.

3. Emission properties of carbon nanotubes

The results of the study of field emission properties of the material, where the

nanotubes are oriented perpendicular to the substrate, have been very interesting for practical

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use. Attained values of the emission current density is of the order of 0.5 mA/mm2. The

obtained value is in good agreement with the Fowler-Nordheim:

I = (2)

Where C and k - constant, I - a work function, E* - the electric field in the places where the

electrons (for the test material is top of the nanotubes). Evaluation of E* can be obtained,

knowing that E * U/r, where U - the voltage between cathode and anode in volts, and r -

radius of curvature of the upper portion of the nanotube. Assuming that r = 6 - 10 cm and U =

500, we obtain E * = 5 '108 V/cm. This electric field is sufficient to extract electrons j = work

function of 5 eV. Thus, in this case, the field emission is provided by the configuration of the

surface from which electrons are extracted [Hutchison K., and all, 1999].

High current emission values were obtained when the single-walled nanotubes in the

0.8-1 nm twisted bundles 10-30 nm in diameter (during growth) and was applied onto a

silicon substrate. The anode used molybdenum rod spaced from the surface of the film at a

distance of 15 microns. Field emission was observed when the electric field E*, equals to 1.6

"104 V/cm. Estimates made by the Fowler-Nordheim equation showed that the work function

of the nanotubes in this case is equal to 1 eV. These data allow us to consider the carbon

nanotubes as the best material for field emission cathodes [Saito, R., and all, 1992].

11

The Use of Nanotubes in Electronics

Despite the fact that the nanotubes have low surface tension can be used in many

areas of modern technology, there are promising ways of using nanotubes associated with

developments in the various fields of modern electronics. Nanotubes inherent advantages

such as small size, changing a different range depending on the synthesis conditions,

electrical conductivity, mechanical strength and chemical stability. This makes it possible to

consider the nanotube basis for future nano electronics components. According to

calculations of the electronic properties, and the chirality of the ideal structure changes in the

implementation of a single-walled nanotube as a defect pair of pentagon - heptagon.

Semiconductor heterojunctions with different band gap can be obtained in the same way by

introducing a defect. Therefore, the nanotube is incorporated in the defect can be regarded as

a metal-semiconductor heterojunction. This heterojunction semiconductor device can realize

a very small size [Dillon, A. C. and all, 1995].

The creation of new types of miniature elements of electronic circuits based on

nanotubes - is not the only use of nanotubes in electronics. In addition, the nanotubes can be

used to create on their basis the finest measuring tool that is used to control the heterogeneity

of surfaces such schemes. In one of the studies in this area to study the surface at the

nanometer level, as a probe was used multi-walled nanotubes [Schmidt M.W., and all, 1993].

The nanotubes have a very high mechanical strength, so they are useful for this purpose. High

strength nano confirmed by the results of direct measurements, according to which the

Young's modulus of the nanotubes in the axial direction of the order of 7000 GPa. At the

same time, steel and iridium, which are commonly used for making such probes have this

value in the range 200 and 500 GPa, respectively.

12

Transistors of One Carbon Nanotube Operating at Gigahertz Frequencies

In addition to the usual properties nanomaterial’s often associated with quantum

effects, transistors based on carbon nanotubes exhibit new opportunities for rapid detection of

the charge. This is possible due to the unique combination of small-time response and high

sensitivity of the charge. Currently, the charge accumulation experiments performed on the

nanotube or semiconductor quantum dot, or use a single-electron transistor, a touch sensor or

a quantum dot, which run on the microsecond time scale of measurement. The high

sensitivity of the field-effect transistors based on carbon nanotubes has recently been

demonstrated by the observation of the process of tunneling between the nanotubes and is

located near the particle. Such effects can play a key role in the construction of the entire

quantum electronics. Theoretical calculations in the article show that the frequency of the

signal through such transistor can exceed 40 GHz.

Scientists from France created an electron-beam lithography in the form of strips

based on carbon nanotubes with two symmetrically arranged on the top gate oxide high-

resistance silicon substrate (resistance 3 - 5K). Nanotubes were prepared by conventional

CVD technique using a nanostructured catalyst. Palladium contacts were sprayed after the

multistep process of aluminum oxidation (the thickness of the oxidized layer 6 nm), and were

then precipitated from the gates of gold. The work found that large slope direct transfer of the

FET based on carbon nanotubes retained up to frequencies of 1.6 GHz. It also found that high

sensitivity remains almost unchanged and that the gate capacitance is related to its length up

to the size of ~ 300 nm.

13

СNT-FETs

Carbon nanotube field effect transistors (CNT-FETs) are very attractive as ultimate,

quantum limited devices. In particular, ballistic transistors have been predicted to operate in

the sub-THz range. Experimentally, a state-of-the-art cutoff frequency of 30 GHz has been

reached in a low impedance multi-nanotube device 3 whereas 8 GHz was achieved with

mitigating single nanotube transistor [Hutchison K., 1993].

Fig. 4. Panel A: scanning electron microscope image of a single carbon nanotube double-

gate transistor. The gate length is 300 nm. Panel B: small-signal-equivalent circuit of the

carbon nanotube transistor. Extrinsic components are displayed outside of the dashed

rectangle. Notations are explained in the text.

