Internal Thermoelectric Effects andInternal Thermoelectric Effects andScanning Probe Techniques forScanning Probe Techniques forInorganic and Organic DevicesInorganic and Organic Devices

Kevin PipeKevin PipeDepartment of Mechanical EngineeringDepartment of Mechanical Engineering

University of MichiganUniversity of Michigan

e-

h+cool

cool

heat

cool

heat

cool

n

p

EC

EV

EFn

EFp

cool

cool

heat

cool

heat

cool

n

p

EC

EV

EFn

EFp

10 m10 m

CollaboratorsRajeev Ram (MIT) Ali Shakouri (UCSC) Li Shi (UT) Max Shtein (UM)

OutlineOutline

Heating in Electronic Devices

Thermoelectric Effects in DevicesThermoelectric cooling backgroundMicroscale thermoelectric coolersInternal cooling / integrated energy harvesting

Scanning Probe Techniques for Energy TransferScanning probes with active organic heterostructuresOLED probesExciton injection probes

Heating in ElectronicsHeating in Electronics

M. J. Ellsworth, (IBM), ITHERM 2004

Increasing transistor density and increasing clock speed have led to rapidly increasing chip temperature.

CMOS chips can have microscale hot spots with heat fluxes greater than 300 W/cm2.

Heating in power electronics and optoelectronics can be >1000 W/cm2.

Traditional thermoelectric coolers cool only ~ 10 W/cm2.

Intel Pentium® III Processor

Intel Itanium® Processor

Hotspots

C.-P. Chiu (Intel), “Cooling challenges for silicon integrated circuits”, SRC/SEMATECH Top. Res. Conf. on Reliability, Oct. 2004

Is it possible to generate targeted cooling or harvest

waste heat energy?

Hot plate

Nuclear reactor

P. B. M. Wolbert et al, IEEE Trans. Comp.-Aid. Des.Int. Circ. Sys. 13, 293 (1994)

Teng, H.-F. and S.-L. Jang, Solid-State Elect. 47, 815 (2003)S. J. Sweeney et al., IEEE J. Sel. Top. Quantum Elect. 9, 1325 (2003)

SOI MOSFET(lattice temperature)MOSFET channel (carrier

temperature)

Device-Internal Temperature GradientsDevice-Internal Temperature Gradients

GaAs/AlGaAs high-powerlaser (facet temperature)

Can energy from hot electrons in transistors or lasers (Auger) be harvested in an analogous manner to techniques in solar cells?

Large variation in carrier temperature (T≈1000K) and lattice temperature (T≈100K) can arise within active devices during operation.

TransistorIntel 90nm MOSFET

S. Sinha and K. E. Goodson, "Thermal conduction in sub-100 nm transistors," THERMINIC 2004

Predicted temperature distribution

5W/m3 heatsource over a

radius of 20nm

Semiconductor Laser

Facetheating

Bulkheating

10 mSEM

Facet temperaturecross-section

P. K. L. Chan et al., Appl. Phys. Lett. 89, 011110 (2006)

Can microscale hot spots be

cooled efficiently?

Device-Internal Temperature GradientsDevice-Internal Temperature Gradients

Heat sink

Cooling Methods for DevicesCooling Methods for Devices

Heat sink

Junction-down mounting(better device performance and lifetime

but has practical difficulties withelectrical contacts, etc.)

Junction-up mounting withdevice-internal thermoelectric cooling

(microscale cooling source with minimal processing impact)

Electronic structure of device optimized for

internal thermoelectric cooling

Heat sink

Junction-up mounting(difficult to remove heat)

Device

Substrate

Large heat sinks inefficient at cooling microscale hot spots

Heat sink

Monolithic integration with TE cooler(complicated processing)

Device

Substrate

Integratedthermoelectric

cooler

C. LaBounty, Ph.D. thesis, UC Santa Barbara (2001).

p-i-n diode

HIT cooler

Cooling Methods for DevicesCooling Methods for Devices

The operating current of a device causes thermoelectric heating/cooling at

every internal device layer junction

Internal thermoelectric effects in active devices can be used for both:

Targeted cooling of a critical region of the device, moving heat sources to the edge of the device where they are more easily conducted away

