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Nanoelectronic Nanoelectronic Devices based on Devices based on

Silicon MOS Silicon MOS structurestructure

Prof.C.K.SarkarIEEE distinguish lecturer

Dept of Electronics and Telecommunication EngineeringJadavpur University

Kolkata- 700032.

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Nanotechnology explores and benefit from quantum phenomenology in the ultimate limit of miniaturization.

At length-scales comparable to atoms and molecules, quantum effects strongly modify properties of matter like “color”, reactivity, magnetic or dipolar moment, … Besides, phenomena characteristic of systems with low dimensionality can be use to control macroscopic properties.

Leading Research efforts in Nanotechnology

1. Quantum confinement2. Electronic Transport3. Quantum confinement

FUNDAMENTALS OF NANOTECHNOLOGY

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NanoparticlesNanoparticles What Is Nanocrystalline Silicon?What Is Nanocrystalline Silicon?

1.1. It is similar to amorphous silicon (a-Si)It is similar to amorphous silicon (a-Si)2.2. It consists solely of crystalline silicon grains, separated It consists solely of crystalline silicon grains, separated

by grain boundariesby grain boundaries3.3. Nanocrystalline silicon (nc-Si) is an allotropic form of Nanocrystalline silicon (nc-Si) is an allotropic form of

siliconsilicon Advantages of nanosilicon over SiliconAdvantages of nanosilicon over Silicon

1.1. It can have a higher mobility due to the presence of the It can have a higher mobility due to the presence of the silicon crystallites.silicon crystallites.

2.2. Higher dielectric constant than bulk silicon. Higher dielectric constant than bulk silicon. 3.3. One of the most important advantages of One of the most important advantages of

nanocrystalline silicon, however, is that it has increased nanocrystalline silicon, however, is that it has increased stability over a-Sistability over a-Si

4.4. Mainly used in optoelectronics due to direct band gapMainly used in optoelectronics due to direct band gap..

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nc-Si Embedded MOS nc-Si Embedded MOS structurestructure

This model This model consists of Si consists of Si substrate/ pure substrate/ pure SiO2/ Embedded SiO2/ Embedded nc-Si layer/ Gate nc-Si layer/ Gate electrodeelectrode

Voltage applied at Voltage applied at the gate Terminalthe gate Terminal

Electrons tunnel Electrons tunnel from Si-substrate from Si-substrate to gate through to gate through these dielectrics.these dielectrics.

Gate Metal

nc- Si Layer

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Methodology to be adopted Methodology to be adopted and Innovative aspectsand Innovative aspects

Effective dielectric constantEffective dielectric constant

Effective barrier heightEffective barrier height

Effective massEffective mass

Modification of tunneling probabilityModification of tunneling probability

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1

ox oxeff

ox nc SiO

d d dd d

( ) 2gsib geffE E

2nc SiO oxox ox

eff

m d dm dm

d d

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Maxwell – Garnett Effective medium Approximation theory

Inclusion Inclusion particles particles randomly randomly dispersed in dispersed in dielectric dielectric mediummedium

Silicon Silicon nanocrystallites nanocrystallites spherical in spherical in shapeshape..

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Maxwell Garnett Theory Maxwell Garnett Theory embedded systemsembedded systems

In a binary composite, if the density of silicon nanocrystals is small, each particle of the component can be treated as being embedded in a large medium of SiO2.

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Mathematical Mathematical formulationformulation

The effective dielectric function of the composite could be expressed as

eff ba ba

a b eff b

f

=Screening factor depends upon the size and orientation of particle. For spherical it is 2 .

fa = volume fill fraction of the particle

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Tunneling in the modelTunneling in the model

Low Applied Gate voltage Direct tunneling

High Applied Gate voltage Fowler-Nordheim tunneling

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Direct TunnelingDirect Tunneling

V < 0b E

q

At low field when

The barrier becomes Trapezoidal in Shape.

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Direct tunneling Direct tunneling ExpressionExpression

1/2

20 0

2

2 2 2exp

eff b eff bD

m E q V m EJ d

d

From Simmon’s model modified at low field

nc oxox oxeff

m d dm dm

d d

Where

α = unit less adjustable parameter depends on effective mass and barrier height.

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Fowler – Nordheim Fowler – Nordheim TunnelingTunneling

At high field when

b oE

q

V>

The barrier becomes triangular in shape

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Different conditions for Different conditions for Fowler – Nordheim Fowler – Nordheim

equationequation

qFeffd< Фb-E0

For this condition

Tunneling probability

1/ 2

0

2exp 2 ( )

a

n nD E m V x E dx

Where V(x) = -qFs.x x<0

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For this condition

V(x)=Фb-qFeff x 0<x<d

Фb-E0< qFeffd< Ф-E0

Tunneling probability becomes

12 2 2 2

2 3 1 2 3 1sin cosh ( ) cos cosh ln(4)nD E

1

1/2

*02

i

i

x

ix

m V x E dx

where

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0 5 10 15 20 25 301E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

Direct tunneling FN tunneling

Gat

e cu

rren

t (A

)

Gate voltage (V)

nc Si total current SiO

2 total current

nc Si FN current SiO

2 FN current

FN tunneling current increases

FN onset voltage decreases

Field emission starts at the low applied voltage.

plot of I g-Vg curve for 30 nm thickness for both pure SiO2 and proposed dielectric.

