the materials science of nano electronics

8/2/2019 The Materials Science of Nano Electronics

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The Materials Science of Nanoelectronics

MAT 791Q, Fall Semester 2003

Oct. 31, 2003


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CNT structure

CNT synthesis

CNT electronic properties

Carbon Nanotubes (CNT)


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Carbon Nananotubes: What are they?

Carbon nanotubes (NT) are hollowelongated cylindrical arrangements of

atoms, closed at the ends withhemispherical caps

They can be a single-wall or a multi-wall variety, when several cylinders

(and the corresponding caps) form anested Russian doll structure.

A perfect wall of an NT is essentially

a rolled up monoatomic layer ofgraphite, that is a hexagonalhoneycomb lattice of covalent C-Cbonds.


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CNT is a tubular form of carbon with diameter as small as 1 nm.

Length: few nm to microns.

CNT is configurationally equivalent to a two dimensional graphene

sheet rolled into a tube.

CNT can be metallic orsemiconducting, depending on

chirality.


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Single-walled and Multi-walled CNT

There are two main types of Carbon Nanotubes, Multi-Walled Nanotubes and Single-

Walled Nanotubes. MWNTs contain overlapping cylindrical tubes, like a coaxial cable.

Their diameters range from a few nanometers to around 40nm, depending on thenumber of concentric tubes. SWNTs, on the other hand, consist of one tube with a

diameter of approximately 1.4nm.

Multi-Walled CNT Single-Walled CNT


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Multi-Walled CNT


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What are the major structural characteristics?

NT diameter ranges from 1 nm to 20-30 nm, and is much less than itslength

Interlayer separation in a multi-wall NT is close to 0.34 nm (like ingraphite).

A crucial property is the corkscrew symmetry of NT depending on theorientation of the hexagonal pattern of the wall with respect to the

molecular axis, often called helicity and specified by the angle

between the NT circumference and the zigzag atomic motif of thehexagonal wall. Another conventional way to identify the helicity is bythe vector (c1,c2) on the graphene lattice corresponding to the NT

perimeter. NTs with c2 = 0 ( = 0) are called zigzag, and those with c2

= c1 ( = /6) are armchair. These types are achiral, while any tubulewith 0 < < /6 is chiral.


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Fabrication of Carbon Nanotube Materials:

Arc-discharge method

I

He

Arc-

discharge

Fullerenes:

P500Torr

SWNT: Catalyst required

The process involves striking a plasma between two graphite electrodes in a vacuumchamber filled with He. Relatively large quantities (102 -103gram/day) of MWNTs can be

produced when the system is optimized. Materials produced typically consist of 50-70%MWNTs with large variations in outer (5-50nm) and inner (2-10nm) shell diameters and

length. The impurities are multi-walled nanotube particles and amorphous carbonstructures.


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Fabrication of Carbon Nanotube

Materials: Laser-ablation

Laser ablation

Oven T~ 1200oC

SWNT:Catalyst required

Yield in soot ~90% (with

two lasers)

SWNTs with relatively high purity (60-70%) and narrow diameter distribution(1.2-1.8nm) can be fabricated by ablating a graphite target mixed with metalcatalysts at high temperature. Both metallic and semiconducting nanotubes are

produced. The laser ablation method affords more experimental control but at aslower production rate compared to that of the arc-discharge method.


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Fabrication of Carbon Nanotube

Materials: Chemical Vapor Deposition

CnHm

CVD

Oven T~ 500-1000oC

catalyst

MWNT: low quality

Catalyst required

~60% yield of all carbon feed

SWNT

Catalyst required

Yield 200% in excess of

Catalyst weight

CVD provides a low temperature alternative to the two high temperature

processes discussed above and can potentially lead to economical highvolume production.


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CNT has been grown by laser ablation

(pioneering at Rice) and carbon arc process

(NEC, Japan) - early 90s.- SWNT, high purity, purification methods

CVD is ideal for patterned growth

(electronics, sensor applications)

- Well known technique frommicroelectronics

- Hydrocarbon feedstock

- Growth needs catalyst

(transition metal)

- Multiwall tubes at500-800 deg. C.

