Transcript Slide 1

Nanoelectronic Devices
based on Silicon MOS
structure
Prof.C.K.Sarkar
IEEE distinguish lecturer
Dept of Electronics and Telecommunication Engineering
Jadavpur University
Kolkata- 700032.
1
FUNDAMENTALS OF NANOTECHNOLOGY
 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 confinement
2. Electronic Transport
3. Quantum confinement
2
Nanoparticles

What Is Nanocrystalline Silicon?
1. It is similar to amorphous silicon (a-Si)
2. It consists solely of crystalline silicon grains, separated
by grain boundaries
3. Nanocrystalline silicon (nc-Si) is an allotropic form of
silicon
 Advantages of nanosilicon over Silicon
1. It can have a higher mobility due to the presence of the
silicon crystallites.
2. Higher dielectric constant than bulk silicon.
3. One of the most important advantages of
nanocrystalline silicon, however, is that it has increased
stability over a-Si
4. Mainly used in optoelectronics due to direct band gap.
3
nc-Si Embedded MOS structure



This model consists of
Si substrate/ pure
SiO2/ Embedded ncSi layer/ Gate
electrode
Voltage applied at the
gate Terminal
Electrons tunnel from
Si-substrate to gate
through these
dielectrics.
Gate Metal
nc- Si Layer
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Methodology to be adopted and
Innovative aspects
 Effective dielectric
constant


 eff   dox  d  dox 
  ox d  ncSiO d 

2
1

 Effective barrier height
b  ( Egeff  Egsi ) 2
 Effective mass
 m d mnc  SiO2  d  dox  
meff   ox ox 

d
d


 Modification of tunneling probability
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Maxwell – Garnett Effective medium
Approximation theory


Inclusion particles
randomly dispersed in
dielectric medium
Silicon nanocrystallites
spherical in shape.
6
Maxwell Garnett Theory embedded
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.
7
Mathematical formulation

The effective dielectric function of the composite
could be expressed as
 eff   b
 a  b
fa

 a   b  eff   b
=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 model
Low Applied Gate voltage  Direct tunneling
High Applied Gate voltage  Fowler-Nordheim
tunneling
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Direct Tunneling
At low field when
V<
b  E0
q
The barrier becomes
Trapezoidal in Shape.
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Direct tunneling Expression
From Simmon’s model modified at low field
JD



2meff b  E0 
2
d
1/2
 q 2V

 2
exp 


2meff b  E0  
d


 mox dox mnc  d  dox  
meff  


d
 d

α = unit less adjustable parameter depends on
effective mass and barrier height.
Where
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Fowler – Nordheim Tunneling
At high field when
V>
b  Eo
q
The barrier becomes triangular
in shape
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Different conditions for Fowler –
Nordheim equation
For this condition
qFeffd< Фb-E0
Tunneling probability
 2 a

1/ 2
D  En   exp   2m V ( x)  En  dx 
 0

Where V(x) = -qFs.x x<0
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For this condition
Фb-E0< qFeffd< Ф-E0
Tunneling probability becomes


D  En   sin 2 cosh (3 1)  cos 2 cosh 3 1  ln(4)
2
2
where
2
2
i 
xi

*
V
2m 

xi1 

1
1/ 2
 x   E0  


V(x)=Фb-qFeff x
dx
0<x<d
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Observation
 FN
tunneling
current
increases
nc Si total current
SiO2 total current
nc Si FN current
SiO2 FN current
1E-4
1E-5
Gate current (A)
1E-6
FN onset
voltage
decreases
1E-7
1E-8
1E-9
FN tunneling
Direct tunneling
1E-10
1E-11
1E-12
0
5
10
15
20
25
30
Field emission
starts at the low
applied voltage.
Gate voltage (V)
plot of I g-Vg curve for 30 nm thickness for both pure
SiO2 and proposed dielectric.
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1 nm
3 nm
5 nm
-60
1 nm
3 nm
5 nm
-15
-20
-80
-25
2
ln (J/F ) A/v
2
2
ln(Jo/F ) A/V
2
-30
-100
-120
-140
-35
-40
-45
-50
-55
-160
-60
-65
-180
0.1
0.2
0.3
0.4
0.5
0.6
0.1
0.2
0.3
Volume fraction
0.5
0.6
0.7
b
a
1 nm
3 nm
5 nm
-15
-20
-25
2
-30
-35
2
ln (JFN/F ) A/v
0.4
volume fraction
-40
-45
-50
The plot of ln(JFN/F2) vs.
volume fraction at
different applied voltages
a) 5v b) 10v and c) 15v
-55
-60
-65
0.1
0.2
0.3
0.4
0.5
0.6
0.7
volume fraction
c
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FN Tunneling current probability
 2 a