Observation

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0.1 0.2 0.3 0.4 0.5 0.6-180

-160

-140

-120

-100

-80

-60

ln(J

o/F

2 ) A

/V2

Volume fraction

1 nm 3 nm 5 nm

0.1 0.2 0.3 0.4 0.5 0.6 0.7-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

ln (

J/F2 )

A/v

2

volume fraction

1 nm 3 nm 5 nm

0.1 0.2 0.3 0.4 0.5 0.6 0.7-65

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

ln (

J FN/F

2 ) A

/v2

volume fraction

1 nm 3 nm 5 nm The plot of

ln(JFN/F2) vs. volume fraction at different applied voltages a) 5v b) 10v and c) 15v

a b

c

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1/ 2

0 0

0

2exp 2 ( )

a

D E m V x E dx

0 0FN IJ qN V D E

FN Tunneling current probability

Tunneling current density

Direct Tunneling current density

1/ 2 2

0

2

0

2

2 2exp

eff b

D

eff b

m E q VJ

d

m Ed

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Variation of dielectric Variation of dielectric constantconstant

2.0x1028 4.0x1028 6.0x1028 8.0x1028 1.0x10293

4

5

6

7

8

9

diel

ectri

c co

nsta

nt

nc-Si concentration ( / m3)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

4.0

4.2

4.4

4.6

4.8

5.0

5.2

5.4

eff

volume fraction ()

1 nm 3 nm 5 nm

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Carbon NanotubesCarbon NanotubesThe Carbon nanotubeThe Carbon nanotube Electronic structure of Carbon nanotubeElectronic structure of Carbon nanotube The geometry of Carbon nanotubeThe geometry of Carbon nanotube Electronic properties of carbon nanotubeElectronic properties of carbon nanotube Quantum Modeling & Proposed Design of Quantum Modeling & Proposed Design of

CNT-Embedded Nanoscale MOSFETsCNT-Embedded Nanoscale MOSFETs CNT band structure and electron affinityCNT band structure and electron affinity CNT mobility modelCNT mobility model Carrier concentrationCarrier concentration Effective potential due to CNT-Si barrierEffective potential due to CNT-Si barrier

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Electronic structure of Electronic structure of Carbon nanotubeCarbon nanotube

a single atomic layer of a single atomic layer of graphite consists of 2-D graphite consists of 2-D honeycomb structurehoneycomb structure

it has conducting states at, it has conducting states at, but only at specific points but only at specific points along certain directions in along certain directions in momentum space at the momentum space at the corners of the first Brillouin corners of the first Brillouin zonezone

Choosing different axes it Choosing different axes it can be used as typical can be used as typical metal or semiconductormetal or semiconductor

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The geometry of Carbon The geometry of Carbon nanotubenanotube

** The lattice constant a= |a1| = |a2| =3ac-c

Where ac-c is carbon carbon bond

length** The vector describe the circumference of a nanotube Ch = na1 + ma2

**The chiral angle = sin-1{3m / 2(n2+m2+mn)}

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Different types of carbon Different types of carbon nanotubesnanotubes

The construction of a nanotube through the rolling up of a graphene sheet leads to three direct verities These are armchair nanotubes which have = 30o

These have an indices of the form (n,n)[n = m]. For = 0o zigzag nanotubeThe indices of the form (n,0)For 00 < < 300 chiral nanotubeIndices of the form (n, m)

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From graphene to carbon nanotubeFrom graphene to carbon nanotube The only discrete wave-vectors are allowed in radical direction and The only discrete wave-vectors are allowed in radical direction and

the following condition isthe following condition is

CChh . k = 2 . k = 2qq For an armchair nanotube the circumferential axis lies along x For an armchair nanotube the circumferential axis lies along x

direction,direction,

|C|Chh| |k| |kxx| = 2| = 2q q

kkxx = 2 = 2q / q / 3na3na For a zigzag nanotube the azimuthal direction lies along the y For a zigzag nanotube the azimuthal direction lies along the y

direction. direction.

|C|Chh| |k| |kxx| = 2| = 2q q

kkxx = 2 = 2q / naq / na

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Electronic propertyElectronic property

the nanotube is metallic or not can be described by the m and n indices with the following rule n = m metallic n – m = 3j metallic n – m 3j semiconducting

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Dependence of semiconducting Dependence of semiconducting band gap with diameterband gap with diameter

The energy gap of semiconducting single walled nanotubes is predicted to be inversely proportional to the diameter of the nanotube The best fit equation is of the form is Eg = 2oac-c / d

o = 2.25 0.06 eV is a good arrangement shows a

fundamental energy gap 0.4 – 0.9 eV which lie in the infrared range

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CNT-Embedded Nanoscale CNT-Embedded Nanoscale MOSFETsMOSFETs

New design a methodology has been developed for modeling nanoscale CNT-MOS-FETs

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Fabrication ProcedureThin HfAlO film was deposited on the Si substrate by the laser molecular beam epitaxy (MBE)

The ratio of Hf to Al for the ceramic target is 1:2

The commercial CNTs were synthesized by chemical vapor deposition

The diameter and length are about 2 nm and 1.5µm respectively.

