Smart Metrics for High Performance Material Design
PhD_10_2011_Abhijeet_Paul
1. Network for Computational
Nanotechnology (NCN)Purdue, Norfolk State, Northwestern, MIT, Molecular Foundry, UC Berkeley, Univ. of Illinois, UTEP
Electronic and Thermal properties of
semiconductor nanostructures:
A modeling and simulation study
Abhijeet Paul
Network for Computational Nanotechnology
(NCN),
Electrical and Computer Engineering
Purdue University
email: abhijeet.rama@gmail.com
2. Abhijeet Paul
Acknowledgements
• Overall guidance and direction
» Prof. Gerhard Klimeck, Prof. Mark Lundstrom, Purdue University, USA.
» Prof. Timothy Boykin, University of Alabama at Huntsville, USA (PhD committee
member).
» Prof. Leonid Rokhinson, Purdue University, USA (PhD committee member).
• Theory and Code development
» Dr. Mathieu Luisier, ETH Zurich, Switzerland (OMEN/OMEN-BSLAB development).
» Prof. Timothy Boykin, University of Alabama Huntsville, USA (Tight-Binding and solid
state phys. theory)
» Dr. Neophytos Neophytou, TU Wien, Austria (Initial MATLAB codes)
• Discussions and work
» Saumitra Mehrotra, Parijat Sengupta, Shuaib Salamat, Sunhee Lee, Lang Zeng, Mehdi
Salmani, Kai Miao, Dr. Raseong Kim and Changwook Jeong, Purdue University.
• .Experimental Collaborators
» Dr. Giuseppe Tettamanzi, TU Delft, Netherlands, Shweta Deora, IIT Bombay, India, Dr.
Subash Rustagi, IME, Singapore, Dr. Mark Rodwell, UCSB, USA.
• Summer Undergrad students (for nanohub tools)
» Junzhe Geng, Victoria Savikhin, Mohammad Zulkifli, and Siqi Wang, Purdue University.
• Funding and Computational Resources
» MSD-FCRP, SRC, NSF and MIND for funding.
» NCN and nanoHUB.org for computational resources.
2
3. Abhijeet Paul
PhD timeline and progress
[A] N. Singh et. al, EDL 2006 [B] A. Hochbaum et. al, Nature, 2008
[C] Yu et. al, Nature, 2010. [D] Pernot et. al, Nature, 2010.
Important experimental works that guided this PhD work.Important experimental works that guided this PhD work.
3
4. Abhijeet Paul
Outline of the talk
• Motivation
» Why the present work is important ?
» Need for integrated atomistic simulation framework
• Computational modeling and simulation approaches.
• Application of the methods to Si nanowires (SiNWs).
• Application to non-Si system GaAs a quick look !!
• Global dissemination of findings nanoHUB.org
• Summary
• Future directions
4
5. Abhijeet Paul
MoreMoore(MM)BeyondSi
More than Moore(MtM) Beyond CMOS
90
nm
90
nm
65
nm
65
nm
45
nm
45
nm
32
nm
32
nm
22
nm
22
nm
BaselineCMOS:CPU,Memory,Logic
Analog
RF
Analog
RF
SensorSensor
Thermo
electricity
Thermo
electricity
Bio-chipsBio-chips
Added Functional Diversity: More interaction
Sensory parts:
Measuring and
sense surrounding
Brain:
Computation and
Calculations
HigherValueSystems
Faster processing and better interaction with
environment holds the key to next-gen technologies.
MM and MtM are the solutions !!!
Faster processing and better interaction with
environment holds the key to next-gen technologies.
MM and MtM are the solutions !!!
How to get more ???
5
6. Abhijeet Paul
ITRS
CMOS scaling challenges
Intel
Need
higher
processing
Speed
!!!
Need
for faster
Transistors!!!
CMOS challengesCMOS challenges
New MaterialsNew Materials SiNW FET
New device
structure
New device
structure
Next-gen CMOS scaling solutions More MooreNext-gen CMOS scaling solutions More Moore
6
Revolutionary Evolutionary
7. Abhijeet Paul
www.tellurex.com
Choudhary et. al,
Nature nano. (2009)
Why thermoelectricity ???
Increasing
pollution!!!
IEA, WEO,
2008
Increasing
energy
dem
and!!!
Gelsinger et. al ISSCC 2001
Increasing
IC
heat!!!