Energy harvesting using large gradients in lattice and carrier temperatures to reclaim electrical power

Heat sink

Junction-up mounting withdevice-internal thermoelectric cooling

(microscale cooling source with minimal processing impact)

Electronic structure of device optimized for

internal thermoelectric cooling

Recent Convergence ofRecent Convergence ofThermoelectric / Device MaterialsThermoelectric / Device Materials

(m*)3/2

(bulk thermoelectric figure-of-merit)

A. Shakouri and C. LaBounty, ICT, Baltimore, 1999.750 W/cm2 at 300KBiTe/SbTe SL

R. Venkatasubramanian et al., Nature 413 (2001)

680 W/cm2 at 345KSiGe/Si SL

A. Shakouri et al., IPRM (2002)

300 W/cm2 cooling at 300KInGaAs/InGaAsP SL

C. LaBounty et al., J. Appl. Phys. 89 (2002)

InGaAs/InGaAsP BarrierA. Shakouri et al., Appl. Phys. Lett 74 (1999)

Detectors, Mid-IR lasers

4x larger figure-of-meritHgCdTe Superlattice

R. Radtke et al., J. Appl. Phys. 86 (1999)

12x larger figure-of-meritGaAs/AlAs Superlattice

T. Koga et al., J. Comp.-Aid. Mat. Des. 4 (1997)

Thermoelectric Coolers

Transistors, lasers

High-speedtransistors, lasers

High-speed, high-power transistors

Active Devices

High-performance semiconductors have recently beenused to create superior thermoelectric devices

Conventional TE Cooler

Tcold

ThotH

ole

s

Ele

ctro

ns

np

I

I

_ECEF

Heatabsorbed

EV

EF

Heatabsorbed

+

_ECEF

Heatreleased

IEV

EF

Heatreleased

+

Z = 2

T2 (Thot-Tcold)max= ZT212

Optimump,n doping

• Electrical Conductivity (maximize current)• Thermal Conductivity (minimize thermal conduction)• Peltier Coefficient (maximize energy difference at contacts)

Thermoelectric figure-of-merit(sometimes written as ZT)

Internal Cooling of DevicesInternal Cooling of Devices

The operating current of a device causes thermoelectric heating/cooling at every internal device junction.

emitterbase

collector

n p n

Heterojunction Bipolar Transistor

cool

EC

EV

cool

cool

heat

cool

heat

cool

n

p

EC

EV

EFn

EFp

P-N Diode

EC

EV

EFnEFpp nn+

heat

heat

cool

cool

heat

heat

Semiconductor Laser Diode

_EC

EV

EF

metal n-type

cool

Thermoelectric Cooling

EC

EF

cool

HFET Channel

Diode Thermoelectric EffectsDiode Thermoelectric Effects

Tcold

ThotI

hole

s

elec

tron

s

np I

Iholes

electrons

npI I

TcoldThot Thot

The diode is the fundamental building block of most electronic and optoelectronic devices(transistors, lasers, amplifiers, etc.)

EC

EF

EV

n

p cool

cool

heat

heat

cool

cool

heatcool

heat

cooln

p

EC

EV

EFn

EFp

Conventional TE Cooler P-N Diode

K. P. Pipe, R. J. Ram, and A. Shakouri, "Bias-dependent Peltier coefficient and internal cooling in bipolar devices", Phys. Rev. B 66, 125316 (2002).

-150 -100 -50 0 50-20

-10

0

10

20

-150 -100 -50 0 5010

-10

100

1010

1020

-150 -100 -50 0 50-2

-1.5

-1

-0.5

0

0.5

1

1.5

Measurement of Bipolar Thermoelectric EffectMeasurement of Bipolar Thermoelectric Effect

Unbiased GaAs diode: ND = 5×1018 cm-3, NA = 1×1019 cm-3

Ene

rgy

(eV

)

Position (nm)

Car

rier

Con

cent

ratio

n (c

m-3)

holes

electronsPosition (nm)

Th

erm

oele

ctric

Vol

tag

e (

mV

)

Measurement

Theory

Voltage measured using SThEM,an STM-based technique

Carrier transport calculated with self-consistentdrift-diffusion / Poisson equation software

