PhD_10_2011_Abhijeet_Paul

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 PaulNetwork for Computational Nanotechnology

(NCN),Electrical and Computer Engineering

Purdue Universityemail: [email protected]

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

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

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

Abhijeet Paul

Mo

re Mo

ore(M

M)

Beyo nd

Si

More than Moore(MtM) Beyond CMOS

90nm90nm

65nm65nm

45nm45nm

32nm32nm

22nm22nm

Baselin

e CM

OS

: CP

U, M

emo

ry, Lo

gic

Analog RF

Analog RF SensorSensorThermo

electricityThermo

electricity Bio-chipsBio-chips

Added Functional Diversity: More interaction

Sensory parts:

Measuring and

sense surrounding

Brain:Computation and

Calculations

Hig

her

Valu

e S

yste

ms

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

Abhijeet Paul

ITRS

CMOS scaling challenges

Intel

Need higher

processing

Speed !!!

Need for f

aster

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 Moore6

Revolutionary Evolutionary

Abhijeet Paul

www.tellurex.com

Choudhary et. al, Nature nano. (2009)

Why thermoelectricity ???

Increasing

pollutio

n!!!

IEA, WEO, 2008

Increasing energy

demand!!!

Gelsinger et. al ISSCC 2001

Increasing IC

heat!!!

Nasty Problems Nasty Problems

On chip thermoelectric

cooling (BiTe SL)

DEER

Automobile wasteheat thermoelectricpower generation Green energy

Production bythermoelectricity

Green Solutions from thermoelectricity More than MooreGreen Solutions from thermoelectricity More than Moore7

Abhijeet Paul

Dimensional Scaling: CMOS 20

03

2005

2007

2009

2011

Strain Tec hno

lo gy

High- k

Meta

l Gate 3DF

ET

S

SiGe strained

High-KMG

From bulk planar to 3D nano-scale

SiNW FET

III-V FET

Graphene

CNT-FET

Non-Si3D FETs

are a solution

Non-Si3D FETs

are a solution

???

8

IBM

Abhijeet Paul

Dimensional scaling: Thermoelectricity

BiTe/PbTeQwell

Superlattice

SiGe/SiQDot

Superlattice

BiTe,PbTe Bulk

LAST

1950

2000

1990

1980

1970

1960

2010

Thermoelectric Material development Year line

Beg

in S

emico

nd

ucto

r use

(Bi,S

b) ,(Te,S

e ), Pb

Te , P

ho

no

n G

lass Electr o

n cry stal

Dresse lh

aus e t. al,

DO

S en

gg

.

PbTe Ddots

Si /SiGeNW SL

Si Nanowires

ZT ~1

1< ZT < 3ZT > 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 !!

Abhijeet Paul

Need for Atomic level modeling…

Si n-FinFETIMEC

H=65nm,W=25nmG. Tettamanzi et. al, EDL, 2009.

IntelSiGe pMOSFETIEDM 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 DotMaterial variation at atomic scale

Material variation at atomic scale

Atomic scale strain variationAtomic scale

strain variation



Abhijeet Paul

Need for integrated device modeling …

SiGe

SiSiO2

Treatment of multiple valleysTreatment 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

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 quantumbased simulators incl. fundamental parameters linked to electronicbandstructure 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

Abhijeet Paul

Outline of the talk

• Motivation

»Why the present work is important ?

»Need for integrated atomistic simulation framework





• Summary


13

Abhijeet Paul

How to study the nano-scale devices?

Bottom-up Approach

To nano-scale

devices

Bottom-up Approach

To nano-scale

devicesEle

ctro

nic

Stru

ctur

e Lattice Structure

Carrier Transport

AtomisticTight-binding

(TB)model


(TB)model

14

Abhijeet Paul

<111>

Atomistic Tight binding Approach

15

Crystal structureCrystal structure

FEATURES/ADVANTAGESNearest 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/ADVANTAGESNearest 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

OrbitalOrbitalInteractionInteraction

Assemble Assemble 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

Abhijeet Paul

Bulk Bandstructure using Tight-binding

Si

Lent et. al, Superlat. and Microstruc., 1986

Pb

Se

sp3d5s*-SO model

sp3d5-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

ZincblendeRocksalt

LL

16

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

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 = 9nmW = 25nmH = 14 nmSource/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

chargechargepotentialpotential

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

Abhijeet Paul

Experimental validation of atomistic simulator contd.

Electrical Potential Distribution.

Electron charge distribution

Self-consistent SimulationTerminal 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

Abhijeet Paul

How to study the nano-scale devices?