Nasty ProblemsNasty Problems
On chip
thermoelectric
cooling (BiTe SL)
DEER
Automobile waste
heat thermoelectric
power generation
Green energy
Production by
thermoelectricity
Green Solutions from thermoelectricity More than MooreGreen Solutions from thermoelectricity More than Moore
7
8. Abhijeet Paul
Dimensional Scaling: CMOS
2003
2005
2007
2009
2011
StrainTechnology
High-k
MetalGate
3D
FETS
SiGe
strained
High-K
MG
From
bulk
planar to
3D
nano-scale
SiNW FET
III-V FET
Graphene
CNT-FET
Non-Si
3D FETs
are a
solution
Non-Si
3D FETs
are a
solution
?
?
?
8
IBM
9. Abhijeet Paul
Dimensional scaling: Thermoelectricity
BiTe/PbTe
Qwell
Superlattice
SiGe/Si
QDot
Superlattice
BiTe,PbTe
Bulk
LAST
1950
2000
1990
1980
1970
1960
2010
Therm
oelectric Material developm
ent Year line
BeginSemiconductoruse
(Bi,Sb),(Te,Se),PbTe,
PhononGlassElectroncrystal
Dresselhauset.al,
DOSengg. PbTe
Ddots
Si /SiGe
NW SL
Si
Nanowires
ZT ~1
1< ZT < 3
ZT > 3
From
Bulk to Nanostructures …
.
9
Atomic scale interface treatment ??
Phonons in nanostructures ??
Treatment of alloys at atomic level ??
Electronic structure in nanostructures?
???
??
?
Understanding of nano-scale
electronic and thermal properties must !!
Understanding of nano-scale
electronic and thermal properties must !!
10. Abhijeet Paul
Need for Atomic level modeling…
Si n-FinFET
IMEC
H=65nm,W=25nm
G. Tettamanzi et. al, EDL, 2009.
Intel
SiGe
pMOSFET
IEDM 2010
Ultra-scale
geometry with
finite atoms
Ultra-scale
geometry with
finite atoms
An increasing need
for atomic scale
modeling to
simulate ultra-
scaled devices!!!
An increasing need
for atomic scale
modeling to
simulate ultra-
scaled devices!!!
10
http://www.xray.cz/xray/csca/
kol2010/abst/cechal.htm
Quantum Dot
Material variation
at atomic scale
Material variation
at atomic scale
Atomic scale
strain variation
Atomic scale
strain variation
11. Abhijeet Paul
Need for integrated device modeling …
SiGe
Si
SiO2
Treatment of
multiple valleys
Treatment of
multiple valleys
Treatment of
multiple materials
Treatment of
multiple materials
Electron current Phonons
Treatment of
multiple particles
Treatment of
multiple particles
An increasing need
for integrated
modeling to
handle complex
issues in device
modeling !!!
An increasing need
for integrated
modeling to
handle complex
issues in device
modeling !!!
11
12. Abhijeet Paul
ITRS on the future device modeling ...
ITRS 2010, chapter 9, http://www.itrs.net
Physical models for stress induced device performance.
Physical models for novel materials eg. High-k stacks, Ge and
compound III/V channels: … Morphology, Bandstructure,
defects/traps,etc.
General , accurate, computationally efficient and robust quantum
based simulators incl. fundamental parameters linked to electronic
bandstructure and phonon spectra.
Treatment of individual dopant atoms and traps…
Need for an integrated approach to model material, electronic
and lattice properties at the atomic scale.
Need for an integrated approach to model material, electronic
and lattice properties at the atomic scale.
12
13. Abhijeet Paul
Outline of the talk
• Motivation
»Why the present work is important ?
»Need for integrated atomistic simulation framework
• Computational modeling and simulation approaches.
• Application of the methods to Si nanowires (SiNWs).
• Application to non-Si system GaAs a quick look !!
• Global dissemination of findings nanoHUB.org
• Summary
• Future directions
13
14. Abhijeet Paul
How to study the nano-scale devices?
Bottom-up
Approach
To
nano-scale
devices
Bottom-up
Approach
To
nano-scale
devices
ElectronicStructure
LatticeStructure
Carrier Transport
Atomistic
Tight-binding
(TB)
model
Atomistic
Tight-binding
(TB)
model
14
15. Abhijeet Paul
<111>
Atomistic Tight binding
Approach
15
Crystal structureCrystal structure
FEATURES/ADVANTAGES
Nearest neighbor
atomic bond model with
spin orbit (SO) coupling.
Based on localized
atomic orbital treatment.
Appropriate for treating
atomic level disorder.
Strain treatment at
atomic level.