Indirect evidence of microwave operation was also obtained in experiments based on

mixing effects or channel conductance measurement in single nanotubes. The extraordinary

performances of nanotubes as molecular field effect transistors rely on a series of unique

properties. High-mobility “p-doped” single-walled nanotubes can be obtained by CVD

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growth, with a semiconducting gap of Δ ≈ 0.5-1eV (diameter 1-2 nm). Low Schottky barrier

contacts, with Pd metallization, and quasi-ballistic transport result in a channel resistance Rds

approaching the quantum limit, h/4e2) 6.5 kΩ for a four-mode single-walled nanotube. High

saturation currents, limited by optical phonon emission, allow large biases, Ids ≈ µA at Vds ≈ 1

V, in short nanotubes. The above numbers and the good gate coupling explain the large

transconductances, gm ≈ Ids/Δ >10 µS, observed in dc experiments. In the ac, an intrinsic

limitation is given by the transit frequency fT = gm/2πCg, where Cg is the gate-nanotube

capacitance. Here, Cg = CgeoCQ/(Cgeo + CQ) is the series combination of the quantum and

geometrical capacitances, CQ and Cgeo. An ultrathin oxide coating in CNT-FETs allows

approach to the quantum limit with a capacitance per unit gate length of lg, Cgeo/lg > CQ/lg =

4e2/hvf ≈ 400 aF/µm for vf ≈ 4 * 10

5 m/s, a typical value for semiconducting NTs. Beside

basic interest for quantum limited nano devices, single nanotube transistors offer new

opportunities for fast charge detection due to the unique combination of short time response

and high charge sensitivity [Harris P, 2003]. At present, charge counting experiments

performed in nanotube or semiconducting quantum dots use either single electron transistors

or quantum point contact detectors, which operate on microsecond time scales. The high

sensitivity of the NTFET has been demonstrated recently by monitoring tunneling events

between the nanotube and a nearby gold particle at the dc limit. Performing such experiments

with quantum dots at nanosecond time scales would allow one to extend charge counting in

the coherent regime relevant for full quantum electronics. Given the shot noise limitation of a

nanotube detector of bandwidth (1/τ) 1 ns-1

, overestimated by the Poissonian value sqr(Idsτ)/e

) 250 electrons for Ids ≈ 10 µA, single charge resolution requires a charge gain of gmτ/ Cg >

250 or, equivalently, a transit frequency of fT = gm/ 2πCg > 40 GHz. Figure 4 gives a

Scanning Electron Microscopy (SEM) image and a as well as a schematic of a CNT

transistor.

15

CNT is better than Silicon

Researchers at IBM's improved methods of making transistors out of carbon

nanotubes (carbon nanotube, CNT) to the extent that they are able to outperform its key

characteristics of the prototypes -the-art silicon transistors. CNT transistors are formed as

tubes molecule composed of carbon atoms. One such tube has a thickness of 1 nm (about

three atoms) is 50 thousand times thinner than a human hair.

Researchers at the Research Center IBM TJ Watson Research Center have achieved

an extremely high value of the parameter that determines the ability to conduct electricity.

This parameter CNT transistors have twice the (per unit sectional area) than those of the most

advanced existing silicon transistors. As expected, the carbon nanotubes to replace silicon as

the development of the chip reaches the barrier, after which the semiconductor elements of

silicon can no longer be reduced in size. This should happen in 10-15 years. The study

covered issues of creating channel transistors from carbon nanotubes with a structure similar

to the traditional channel MOS transistors. This allowed for a comparison CNT - silicon

transistors with their counterpart [Nanotechnology. Global Strategies, 2005].

New elements have excellent electrical characteristics, easily turned on and off and

operate stably at low voltage. Previous versions of CNT transistors required to enable and

disable the high voltage up to 20 V, while the voltage silicon transistors need only 1 V.

Previous embodiments CNT- transistors created on a silicon substrate, moreover, silicon used

as a gate. Such options have worked, but it meant that all the transistors are switched on and

off simultaneously [Nanotechnology. Global Strategies, 2005].

The new structure of the gate is located on top of the nanotube, as well as a thin

insulating layer between the silicon dioxide is used. However, according to Windows,

externally it looks like a silicon transistor, a flat plate but rather it uses a silicon thin

16

nanotube. The new CNT transistor can be switched on at a voltage of less than 1 due to the

use of the insulating layer and is able to work independently. New technology has yet to be

optimized to reduce the silicon dioxide layer and to reduce the length of the channels, so

Windows calculates that in the future, developers can achieve even better results [Ajayan

P.M., Ebbsen T.W., 1997].

However, CNT transistors will not soon appear in products on store shelves. Scientists

need to find ways to allocate carbon nanotubes in certain places so that they work properly,

and some are still open questions about how to pass an electrical current.

17

Conclusion

One of the most significant achievements of modern science is the discovery of

carbon nanotubes. Although this form of carbon in its structure is intermediate between

graphite and fullerenes, carbon nanotubes have many properties that neither graphite nor

fullerene have. It can therefore be seen and analyzed as an independent nanotube material

physicochemical characteristics of which are unique.

Study of carbon nanotubes represent a serious concern, both fundamental and applied.

Fundamental to this property of interest is due mainly wide range of variation of its

physicochemical properties depending upon the chirality, and the unusual structure.

The problem of studying the fundamental properties of carbon nanotubes leads to the

problem of practical application, and, above all, cheaper ways of producing carbon nanotubes

in large quantities latter decision depends issues. At present, this problem prevents the

possibility of large-scale use of this material. With such features as extra tiny size, good

conductivity, high emission characteristics, high chemical stability of the existing porosity

and the ability to attach itself to a variety of chemical radicals, the nanotubes can be

effectively used in areas such as instrumentation, nano electronics and chemical engineering,

etc. If these problems are solved successfully, the effective impact of basic research on

scientific and technological progress has confirmed another example.

18

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