Bottom-up Approach

To nano-scale

devices

Bottom-up Approach

To nano-scale

devicesEle

ctro

nic

Stru

ctur

e Lattice Structure

Carrier Transport


(TB)model


(TB)model

Modified Valence Force Field (MVFF)

model


model

20

Abhijeet Paul

Phonon dispersion calculation:Modified VFF (MVFF) model

Old KeatingModel [1]

[A]

Bond-stretching(α)

Δr

[B]

Bond-bending(β)

Δθ

[C] Cross-bondstretch bend (γ)

[2] Zunger et. al. 1999ΔθΔr

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,

1993)

[D]

Δr1Δr2

Cross bondStretching (δ)

Short RangeShort Range

[1] Keating. Phys. Rev. 145, 1966.[2] PRB, 59,2881, 1999.[3] PRB, 48, 17938,1993

Long RangeLong 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.

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).

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 Si6 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

Abhijeet Paul

Approaches to study the nano-scale devices

Bottom-up Approach

To nano-scale

devices

Bottom-up Approach

To nano-scale

devicesEle

ctro

nic

Stru

ctur

e Lattice Structure

Carrier Transport


(TB)model


(TB)model


model


model

Landauer’s model (LM)Landauer’s model (LM)

24

Abhijeet Paul

Material A

Material B

How to analyze thermoelectric properties of materials ?

V1

V2

INOUT

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

TkV BLandauer’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

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

VT

Measure of thermoelectric

cooling (High)

Ability of material to conduct electricity `Electrical Conductance’Ability of material to conduct electricity `Electrical Conductance’

V

IG

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. S2G = Electronic Power Factor (PF)

ZT = ‘Thermoelectric Figure of Merit’ by Ioffe in 1949. S2G = Electronic Power Factor (PF)el

TGSZT

2

26

High ZT large G large S and small κ desired !!! High ZT large G large S and small κ desired !!!

Abhijeet Paul

Calculation of thermoelectric parameters

27

)(factor-Pre / lemLf

G,Sκe

(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

eLG 0 ee LLS 01 / ll L1

Abhijeet Paul

A closer look at electrons and phonons

28

max

0

)()()(

dM

T

F

LL BEphml

m Phonon IntegralPhonon Integral

Etop

FDel

m

B

em dEEM

E

EF

L

E

Tk

EfEL )(

)()(Electron IntegralElectron Integral

lemL / •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 !!!.

Abhijeet Paul

The complete approach set

Bottom-up Approach

To nano-scale

devices

Bottom-up Approach

To nano-scale

devicesEle

ctro

nic

Stru

ctur

e Lattice Structure

Carrier Transport


(TB)model


(TB)model


model


model

Landauer’s model (LM)Landauer’s model (LM)

Electronic PropertiesElectronic Properties

Thermal PropertiesThermal

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

Abhijeet Paul

Outline of the talk

• Motivation

»Why the present work is important ?

»Need for integrated atomistic simulation framework





• Summary

• Future direction

30

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.

CathodeLi2S

AnodeSiNW

Batteries [A]

Siliconnanowire

Solar cells [B]

Transistors [C]

Thermoelectricity [D]SiNWs have versatile applicationsand

are highly compatible to CMOS. Interesting system to study!!!

SiNWs have versatile applicationsand

are highly compatible to CMOS. Interesting system to study!!!

31

Abhijeet Paul

Nanoscale solutions in SiNWs

Siliconnanowire

Physical metrologyHow to determine size, shape and

orientation ?

Electrical metrologyHow to determine

interface traps in SiNW FETs?

Thermal propertiesHow to engineer

thermal properties of SiNW ?

ThermoelectricityHow to enhance PF and ZT of SiNW ?

??

32

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?

TemperatureBased 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 !!!

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

SimulatedTri-gated

Si n-FinFET

Schrodinger Eq.: 20 band sp3d5s*

model with spin orbit coupling for Si.

Schrodinger Eq.: 20 band sp3d5s*

model with spin orbit coupling for Si.

2D-Poisson Solution2D-Poisson Solution

ρ(r) V(r)

SimulationApproach SimulationApproach

Performed using OMEN-3Dpar

Channel with ~44K atoms(support from Sunhee Lee)

Goodmatch !!

???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 ?

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

FromCharge

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.

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

bV

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

Abhijeet Paul

ConductanceMeasurement

and simulations.


and simulations.

Siliconnanowire


orientation ?






??

37

Abhijeet Paul

Physical Metrology Raman Spectroscopy: A primer

Fre

qu

ency

(cm

-1)

Intensity (a.u)

Bulk Material

Nanostructure(NS)

Phonon Shift Raman

Spectrometer

Phonon Shift Raman

Spectrometer

Acoustic Phonon

shift

Acoustic Phonon

shift

q Fre

qu

ency

(ω)

Optical Phonon

shift

Optical Phonon

shift

q Fre

qu

ency

(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

Abhijeet Paul

Phonon shifts: Experimental benchmarking.