Structural and material
variation treated easily.
Potential variations can
be accounted for (easily).
FEATURES/ADVANTAGES
Nearest neighbor
atomic bond model with
spin orbit (SO) coupling.
Based on localized
atomic orbital treatment.
Appropriate for treating
atomic level disorder.
Strain treatment at
atomic level.
Structural and material
variation treated easily.
Potential variations can
be accounted for (easily).
Y
Z
<100>
(x)<110>
StructureStructure
OrbitalOrbital
InteractionInteraction
AssembleAssemble
TB HamiltonianTB Hamiltonian
Atomistic Tight Binding (TB)
A reliable way to calculate electronic
structure in ultra-scaled structures.
Atomistic Tight Binding (TB)
A reliable way to calculate electronic
structure in ultra-scaled structures.
15
16. Abhijeet Paul
Bulk Bandstructure using Tight-binding
Si
Lent et. al,
Superlat. and Microstruc.,
1986
Pb
Se
sp3
d5
s*-SO model
sp3
d5
-SO model
Γ VB L
X CB L
Atomistic Tight-Binding method
A robust and accurate electronic structure model
Atomistic Tight-Binding method
A robust and accurate electronic structure model
Zincblende
Rocksalt
LL
16
17. Abhijeet Paul
Application of TB to FETs:
Charge-potential self-consistent approach
Schrodinger-Poisson self-consistent solution
Electron transport analysis in nano-scale FETs.
Schrodinger-Poisson self-consistent solution
Electron transport analysis in nano-scale FETs.
17
18. Abhijeet Paul
Experimental validation of 2D atomistic
Schrodinger-Poisson simulator
Collaboration between Purdue University & Institute of Microelectronics, Singapore (2007-08).
TEM image of
Experimental SiNW FET
Device Dimensions:
Tox = 9nm
W = 25nm
H = 14 nm
Source/Drain doping : n-
type ,1e20cm-3
Intrinsic <100>
oriented Silicon
channel.
Schrodinger using TB
2D FEM Poisson solution
4505 atoms4505 atoms
~20K FEM elements~20K FEM elements
chargecharge
potentialpotential
Self-consistent Simulation
Self-consistent simulation of realistic devices using parallel
C/C++ code.
Self-consistent simulation of realistic devices using parallel
C/C++ code.
18
19. Abhijeet Paul
Experimental validation of atomistic simulator
contd.
Electrical Potential Distribution.
Electron charge distribution
Self-consistent Simulation
Terminal CV benchmarking
Good matchingGood matching
Impact: Work published in IEEE, EDL
VOL. 30, NO. 5,MAY 2009. p.526
Impact: Work published in IEEE, EDL
VOL. 30, NO. 5,MAY 2009. p.526
Simulator benchmarked !!!
Quantum mechanical simulations for realistic FETs possible.
Simulator benchmarked !!!
Quantum mechanical simulations for realistic FETs possible.
19
20. Abhijeet Paul
How to study the nano-scale devices?
Bottom-up
Approach
To
nano-scale
devices
Bottom-up
Approach
To
nano-scale
devices
ElectronicStructure
LatticeStructure
Carrier Transport
Atomistic
Tight-binding
(TB)
model
Atomistic
Tight-binding
(TB)
model
Modified
Valence Force
Field (MVFF)
model
Modified
Valence Force
Field (MVFF)
model
20
21. Abhijeet Paul
Phonon dispersion calculation:
Modified VFF (MVFF) model
Old Keating
Model [1]
[A]
Bond-stretching(α)
Δr
[B]
Bond-bending(β)
Δθ
[C] Cross-bond
stretch bend (γ)
[2] Zunger et. al. 1999Δθ
Δr
Imp. For polar materials [2]
Imp. for
polar
materials [2]
[F]
Coulomb
interaction
[E]
Δθ1
Δθ2
Coplanar bond
bending(τ)
Imp. for non-polar materials
([3] Sui et. al,
[D]
Δr1
Δr2
Cross bond
Stretching (δ)
Short
Range
Short
Range
[1] Keating. Phys. Rev. 145, 1966.
[2] PRB, 59,2881, 1999.
[3] PRB, 48, 17938,1993
Long
Range
Long
Range
21
New combination of Interactions:
Modified Valence Force Field
Calculate phonons in zinc-blende
materials.
New combination of Interactions:
Modified Valence Force Field
Calculate phonons in zinc-blende
materials.
22. Abhijeet Paul
Bulk Si
Expt. (dots) [1]
What is the need for a new phonon model??
