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Thermoelectric Energy Transport in Nanostructures
Ali ShakouriBaskin School of Engineering, University of California, Santa CruzHttp://quantum.soe.ucsc.edu
Int. Workshop on Nanoscale Energy Conversion and Information Processing DevicesNice, France, 24 September 2006
AcknowledgementPostdocs/Students: Zhixi Bian, James Christofferson, Mona Zebarjadi, Rajeev Singh,
Xi Wang, Daryoosh Vashaee, Yan Zhang, Kazuhiko Fukutani, Tammy Humphrey
Collaborators: John Bowers, Art Gossard, Arun Majumdar, Venky Narayanamurti, Rajeev Ram, Tim Sands, Avi Bar-Cohen, Stefan Dilhaire, Ed Croke, Peidong Yang, Holger Schmidt
Sponsors: ONR/MURI, Intel, Canon, National, Packard Foundation, DARPA/Heretic, NSF
2
AS 9/24/2006Motivation: Microprocessor Evolution
Source: IntelSource: Intel
1,000,0001,000,000
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1 Billion 1 Billion TransistorsTransistors
80868086
8028680286i386i386
i486i486PentiumPentium®®
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PentiumPentium®® IIII
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Source: IntelSource: Intel
1,000,0001,000,000
100,000100,000
10,00010,000
1,0001,000
1010
100100
11
1 Billion 1 Billion TransistorsTransistors
80868086
8028680286i386i386
i486i486PentiumPentium®®
KK
PentiumPentium®® IIII
’’7575 ’’8080 ’’8585 ’’9090 ’’9595 ’’0000 ’’0505 ’’1010
PentiumPentium®® IIIIIIPentiumPentium®® 44
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Electronic/Optoelectronic devices → Generate high/ localized heat density
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Possible ApplicationsPossible Applications
• Waste heat recovery
• Electric power generator with no moving part
• Microscale power sources
Direct Conversion of Heat into ElectricityDirect Conversion of Heat into Electricity
Significant amount of heat generated as by product of any energy conversion.
Thermal
Electrical
Optical
Magnetic Mechanical
Chemical/biological
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RejectedEnergy 61%
Total 91.4 quad(↑ x3) 1950
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ab
a
I
Q Q
ab a b Q
I
STdS
dTT
Peltier:
Peltier and Seebeck Effects Peltier and Seebeck Effects
Thomson:
Commercial TE Module• T=72C • Cooling density <10W/cm2
• Efficiency 6-8% of Carnot
RTGs (space power)
S VT
Seebeck: ab
V
T1T2
a
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AS 9/24/2006Efficiency of TE Power GenerationEfficiency of TE Power Generation
ZT = 1.2-3.6
ZT = 0.3-0.9
Z S2
Z (Seebeck)2 (electrical conductivity)
(thermal conductivity)
Efficiency (COP) depends on a single ratio (Z)
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AS 9/24/2006
PbTe/PbTeSe Quantum DotSuperlattices
Ternary: ZT=1.3-1.6Quaternary: ZT=2T=43.7 K, Bulk T=30.8 KT.C. Harman, Science, 2002
T=32.2 K, ZT ~2-2.4R. Venkatasubramanian, Nature, 2001
Nanostructure Bulk
Power Factor (W/cmK2) 25.5 28 40 50.9Thermal Conductivity (W/mK) 0.5 2.0 0.5 1.26
PbTe/PbSeTe Bi2Te3/Sb2Te3 Superlattice Bulk
In-plane geometry Cross-plane geometry
(From M. S. Dresselhaus, Rohsenow Symposium, 2003)
Superlattices/ Quantum Dot Thermoelectrics T. C. Harman (2002) and R. Venkatasubramanian (2001)
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AS 9/24/2006Thermionic Emission for Energy Conversion
Cathode Barrier Anode
Energy
Hot electron
Cold electron
Metal/Semiconductor Superlattice, Embedded Nanostructures
Low work function Vacuum (ions) Low work function
Metal/ Deg. Semicond
Metal/ Deg. Semicond
Solid-State
Vacuum
HotHot ColdCold
• Selective emission of hot electrons over a potential barrier can generate electrical power from temperature difference •Thermodynamic reverse process: evaporative cooling of electrons
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Si/SiGeC Superlattice Structures for Heterostructure Thermionic Filtering
150x SiGeC/Si Superlattice(10nm/10nm)
Barrier
SiCathode
Si (001) SubstrateAnode
• MBE Grown 5” Substrate
• Material and Processing Compatible with SiGe HBTs. 1 µm
Hot Electron
Cold Electron
Funded by ONR and DARPA/ARMY HERETIC
Si
Si0.89Ge0.1C0.01
X. Fan, E.Croke, J.E. Bowers, A. Shakouri, et al., “SiGeC/Si superlattice micro cooler,” Applied Physics Lett. 78 (11), 2001. Featured in Nature Science Update, Physics Today, AIP April 2001
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AS 9/24/2006Microrefrigerator on a chip
• Temperature resolution: 0.006oC• Spatial resolution: submicron
High resolution thermal imaging
Thermal imaging camera; J. Christofferson, A. Shakouri, Review of Scientific Instruments Feb 2005. Nanoscale heat transport and microrefrigerators on a chip; A. Shakouri, Proceedings of IEEE, 2006
• Maximum cooling: 4C (300K), 12 (500K)
• Cooling power density: >500 W/cm2
• Response time: < 20-40s• Materials: SiGe, SiGeC, InGaAs, InP• Fabrication: IC compatible
ZT~0.08-0.1
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J. Snyder (2003) http://www.its.caltech.edu/~jsnyder/thermoelectrics/science_page.htm
I
Z S2
Z (Seebeck)2 (electrical conductivity)
(thermal conductivity)For almost all materials, if doping is increased, electrical conductivity increases but Seebeck coefficient is reduced.