Acoustic hardening or blue-shiftin SiNWs

Acoustic hardening or blue-shiftin SiNWs

Optical softening or red-shift in SiNWs

Optical softening or red-shift in SiNWs

39

d

W

aA 0

Connects to dimensionality

of NS Connects to the

shape of the nanowire in 1D

MVFF compares

with expts. very well

Acousticd <1

for 1D. A >0

Opticald >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

Abhijeet Paul

Physical metrology of SiNWs

40

SiNW shapes under study

d

W

aA 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

Abhijeet Paul


and simulations.


and simulations.

Siliconnanowire


orientation ?






Raman spectroscopyPhonon shift

in SiNWs


in SiNWs

??

41

Abhijeet Paul

Heat SinkHeat Sink

BB

AA

Heat SourceHeat Source

Thermoelectric device

Need for tuning material thermal properties

42

Hea

t F

low

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 CoolingBetter Heat evacuation

in FETs.Improved ZT in

thermoelectric devices

Abhijeet Paul

Strain: Tuning thermal conductivity of SiNWs

Set-up

Expt.Result

Gan et.alPurdue University

MVFF simulations show similar tuning for thermal conductivity

with strain.

MVFF simulations show similar tuning for thermal conductivity

with strain.

SimulationMVFF

A. Paul et. al, APL, 2011.

43

Abhijeet Paul

Engineering κl using strain in SiNW

Phonon energy range

0 -22 meVLow

22-44 meVMid

44-65 meVHigh

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

compressiveuniaxial 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!!

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 !!!

Abhijeet Paul


and simulations.


and simulations.

Siliconnanowire


orientation ?






Strain tunes Phonon thermal

properties

Strain tunes Phonon thermal

properties

??


in SiNWs


in SiNWs

46

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

Abhijeet Paul

Electronic and Phonon dispersion: Porous SiNW

Rh=0.4 nmDsep=0.2 to 1 nm

Hollow SiNW: [100], W=4nm

TightBinding

TightBinding

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

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)(

GSZT

l

2

e

Thermoelectric Efficiency

Thermoelectric Efficiency

PF (S2G)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

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

Abhijeet Paul


and simulations.


and simulations.

Siliconnanowire

Physical metrologyHow to determine size, shape and orientation ?



Thermal propertiesHow to determine


Phonons Lattice thermal

properties

Phonons Lattice thermal

properties


Porous SiNW Enhance ZT

Porous SiNW Enhance ZT


in SiNWs


in SiNWsIntegrated modeling approach sheds light on

many nano-scale aspects of SiNWs.Integrated modeling approach sheds light on

many nano-scale aspects of SiNWs.

51

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

Abhijeet Paul

Outline of the talk







• Summary


53

Abhijeet Paul

GaAs nanostructures: Electronic and thermoelectric enhancement

54

SiNW SiNW

GaAs

[100]/(100)

~38% inc. in ION for 4%

strainp-type.

Integrated Modeling Approach

Integrated Modeling ApproachGa

As

GaAs NW

0%2%5%

kl = 1W/m-K [1]

~10% inc.in ZT for

tensile strainn-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.

Abhijeet Paul

Outline of the talk







• Summary


55

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 ToolResearch Tools

Most popular tool on nanoHUB. Over 3K users.Till now ran 34503 simulations.Has been cited 28 times in research.

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.

Abhijeet Paul

Outline of the talk







• Summary


58

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

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

Abhijeet Paul

Outline of the talk







• Summary


61

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/

http://www.comsol.com/papers/6801/

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 expansionSi bulk

http://www.ioffe.ru/SVA/NSM/Semicond/Si

Abhijeet Paul

Thank you!!!

64

Abhijeet Paul

Appendix A

• References for Acoustic phonon shift»Si-1/Si-2: T. Thonhauser et. al, PRB, 69, 2004. (T)

»Si-3: Hepplestone et. al., APL, 87, 2005. (T)

• References for Optical phonon shift:»Si-1: Hepplestone et. al., APL, 87, 2005. (T)

»Si-2: K. Adu et. al, App. Phys. A, 85, 2006. (E)

»Si-3: Sun et. al, PRB, 72, 2005. (T)

»Si-4: Campbell et. Al, Solid State Comm., 58, 1986. (T)

»Si-5: Zi et. Al, APL, 69, 1996. (T)

»Si-6: Yang et. Al, Jour. Phys. Chem., 112, 2008. (E)

»Si-7: Faraci et. Al, Journ. App. Phys., 109, 2011. (T)

T = Theory , E = Expt.

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