Accurate phonon model crucial for correct calculation of
phonon dispersion in nanostructures.
Accurate phonon model crucial for correct calculation of
phonon dispersion in nanostructures.
Bulk Si
Expt. (dots) [1]
[1] Nelsin et. al, PRB, 6, 3777, 1972. 22
Keating VFF Model
Over estimates
acoustic modes
at zone edges.
Over estimates
optical modes
New MVFF model matchs the
dispersion very well in the entire
Brillouin zone !!!
Expt. Data[1], inelastic
neutron scattering
(80K and 300K).
Expt. Data[1], inelastic
neutron scattering
(80K and 300K).
23. Abhijeet Paul
1D periodic [100] Si
nanowire structure.
Surface atoms free to
vibrate.
1D periodic [100] Si
nanowire structure.
Surface atoms free to
vibrate.
[100] free
standing
SiNW
qx [norm.] X
Bulk Si
6 branches
Phonon dispersion in free-standing nanowires
23
Strong phonon confinement responsible for different lattice
properties in SiNWs compared to bulk.
Strong phonon confinement responsible for different lattice
properties in SiNWs compared to bulk.
Lot of flat bands (zero velocity)
resulting in phonon confinement.
2 branches
1 branch
1 branch
2 branches
24. Abhijeet Paul
Approaches to study the nano-scale devices
Bottom-up
Approach
To
nano-scale
devices
Bottom-up
Approach
To
nano-scale
devices
ElectronicStructure
LatticeStructure
Carrier Transport
Atomistic
Tight-binding
(TB)
model
Atomistic
Tight-binding
(TB)
model
Modified
Valence Force
Field (MVFF)
model
Modified
Valence Force
Field (MVFF)
model
Landauer’s model (LM)Landauer’s model (LM)
24
25. Abhijeet Paul
Material A
Material B
How to analyze thermoelectric
properties of materials ?
V1
V2
IN
O
U
T
Tc Th
IQ
Ie
Ie
Steady-state linear thermoelectric (Onsager’s) relations [1,2]
[1] L. Onsager, Phys. Rev. 37 405 (1931).
[2] G. D. Mahan, Many-body Physics.
lehhch TTTTTTVVV ,2,,21
TT q
Tk
V B
Landauer’s Formula can be used to
evaluate the transport parameters
Landauer’s Formula can be used to
evaluate the transport parameters
TTGSVTGSIQ .. 2
TGSVGIe ..
Electric current Heat current
25
26. Abhijeet Paul
Goodness of thermoelectric materials:
Figure of Merit (ZT)
T
V
S
Generation of potential difference due
to applied temperature
difference`Seebeck Coefficient’.
Generation of potential difference due
to applied temperature
difference`Seebeck Coefficient’.
Generation of temperature difference
due to applied potential difference
`Peltier Coefficient’
Generation of temperature difference
due to applied potential difference
`Peltier Coefficient’
Measure of thermoelectric
power generation (High)
T
V
T
Measure of thermoelectric
cooling (High)
Ability of material to conduct electricity
`Electrical Conductance’
Ability of material to conduct electricity
`Electrical Conductance’
V
I
G
Measure of charge flow
(High)
d
Q
T
1
Ability of material to conduct heat
energy `Thermal Conductance’
Ability of material to conduct heat
energy `Thermal Conductance’
Measure of heat flow (Low)
Both electrons (ke)and
lattice(kl) carry heat.
ZT = ‘Thermoelectric Figure of
Merit’ by Ioffe in 1949.
S2
G = Electronic Power Factor (PF)
ZT = ‘Thermoelectric Figure of
Merit’ by Ioffe in 1949.
S2
G = Electronic Power Factor (PF)el
TGS
ZT
2
26
High ZT large G large S and small κ desired !!!High ZT large G large S and small κ desired !!!
27. Abhijeet Paul
Calculation of thermoelectric parameters
27
)(factor-Pre /le
mLf
G,S
κe
(Electronic)
Landauer’s approach
A suitable approach to calculate
thermoelectric transport parameters in nanostructures.
Landauer’s approach
A suitable approach to calculate
thermoelectric transport parameters in nanostructures.
κl (Lattice)
Landauer’s IntegralLandauer’s Integral
Under zero current condition
e
LG 0
ee
LLS 01 /
l
l L1
28. Abhijeet Paul
A closer look at electrons and phonons
28
max
0
)(
)()(
dM
T
F
L
L BEphml
m Phonon IntegralPhonon Integral
Etop
FDel
m
B
e
m dEEM
E
EF
L
E
Tk
EfE
L )(
)()(
Electron IntegralElectron Integral
le
mL / •No. of modes, M(E).