1/ 2
D  E0   exp 
2
m
V
(
x
)

E
dx




0


 0

Tunneling current density
J FN  qN I V0 D  E0 
Direct Tunneling current density
1/ 2
2
2
m


E

q
V

 eff  b

0
JD 
 2
exp  


2
2meff
d

b
 E0 
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
d


Variation of dielectric constant
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1 nm
3 nm
5 nm
5.4
5.2
5.0
7
4.8
6
eff
dielectric constant
8
5
4.6
4.4
4.2
4
4.0
3
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2.0x10
28
4.0x10
28
28
6.0x10
8.0x10
3
nc-Si concentration ( / m )
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1.0x10
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
volume fraction ()
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Carbon Nanotubes
The Carbon nanotube

Electronic structure of Carbon nanotube

The geometry of Carbon nanotube

Electronic properties of carbon nanotube

Quantum Modeling & Proposed Design of CNTEmbedded Nanoscale MOSFETs

CNT band structure and electron affinity

CNT mobility model

Carrier concentration

Effective potential due to CNT-Si barrier
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Electronic structure of Carbon
nanotube



a single atomic layer of
graphite consists of
2-D
honeycomb structure
it has conducting states at, but
only at specific points along
certain
directions
in
momentum space at the
corners of the first Brillouin
zone
Choosing different axes it can
be used as typical metal or
semiconductor
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The geometry of Carbon nanotube
** 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
nanotubes
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 nanotube
The indices of the form (n,0)
For 00 < < 300 chiral nanotube
Indices of the form (n, m)
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From graphene to carbon nanotube



The only discrete wave-vectors are allowed in radical
direction and the following condition is
Ch . k = 2q
For an armchair nanotube the circumferential axis lies along x
direction,
|Ch| |kx| = 2q
kx = 2q / 3na
For a zigzag nanotube the azimuthal direction lies along the y
direction.
|Ch| |kx| = 2q
kx = 2q / na
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Electronic 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 band 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 = 2oac-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
MOSFETs
New design a methodology has been
developed for modeling nanoscale CNTMOS-FETs
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Fabrication Procedure
Thin 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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Pt/8nmHfAlO/CNT/3nmHfAlO/Si
IV measured at 77K
Actual structure
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Nanotube Parameters
The dielectric constant of CNT is dependent on its symmetry and tube radius
Where
C~ 1.96
2.15
For metallic
For Semiconducting
According to Maxwell- Garnett Theory the effective dielectric constant can be
written as
Where f is the volume fraction and εox is the
dielectric constant of HfAlO εox =16
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C-V measurement of Embedded Carbon Nanotubes
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.
Typical C-V hysteresis
characteristics of the CNT based
MOS memory devices
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Observation
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.
Direct tunneling gate leakage current
density at low gate voltage
SWCNTs stored charges,
breaks tunneling paths from
channel to gate and current
density decreases.
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Observations
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
F-N Tunneling current as a function of
high gate voltages
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Observation
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.
F-N plot of pure HfAlO and CNT
embedded HfAlO dielectric
The dielectric constant is
higher in CNT embedded
dielectric than pure HfAlO
dielectric
33
Observation
Gate leakage
current is direct
tunneling current
As applied voltage
increases tunneling
current decreases
As the diameter of
nanotube decreases
direct tunneling
current decreases.
Direct tunneling current with different
nanotube diameters
34
Observation
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.
F-N Tunneling current with the variation
of nanotube diameters
35
Observation
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.
F-N tunneling current of different
pure and embedded dielectric
Embedded CNT has highest
dielectric constant.
36
Observation
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.
F-N plot with different pure and embedded
dielectrics
37
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.
38
Observation
39
Observation
40
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
41
Conclusion




CNT-MOSFET device appears to yield better
performance than the conventional MOSFET
The current voltage characteristics predicts that
the device current of CNT-MOSFET is higher
than the conventional one.
The narrow diameter tube shows similar
performance compared to conventional one.
CNT-MOSFET may represent the new paradigm
for devices in the 21st century
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