EF

EC

EV

Position (nm)

p n

Built-inpotential

>0for holes

<0for electrons

4x bulkvalue

10x bulkvalue

• First observation of enhanced thermoelectric effect due to minority carriers• Most active devices use minority carriers for operation

H.-K. Lyeo, A.A. Khajetoorians, L. Shi, K.P. Pipe, R.J. Ram, A. Shakouri,and C.K. Shih. Science 303, 816 (2004)

Alloys in DevicesAlloys in DevicesAlloys with different bandgaps are addedbetween the p-type and n-type regions:

• One alloy traps electrons and holes so that they overlap and recombine to emit light. • Another alloy provides refractive index contrast so that light is confined.

n

pEC

EV

electrons

holes

Semiconductor Laser

Lasers are typicallybiased to “flat-band”

EC

EV

EFn

EFpP N

N+

Electroninjection

QW

Electronleakage

Hole leakageHole injection

radiation

(substrate)

Electron/hole injection current Thermoelectric heatingElectron/hole leakage current Thermoelectric cooling

Quantum well temperatureis critical to laser operation

Optimizing Thermoelectric HeatOptimizing Thermoelectric HeatExchange DistributionExchange Distribution

x

The

rmoe

lect

riche

at

exch

ange

Conventional Design Injection Current Internally Cooled Light Emitter

x

Active region cooling

K. P. Pipe, R. J. Ram, and A. Shakouri, “Internal cooling in a semiconductorlaser diode”, IEEE Phot. Tech. Lett. 14, 453 (2002).

EC

EV

EFnEFpP NN+

heat

heat

cool

cool

heat

heat

EC

EV

EFn

EFp

P

N N+

cool

cool heat

heat less cool

less cool

The

rmoe

lect

riche

at

exch

ange

QW

0 200 400 600 800 1000 1200

20

25

30

35

40

Current Density (A/cm2)

Tem

pe

ratu

re (

oC

)

Conventional

Optimized

Optimizing Thermoelectric HeatOptimizing Thermoelectric HeatExchange DistributionExchange Distribution

K. P. Pipe, R. J. Ram, and A. Shakouri, “Internal cooling in a semiconductorlaser diode”, IEEE Phot. Tech. Lett. 14, 453 (2002).

18% reduction in operating temperature

GaInAsSb-based laser simulation

QW

Injection Current Internally Cooled Light Emitter

x

Active region cooling

EC

EV

EFn

EFp

P

N N+

cool

cool heat

heat less cool

less cool

The

rmoe

lect

riche

at

exch

ange

Internal Cooling of TransistorsInternal Cooling of Transistors

Optimizing for thermoelectric/thermionic cooling could reduce device heating.

W. Y. Zhou, Y. B. Liou and C. Huang, Solid-State Electron. 38, 1118 (1995)

E. Pop, S. Sinha, and K. E. Goodson, IMECE 2002

cooling

Boltzmann transport simulationof AlGaAs/GaAs HBT

(he

ats

ink

at

em

itte

r)

(he

ats

ink

at

co

lle

cto

r)

EC

EF

HFET Channel

emitterbase

collector

n p n

Heterojunction Bipolar Transistor

cool

EC

EV

Remove hot electronsby thermionic emission

Could energy from microscaledevice waste heat be harvested?

Thermoelectric PowerThermoelectric PowerGenerationGeneration

A temperature difference applied across a material causes a net motion of charge and hence an open-circuit voltage to develop.

n

n-type material: electrons are majority carriers, Sn < 0

T

T+T

elec

tro

ns

V = SnT+

p

T

T+T

ho

les

V = SpT+

p-type material: holes are majority carriers, Sp > 0

S = “Seebeck coefficient” [V/K]

= “Peltier coefficient” = TS [V]

Induced voltage measured from cold to hot end

Thot

n

Tcold

+ pVn

Rn + Vp

Rpn p

Thot

n

Tcold

+ pVn

Rn + Vp

Rpn p

THot

TColdRLoad RLoad

+_

Vtot = a×(Vn+Vp)

Rtot = a×(Rn+Rp)

Attaching a load to a thermoelectric generator causes current to flow.