•Mean free path (λ).
Both need
•Moment calculation near Fermi Level
•Fermi Dirac distribution (fermions!!)
•M(E) Electronic bandstructure.
Electrons need
•No Fermi Level
•Bose Einstein distribution (bosons!!)
• M(ω) Phonon dispersion.
Phonons need
Accurate electronic &
phonon dispersions must !!!.
Accurate electronic &
phonon dispersions must !!!.
29. Abhijeet Paul
The complete approach set
Bottom-up
Approach
To
nano-scale
devices
Bottom-up
Approach
To
nano-scale
devices
ElectronicStructure
LatticeStructure
Carrier Transport
Atomistic
Tight-binding
(TB)
model
Atomistic
Tight-binding
(TB)
model
Modified
Valence Force
Field (MVFF)
model
Modified
Valence Force
Field (MVFF)
model
Landauer’s model (LM)Landauer’s model (LM)
Electronic
Properties
Electronic
Properties
Thermal
Properties
Thermal
Properties
ThermoelectricityThermoelectricity
An ‘integrated approach’ to study electronic, physical
and thermal properties of nanostructures !!!
An ‘integrated approach’ to study electronic, physical
and thermal properties of nanostructures !!!
29
30. Abhijeet Paul
Outline of the talk
• Motivation
»Why the present work is important ?
»Need for integrated atomistic simulation framework
• Computational modeling and simulation approaches.
• Application of the methods to Si nanowires (SiNWs).
• Application to non-Si system GaAs a quick look !!
• Global dissemination of findings nanoHUB.org
• Summary
• Future direction
30
31. Abhijeet Paul
SiNW
Explosive -sensor [E]
Silicon nanowires (SiNW): The vast potential
[A] Yang et. al, 2010, Nanoletters.
[B] Kalzenberg et. al, 2008, Nanoletters.
[C] Chin et. al, 2009, IEEE, TED.
[D] Hochbaum et. al, 2008, Nature.
[E] Patlosky et. al, 2010, Verlag, Germany.
Cathode
Li2S
Anode
SiNW
Batteries [A]
Silicon
nanowire
Solar cells [B]
Transistors [C]
Thermoelectricity [D]SiNWs have versatile applications
and
are highly compatible to CMOS.
Interesting system to study!!!
SiNWs have versatile applications
and
are highly compatible to CMOS.
Interesting system to study!!!
31
32. Abhijeet Paul
Nanoscale solutions in SiNWs
Silicon
nanowire
Physical metrology
How to determine
size, shape and
orientation ?
Electrical metrology
How to determine
interface traps in SiNW
FETs?
Thermal properties
How to engineer
thermal properties of
SiNW ?
Thermoelectricity
How to enhance PF
and ZT of SiNW ?
??
32
33. Abhijeet Paul
Peeking into the channel of Si trigated n-FinFETs
33
Collaboration between Purdue University ,TU Delft, Netherlands and IMEC, Belgium (2009-2011).
TEM image of tri-gated n-FinFETs
Active
Area(SAA)
Where do the
charges flow ?
source
Channel
Barrier
Height (Eb)
How easily
charges go from
source to channel?
Temperature
Based G-V
measurement
Experiment
From slope
From
intercept
Sub-threshold thermionic current provides information
about undoped channel Si FinFETs !!!
Sub-threshold thermionic current provides information
about undoped channel Si FinFETs !!!
34. Abhijeet Paul
Trends of Eb and SAA in Si n-FinFET:
Experiment vs. Simulation
34
Experimental ResultsEb and SAA
decrease with Vgs
volume to surface
inversion
Simulated
Tri-gated
Si n-FinFET
Schrodinger Eq.:
20 band sp3
d5
s*
model with spin orbit
coupling for Si.
Schrodinger Eq.:
20 band sp3
d5
s*
model with spin orbit
coupling for Si.
2D-Poisson Solution2D-Poisson Solution
ρ(r) V(r)
Simulation
Approach
Simulation
Approach
Performed using
OMEN-3Dpar
Channel with ~44K atoms
(support from Sunhee Lee)
Good
match !!
???
Simulations give good qualitative match !!
What is the reason for mismatch in SAA ?
Simulations give good qualitative match !!
What is the reason for mismatch in SAA ?