Finally another layer of HfAlO was deposited to cover these CNTs and form the structure of HfAlO/CNT/HfAlO/Si.

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Actual structure

Pt/8nmHfAlO/CNT/3nmHfAlO/Si

IV measured at 77K

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The dielectric constant of CNT is dependent on its symmetry and tube radius

Where

C~ 1.96 For metallic

2.15 For Semiconducting

According to Maxwell- Garnett Theory the effective dielectric constant can be written as

Nanotube Parameters

Where f is the volume fraction and εox is the dielectric constant of HfAlO εox =16

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Typical C-V hysteresis characteristics of the CNT based MOS memory devices

Backward C-V curve overlaps forward C-V curve without CNT

A clear hysteresis between subsequent forward and backward C-V curves containing CNTs.

This curve suggests small number of charge carriers are stored inside CNTs.

C-V measurement of Embedded Carbon Nanotubes

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Observation

Direct tunneling gate leakage current density at low gate voltage

Gate leakage current is direct tunneling current

Two different dielectric, pure HfAlO and HfAlO embedded with SWCNTs.

As gate voltages increases tunneling current density decreases.

Tunneling current is lower in embedded CNTs than pure HfAlO dielectric.

SWCNTs stored charges, breaks tunneling paths from channel to gate and current density decreases.

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Observations

F-N Tunneling current as a function of high gate voltages

Field emission or F-N tunneling current as a function of applied gate voltage.

The F-N tunneling onset voltage is lower in CNT embedded dielectric than pure HfAlO oxide dielectric

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Observation

F-N plot of pure HfAlO and CNT embedded HfAlO dielectric

F-N plot is straight line.

Slopes of the two different dielectrics pure and embedded are different

For a particular applied field the F-N tunneling current density is higher in CNT embedded dielectric than pure HfAlO oxide dielectric.

The dielectric constant is higher in CNT embedded dielectric than pure HfAlO dielectric

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Observation

Direct tunneling current with different nanotube diameters

Gate leakage current is direct tunneling current

As applied voltage increases tunneling current decreases

As the diameter of nanotube decreases direct tunneling current decreases.

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Observation

F-N Tunneling current with the variation of nanotube diameters

F-N tunneling current with different diameters of nanotubes

The F-N tunneling onset voltage decreases with the increase of the nanotube diameter.

The diameter in nanometer regime can cause a highly localized field across the nanotube surface. This helps to increase the Field emission current.

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Observation

F-N tunneling current of different pure and embedded dielectric

High positive gate voltage

nc-Si embedded in SiO2 matrix

SWCNT embedded in high-k dielectric

High-k dielectric is HfAlO

F-N onset voltage is maximum in case of pure SiO2 and minimum in case of embedded CNTs in HfAlO

Embedded CNTs have better Field emission properties than embedded nc-Si.

Embedded CNT has highest dielectric constant.

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Observation

F-N plot with different pure and embedded dielectrics

F-N tunneling current higher in embedded dielectric than pure oxide

Tunneling current in embedded CNTs is higher than in embedded nc-Si

The value of dielectric constant is higher in HfAlO than Pure SiO2

Tunneling current increases with the increase of dielectric constant value.

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Observation

F-N onset voltage is highest in case of pure SiO2

Onset voltage decreases with the introduction of nanoparticles.

Onset voltage is lower in case of CNT than in nc_si.

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Observation

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Observation

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Observation

Leakage current is lower in high-k dielectric HfO2, than pure SiO2

With embedded nanoparticles direct tunneling current also decreases

It is lowest in Hf)2 embedded with CNTs

All this is due to the higher value of dielectric constant of gate oxide

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ConclusionConclusion

CNT-MOSFET device appears to yield better CNT-MOSFET device appears to yield better performance than the conventional performance than the conventional MOSFETMOSFET

The current voltage characteristics predicts The current voltage characteristics predicts that the device current of CNT-MOSFET is that the device current of CNT-MOSFET is higher than the conventional one.higher than the conventional one.

The narrow diameter tube shows similar The narrow diameter tube shows similar performance compared to conventional one.performance compared to conventional one.

CNT-MOSFET may represent the new CNT-MOSFET may represent the new paradigm for devices in the 21paradigm for devices in the 21stst century century

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