1 Nanoelectronic Devices based on Silicon MOS structure Prof.C.K.Sarkar IEEE distinguish lecturer...
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Transcript of 1 Nanoelectronic Devices based on Silicon MOS structure Prof.C.K.Sarkar IEEE distinguish lecturer...
1
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
2
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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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.
36
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.
37
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
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
42
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