S
S2
How to improve ZT?How to improve ZT?
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Energy
Density of States
Ef High doping
Doped Bulk Semiconductor/ Metal
Highly-Doped Tall Barrier Superlattice
Ef
Ebarrier
Metallic Superlattices for Thermionic Energy Conversion
Distance
Energy
Symmetry of DOS near Fermi energy is the main factor determining Seebeck coefficient.
Ef Low doping
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Program Manager: Mihal Gross
D. Vashaee., A. Shakouri, Physical Review Letters March 12, 2004
Non-planar Barrier
UCSC Berkeley Harvard MIT NCSU Purdue UCSB
Director:A. Shakouri
ZT for metallic superlattices with non-planar barriersZT for metallic superlattices with non-planar barriers
Thermionic Energy Conversion Center MURIThermionic Energy Conversion Center MURI Assume: lattice=1W/mK, mobility ~10 cm2/Vs
Planar Barrier
Planar barriers are not ideal for hot electron filtering. ZT>5 is possible with metallic structures with non-planar barriers.
Hot and cold electrons in equilibrium
Hot electron filter
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TEM/HAADF of Semimetallic ErAs Nanoparticles in InGaAs Matrix
In,GaAsEr
STEM images show that the ErAs particles have the rock salt structure. The As sublattice is continuous across the interface.
STEM images show that the ErAs particles have the rock salt structure. The As sublattice is continuous across the interface.
110
001
1nm
D. O. Klenov, D. C. Driscoll, A. C. Gossard, S. Stemmer, Appl. Phys. Lett. 86, 111912 (2005)
HAADFHAADF
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0.4 ML 40 nm
0.1 ML 10 nm
In0.53Ga0.47As
Kim et al., Physical Review Letters, 30, 045901 (2006)
In0.53Ga0.47As
0.3 % ErAs
3.0 % ErAs
3.0 % ErAs:In0.53Ga0.28Al0.19As
Thermal Conductivity of ErAs:InThermal Conductivity of ErAs:In0.530.53GaGa0.470.47AsAs
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AS 9/24/2006ErAs: InGaAs/InGaAlAs SL
n-InP substrate
50nm 5E18 n-InGaAs
20nm n-InGaAs/ErAs 0.3%
10nm InGaAlAs
20nm n-InGaAs Cap layer
• Sample 1– 1E19
• Sample 2 – 4E18
• Sample 3 – 2E18
Add superlattice energy filtering to increase the thermoelectric power factor.