- Numerous parameters

influence CNT growth


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Large scale computer simulations based on ab initio

methods enable understanding nanotube characteristics

and serve as design tool- Evaluation of mechanical properties

- Evaluation of electronic properties

- Electron transport in CNT devices

- Functionalization of the nanotubes- Design of electrical and mechanical devices

- Evaluation of storage potential (H2, Li)CNT Molecular Network


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Representative images of CNT bundles


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Carbon nanothermometer containing gallium

YIHUA GAO AND YOSHIO BANDO


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Energy band structure of CNT

CNT can be either metallic or semiconducting, depending on the 1st

Brillouin Zone. The 1st Brillouin Zone is the primitive cell of the lattice

in reciprical space, and it exhibits the wavevectors of the lattice. From

this, a simple relationship stating that if:

n-m is divisible by three, metallic

n-m not divisible by three, semiconducting, with a gap of~0.5 eV.


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How do NT conduct?

for the same elemental composition, the answer dependson helicity.

Armchair NT are metallic, with zero gap and an estimatedcarrier density ~1022 cm-3 for equivalent bulk material;

chiral tubes with c2 - c1 multiple of 3 have a very small(meV) curvature-induced gap ~1/d2, and other types are

semiconductors with a gap ~1/d in the range of 1 eV.

Conductivity can be vulnerable to minor deformations.Besides these elastic perturbations, any atomic disorder or

doping with other elements can change the electronicssignificantly.


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High-resolution STM image of an individual nanotube,showing the chiral winding of the hexagon lattice along

the tube axis (Dekker)


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Band gap measurements


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Energy band in metallic CNT (Lieber)


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Electrical Characterization of CNT

Individual single-wall nanotube

deposited on 2 Pt electrodes (C.Dekker)

4-probe measurements(Schoenenberger)


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Electrical characteristics of CNT

Author SampleMethod

Min. effective

resistivity at

300 K, cm

Current-

carrying

capacity,

A/cm2

Song et

al.

Bundles of

MWNT2-t 6.5x10-3

De Heer

et al.

Oriented film

of MWNT2-t 20x10-3

Frank et

al.

Individual

MWNT 2-t >10

7

Dai et

al.

Individual

MWNT2-t(STM)

8x10-4

De Pablo

et al.

Individual

MWNT

2-t(planar)

1.5x10-5 2.4x108

Ebbesson

et al.

Individual

MWNT4-t(planar)

5.1x10-6 6x106

Bachtold

et al.

Individual

MWNT 4-t(planar) 3x10-5


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Continuous Quantum Wire?

=== ke

h

e

hR 5.6

422

122min

h

e

h

eG

22

42

2 ==

L=200 nm

Liang et al.


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CNT/metal Schottky diodes

Dekker


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Bandgap modulation of CNT by

encapsulated metallofullerenesLee et al

Gd@C82


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H. Dai: Chemical Doping of CNT

Esaki tunnel diode

pn

junctiondiodes


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CNT p-n-p structure


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Oxygen desorption/adsorption doping

(Avouris)


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P. Avouris: Engineering CNT and CNT circuits by

electrical breakdown


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Architecture

Non-classical

CMOS

Memory

Logic

Time

Emerging Technology Sequence

StrainedSi

VerticalTransistor

FinFET Planardouble gate

Phase Change

Nano FG SET Molecular

Magnetic RAM

SETRSFQ QCA Molecular

RTD-FET

Quantum

computing

CNNDefect

Tolerant

QCA

3DIntegration

FD SOI

Molecular

Emerging

TechnologyVectors


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Summary

Carbon nanotube potentially could be used as nanoelectronic

components

Naturally structured

Electronic properties can be controlled by geometry and surface

adsorbates

Ballistic conductance demonstrated at length up to 0.2 m

BUT There are no methods allowing for reproducible synthesis of

CNTs with desired properties (e.g. geometry)

No methods for large scale integration of CNTs in useful devices

and circuits

the materials science of nano electronics

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