35. Abhijeet Paul
Mismatch in SAA :
Interface trap density (Dit) extraction
35
A
B
3D FinFETs
bad sidewall etch [1]
interface traps
gate screened
from channel
mismatch in SAA
[1] Kapila et. al, IEEE, EDL, 2008
From
Charge
Neutrality
~2X~2X
No H2 anneal
More mismatch!!
No H2 anneal
More mismatch!!
A. Paul et. al, JAP, 2011
Difference in expt. and simulated SAA Dit extraction
Method 1
H2 anneal reduce traps by ~2X.
Difference in expt. and simulated SAA Dit extraction
Method 1
H2 anneal reduce traps by ~2X.
36. Abhijeet Paul
Mismatch in Eb :
Interface trap density (Dit) extraction
36Gate Voltage (V)
Eb(meV)
H2 anneal
H2 anneal
3D FinFETs
bad sidewall etch [1]
interface traps
gate screened
from channel
mismatch in Eb
g
b
V
E
Gate to Channel coupling.
Suppressed by interface
traps
Dit ~18.1x1011
#/cm2
Dit ~15.3x1011
#/cm2
[110]
Dit
~10.3x1011
#/cm2
[100]Difference in expt. and simulated α Dit extraction
Method 2
[110] sidewall Dit > [100] sidewall Dit.
Difference in expt. and simulated α Dit extraction
Method 2
[110] sidewall Dit > [100] sidewall Dit.
A.Paul et. al, JAP, 2011
38. Abhijeet Paul
Physical Metrology Raman Spectroscopy:
A primer
Frequency(cm-1
)
Intensity (a.u)
Bulk Material
Nanostructure
(NS)
Phonon Shift
Raman
Spectrometer
Phonon Shift
Raman
Spectrometer
Acoustic
Phonon
shift
Acoustic
Phonon
shift
q
Frequency(ω)
Optical
Phonon
shift
Optical
Phonon
shift
q
Frequency(ω)
Bulk
/
NS
// acoptacoptacopt
2 types of
shifts
∆ω > 0 Blue-shift
∆ω < 0 Red-shift
Info on
size, dimensionality,
crystallanity
of nanostructuresPhonon shifts provide vital information about
Physical properties of nanostructures!!!
Phonon shifts provide vital information about
Physical properties of nanostructures!!!
38
39. Abhijeet Paul
Phonon shifts: Experimental benchmarking.
Acoustic hardening or blue-shift
in SiNWs
Acoustic hardening or blue-shift
in SiNWs
Optical softening or red-shift
in SiNWs
Optical softening or red-shift
in SiNWs
39
d
W
a
A 0
Connects to
dimensionality
of NS
Connects to the
shape of the
nanowire in 1D
MVFF
compares
with expts.
very well
Acoustic
d <1
for 1D. A >0
Optical
d >1
for 1D.
A < 0
MVFF provides correct trend for
phonon shifts ‘A’ and ‘d’ correlation can connect to
SiNW shape
MVFF provides correct trend for
phonon shifts ‘A’ and ‘d’ correlation can connect to
SiNW shape
40. Abhijeet Paul
Physical metrology of SiNWs
40
SiNW shapes under study
d
W
a
A 0
‘A’ and ‘d’ from
acoustic and optical phonon shifts correlate to
SiNW shape nanoscale metrology
‘A’ and ‘d’ from
acoustic and optical phonon shifts correlate to
SiNW shape nanoscale metrology
42. Abhijeet Paul
Heat SinkHeat Sink
BB
AA
Heat SourceHeat Source
Thermoelectric device
Need for tuning material thermal properties
42
HeatFlow
Thermal Capacitance
VCV thC
Equivalent
thermal circuit
Thermal Resistance
L
A
th
thR
Engineering material thermal properties can improve
system performance!!!
Engineering material thermal properties can improve
system performance!!!
Better Laser Cooling
Better Heat evacuation
in FETs.
Improved ZT in
thermoelectric devices
43. Abhijeet Paul
Strain: Tuning thermal conductivity
of SiNWs
Set-up
Expt.
Result
Gan et.al
Purdue University
MVFF simulations show similar
tuning for thermal conductivity
with strain.
MVFF simulations show similar
tuning for thermal conductivity
with strain.
Simulation
MVFF
A. Paul et. al, APL, 2011.
43
44. Abhijeet Paul
Engineering κl using strain in SiNW
Phonon
energy
range
0 -22 meV
Low
22-44 meV
Mid
44-65 meV
High
Strain type
Compressive Tensile (-2%+2%)
Uniaxial 36%34% 52%50% 12%13%
Hydrostatic 32%37% 56%45% 11%16%
44
Energy Spectral Contribution κl
κl increases
under
compressive
uniaxial strain
κl is weakly
sensitive to
hydrostatic
strain
Low and mid
range bands
responsible.