Joshua Zide, Daryoosh Vashaee, Gehong Zeng et al., submitted to PRB 2006
70x
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AS 9/24/2006Cross-plane/ In-plane Seebeck Characterization
J. Zide et al., (UCSB, UCSC) submitted to Physical Review B, 2006
Theory/Experiment Seebeck II, ┴ (300K)
ErAs: InGaAs/InGaAlAs Superlattices
0
0.5
1
1.5
2
300 400 500 600 700 800
2e184e186e188e181e19
ZT
Temperature (K)
1e19 cm-3
2e18 cm-3
6e18 cm-3
Theoretical ZT
ZT
Temperature (K)
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AS 9/24/2006Thermoelectric single element characterizationThermoelectric single element characterization
• ErAs generates significantly more power despite the lower effective
Seebeck
• BiTe degrades rapidly at higher temperatures while ErAs improves with
temperature
0 50 100 150 200 250 300 3500
1
2
3
4
5
T (K)Pow
er
densi
ty (
W/c
m2)
-ErAs (SL+substrate)-SiGe (SL+substrate)-BiTe (bulk)
160x(10nm (InGaAs)0.6(InAlAs)0.4/20nm (n-InGaAs)0.97Er0.03) on 474 m doped InP substrate
200x(75Å SiGe0.16/ 75Å SiGe0.24) on 403 m doped Si substrate
Thot
V+
V-
Heater
OFHC copper
OFHC copper
TE sample
Al cold plate
Chilled water
Ceramic rails(insulating)
Peter Meyer, Rajeev Ram (MIT)
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AS 9/24/2006Thin film array generator 200 n-p couples, 5-10 microns ErAs:InGaAs/InAlAs superlattice thin films, 120x120m2, 12 ohm load
G. Zeng, J. Bowers, et al. (UCSB)Appl. Physics Letters 2006
0
0.2
0.4
0.6
0.8
1
1.2
0 20 40 60 80 100 120 140
400 element generator array10 m x 120 m x 120 m (R
L = 12 Ohm)
heat up (W/cm2)cooling down (W/cm2)
Po
wer
(W
/cm
2 )
T (K)
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AS 9/24/2006Monte Carlo + Poisson Equation
InGaAs InGaAsP InGaAsH
eat Sin
kA
nod
eBias
Hot
Sou
rceC
athod
e
Cathode contactlayer
Anode contact layer
Barrier (main-layer)
+ Electron-phonon energy exchange (S) TJ
SJ
ph
ph
.
Goal: Range of validity for thermoelectric and thermionic transport formalisms
Mona Zebarjadi, Keivan Esfarjani, Ali Shakouri (UCSC)
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-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.25 0.5 0.75 1 1.25 1.5 1.75
length (um)
En
erg
y e
xch
an
ge
(1
E-9
W/C
m^2
)Electron-phonon energy exchange (S)
Peltier Cooling Peltier Heating
Non-equilibrium transport in the barrier
Energy relaxation length in cathode
Energy relaxation length in anode
Mona Zebarjadi, Keivan Esfarjani, Ali Shakouri (UCSC)
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Effective Seebeck Coefficient vs. Barrier Thickness
200
220
240
260
280
300
320
340
0 500 1000 1500 2000
Length(nm)
seeb
eck
(uv/
K)
Convectional Thermoelectric transport
Convectional Thermionic transport
Mona Zebarjadi, Keivan Esfarjani, Ali Shakouri (UCSC)
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Summary• Micro-refrigerator on a chip• Cooling 4 -7C , >500W/cm2, 20-40s
• Solid-state thermionic energy conv.Metallic SL and embedded nanoparticles
• ErAs: lattice thermal conductivity 6→ 2-3 W/mK • Increase ┴ Seebeck coefficient 200→600V/K • Power generation 1 element >5W/cm2 for T=300C
• Improvement in ZT: decouple S, , k
• Microscopic origin of TE/TI– Location and spatial extent of regions where
Peltier cooling/ heating occurs– Transition from TE to TI transport
• Statistical properties of reservoirs
Students/postdocsUCSC Zhixi Bian, Rajeev Singh, Mona Zebarjadi, Yan Zhang Younes Ezzahri, Daryoosh Vashaee,Tammy Humphrey
Berkeley Woochul KimSusanne Singer
Harvard Kasey Russel
MIT Peter Mayer
Purdue Vijay Rawat
UCSB Josh Zide, Gehong Zeng, J-H Bahk
Acknowledgement: ONR MURI (Dr. Mihal Gross), Packard, DARPA, Intel, Canon
SUMMARY
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kBTh ~ 75 meV kBTc ~ 25 meV
Why there is Carnot limit?
T=900K T=300K
Average Random Kinetic Energy of Carriers
If an electron is moved from hot reservoir to cold reservoir with “no dissipation”, on the average the maximum amount of energy per electron available to do work is: (KBTh-KBTc)/KBTh = (Th-Tc)/Th Carnot limit
Ali Shakouri, TE, TI and TPV energy conversion, MRS Fall 2005
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kBTh ~ 75 meV kBTc ~ 25 meV
hBlackBody~ 400 meV
Thermoelectric/Thermionic vs. TPV
0
2000
4000
6000
8000
10000
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
I (W
m-2
m
-1)
E (eV)
np
hBlackBody~ 125 meV
T=900K T=300K
Photons emitted from hot source have higher average energy than electrons emitted at the same temperature.
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T=625C T=25C
Photon-Assisted Thermionic Power Generation
Solid State TI
Possibility to use both hot electrons and hot photons?Ali Shakouri, TE, TI and TPV energy conversion, MRS Fall 2005
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Possibility to use phase transition (change in internal degrees of
freedom, latent heat) in electron gas to improve TE energy conversion
efficiency?
Is there room temperature phase change for electrons?