Low and high bands
oppose mid bands
overall negligible
change
Uniaxial strain
tunes κl more than
hydrostatic strain
in SiNWs!!
Uniaxial strain
tunes κl more than
hydrostatic strain
in SiNWs!!
45. Abhijeet Paul
Tuning Specific heat (Cv) of SiNWs using strain
45
Uniaxial strain
brings neglible
change to Cv
Very less change
In energy
contribution under
strain
Hydrostatic strain
brings large change
to Cv
Higher energy
bands contribute to
the change in Cv.
Hydrostatic strain tunes Cv more than uniaxial
strain in SiNWs !!!
Hydrostatic strain tunes Cv more than uniaxial
strain in SiNWs !!!
47. Abhijeet Paul
Porous crystalline Si for thermoelectricity
Hopkins et.al
Nano. Lett.,
2011.
Tang et.al
Nano Lett., 2010.
Yu et. al
Nature Nanotech.
2010
Electrical
Conductivity[1]
Electrical
Conductivity[1]
~1.5X Drop
Thermal
Conductivity[1]
Thermal
Conductivity[1]
~8X
Reduction
Experimental
structures
Experimental
structures
Experimental
results
Experimental
results
[1] Yu et. al Nature Nanotech., 2010.
Porous Silicon an attractive alternative for
RT thermoelectric material.
How about porous SiNWs ?
Porous Silicon an attractive alternative for
RT thermoelectric material.
How about porous SiNWs ?
47
48. Abhijeet Paul
Electronic and Phonon dispersion: Porous SiNW
Rh=0.4 nm
Dsep=0.2 to 1 nm
Hollow SiNW:
[100], W=4nm
Tight
Binding
Tight
Binding
Increase in Ec
more confinement
More flat bands
Suppression of
heat flow.MVFFMVFF
Increased electron and phonon confinement in
porous SiNWs compared to filled nanowire.
Increased electron and phonon confinement in
porous SiNWs compared to filled nanowire.
48
49. Abhijeet Paul
Electrical and thermal transport parameters
Landauer’s method with scattering
Electrical and thermal transport parameters
Landauer’s method with scattering
Electron and Phonon dispersionElectron and Phonon dispersion
Porous SiNWs:
Electronic and lattice contribution to ZT
~7% drop
~35% drop
T
)(
GS
ZT
l
2
e
Thermoelectric
Efficiency
Thermoelectric
Efficiency
PF (S2
G)
reduction ~49%
~55% drop
kl reduction ~55%
Interplay of PF and κl determine the final ZT !!!Interplay of PF and κl determine the final ZT !!!
49
Solid nanowire
50. Abhijeet Paul
Porous SiNW: Power-factor and ZT
~49%
drop
~7% rise
Large reduction in
Electrical power-factor
due to pores.
ZT improves due to large
suppression on lattice
thermal conductivity.
ZT in porous SiNW improves but at the
expense of electrical performance !!!
ZT in porous SiNW improves but at the
expense of electrical performance !!!
50
51. Abhijeet Paul
Conductance
Measurement
and simulations.
Conductance
Measurement
and simulations.
Silicon
nanowire
Physical metrology
How to determine size,
shape and orientation ?
Electrical metrology
How to determine
interface traps in SiNW
FETs?
Thermal properties
How to determine
thermal properties of
SiNW ?
Phonons
Lattice thermal
properties
Phonons
Lattice thermal
properties
Thermoelectricity
How to enhance PF
and ZT of SiNW ?
Porous SiNW
Enhance ZT
Porous SiNW
Enhance ZT
Raman
spectroscopy
Phonon shift
in SiNWs
Raman
spectroscopy
Phonon shift
in SiNWs
Integrated modeling approach sheds light on
many nano-scale aspects of SiNWs.
Integrated modeling approach sheds light on
many nano-scale aspects of SiNWs.
51
52. Abhijeet Paul
Key new findings and accomplishments
• Developed two new interface trap metrology methods in Si
trigated FinFETs.
»Methods are complimentary and repeatable.
(Published in JAP, 2011, IEEE EDL 2010, IEEE EDL 2009)
• Correlated the shape and size of SiNWs to phonon shifts
guides Raman Spectroscopy.
(Accepted in JAP 2011)
• Strain engineering of lattice thermal conductivity and specific heat
of SiNWs possible. (Published in APL, 2011)
• Possibility of using porous SiNWs for enhanced ZT (~6% rise) at
room temperature shown.
52
53. Abhijeet Paul
Outline of the talk
• Motivation
»Why the present work is important ?
»Need for integrated atomistic simulation framework
• Computational modeling and simulation approaches.
• Application of the methods to Si nanowires (SiNWs).
• Application to non-Si system GaAs a quick look !!
• Global dissemination of findings nanoHUB.org
• Summary
• Future directions
53
54. Abhijeet Paul
GaAs nanostructures:
Electronic and thermoelectric enhancement
54
SiNWSiNW
GaAs
[100]/(100)
~38% inc. in
ION for 4%
strain
p-type.
Integrated Modeling Approach
Integrated Modeling ApproachGa
GaAs NW
0%
2%
5%
kl = 1W/m-K [1]
~10% inc.
in ZT for
tensile strain
n-type
[1] Martin et al,
Nanoletters, 10, 2010
A. Paul et. al,
IEEE Nano,
2011
A. Paul et. al,
IEEE DRC,
2011
Integrated modeling performance enhancement of
GaAs nanostructures.
Integrated modeling performance enhancement of
GaAs nanostructures.
55. Abhijeet Paul
Outline of the talk
• Motivation
»Why the present work is important ?
»Need for integrated atomistic simulation framework
• Computational modeling and simulation approaches.
• Application of the methods to Si nanowires (SiNWs).
• Application to non-Si system GaAs a quick look !!
• Global dissemination of findings nanoHUB.org
• Summary
• Future directions
55
56. Abhijeet Paul
Global scientific outreach using nanoHUB.org
•C/C++ and Matlab based tools.
•Enables research in electronic
structure and thermoelectricity
56
Open research tool for fellow
researchers !!!
Open research tool for fellow
researchers !!!
BandStructure Lab
LANTEST Tool
Research Tools
Most popular tool on nanoHUB.
Over 3K users.
Till now ran 34503 simulations.
Has been cited 28 times in research.
57. Abhijeet Paul
Global semiconductor education
using nanoHUB.org
57
Semiconductor Educational Tools
Crystal Viewer Tool
Periodic Potential lab
• 6 C/C++ and MATLAB based
semiconductor physics tools
developed.
•Used in EE305
(Semiconductor Introduction)
at Purdue University
Users (last 12 months) = 887
Simulations (last 12 months) ~3K
Enabled dissemination of
device physics knowledge
globally.
Enabled dissemination of
device physics knowledge
globally.
58. Abhijeet Paul
Outline of the talk
• Motivation
»Why the present work is important ?
»Need for integrated atomistic simulation framework
• Computational modeling and simulation approaches.
• Application of the methods to Si nanowires (SiNWs).
• Application to non-Si system GaAs a quick look !!
• Global dissemination of findings nanoHUB.org
• Summary
• Future directions
58
59. Abhijeet Paul
Summary
• An integrated modeling approach
developed to study nanoscale devices.
• SiNWs :
»Electrical metrology trap extraction
method.
»Structural metrology Raman
spectroscopy phonon shift
»Thermal property tuning Phonon
confinement.
»Thermoelectricity Porosity control.
59
60. Abhijeet Paul
Summary
• GaAs:
»Compressive strain and body
scaling enhances ION of UTB p-
FETs.
»Tensile strain and orientation
enhances ZT of GaAs
nanowires.
• Global outreach for research
using nanoHUB.org.
60
61. Abhijeet Paul
Outline of the talk
• Motivation
»Why the present work is important ?
»Need for integrated atomistic simulation framework
• Computational modeling and simulation approaches.
• Application of the methods to Si nanowires (SiNWs).
• Application to non-Si system GaAs a quick look !!
• Global dissemination of findings nanoHUB.org
• Summary
• Future directions
61
62. Abhijeet Paul
Future directions
• Combining electrons and phonons for
better eletro-thermal understanding in
nano-scale devices.
• Increased device to system level
interaction for better design
optimizations.
62
http://www.comsol.com/papers/6801/
63. Abhijeet Paul
Future directions
• Inclusion of thermodynamics into
phonon calculations.
• Investigation of source to drain tunneling
for performance evaluation of ultra-short
MOSFETs.
63
Lattice thermal expansion
Si bulk
http://www.ioffe.ru/SVA/NSM/Semicond/Si