a = # of n / p pairs

Thermoelectric Power GeneratorThermoelectric Power GeneratorEfficiencyEfficiency

For an optimized TE device with a matched load (Rload = RTE),

TH

TC

RLoad

QH

I

=I2RLoad

QH

opt=TH - TC

TH

M - 1

TCTH

M +

M = 1 + Z TH + TC

2

where

Z =S2k

Thermoelectric figureof merit ZT averaged

over the operatingtemperature range

Carnot efficiency

Efficiency CurvesEfficiency Curves

TCold (K)TCold (K)

TCold (K)TCold (K)

TH

ot -

TC

old (

K)

TH

ot -

TC

old (

K)

TH

ot -

TC

old (

K)

TH

ot -

TC

old (

K)

Efficiency(%)

ZT = 1 ZT = 2

ZT = 3 ZT = 4

IncreasingCarnot

efficiency

In order to generate significant power density, device must maintain a large T (high ) or have a high heat flux. These two effects are linked.

Efficiency Increase with Increasing Heat FluxEfficiency Increase with Increasing Heat Flux

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Q/A (W/cm2)

(cm

2 K/W

)T

hic

kn

es

s

Th

erm

al C

on

du

cti

vity

Eff

icie

nc

y

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

Q/A (W/cm2)

(cm

2 K/W

)T

hic

kn

es

s

Th

erm

al C

on

du

cti

vity

Th

ick

ne

ss

Th

erm

al C

on

du

cti

vity

Eff

icie

nc

y

ZT = 2

As heat flux Q/A increases, T = Thot -Tcold increases, and therefore the efficiency increases.

Assuming 1D heat flow,

T =LQkA

L: Thickness of TE generatorQ: Heat sourcek: Thermal conductivityA: Cross-sectional area

≈Lk

10-5 to 10-2 cm10-2 to 10-1 W/cmK

For most devices made from (nanostructured) TE materials with high ZT,

Increasing heat flux

Increased Efficiency for Energy ConversionIncreased Efficiency for Energy Conversionfrom Small Hot Spots Using Small TE Generatorsfrom Small Hot Spots Using Small TE Generators

RL2

Small one-leg generatorfor each heat source

TCold

QH

3(each)

Net area reduced to A2

TCold

RL1

One-leg generator

QH

Area A1

In systems with micro/nanoscale heat sources, efficiency can be improved by employing targeted micro/nanoscale thermoelectric generators which only enclose the individual heat sources, reducing the total cross-sectional area and therefore increasing the heat flux QH/A.

Same QH

Wastedheat

Wastedheat Larger

TH-TC

What systems have micro/nanoscaleheat sources with high heat flux?

I2RL1I2RL2

IntelItanium®

Processor

Device-Level ThermoelectricDevice-Level ThermoelectricGeneration MethodsGeneration Methods

Microscale thermoelectric energy harvester monolithically integrated with device

High performance chips typically have strong heat sinking which could maintain a significant temperature gradient across the TE generator.

Increase in device temperature could be outweighed by energy savings.

Device-External

Devices can have large internal heat fluxes and temperature gradients due to high-power operation, low thermal conductivity regions, etc.

Is it possible to perform energy harvesting directly at heat sources by integrating thermoelectric structures into the device design (band structure) itself?

Device-Internal

Device

RLoad

VDevice

+ -

Substrate

Thermoelectric Generator

Device

VDevice

+ -

RLoadQH

Heat sink

C. LaBounty, Ph.D. thesis,UC Santa Barbara (2001)

0 25 50 75 1000

5

10

15

20

Temperature ( C)T

hre

sho

ld C

urr

ent

(mA

)

Slo

pe

Eff

icie

nc

y (W

/A)

0.4

0.3

0.2

0.1

0 25 50 75 1000

5

10

15

20

Temperature ( C)T

hre

sho

ld C

urr

ent

(mA

)

Slo

pe

Eff

icie

nc

y (W

/A)

0.4

0.3

0.2

0.1

flat

QD lasers canhave small

temperaturedependence

(data from P. Bhattacharya)

Until now we have examined energy conversion within active devices.

Now we will look at scanning probe techniques for energy transfer from an active device to a sample.

RadiationWaveguided

Surface Plasmon

Leaky mode (Radiation)

WaveguidedSPP

Si Substrate

Anode

HTL

ETL

Cathode V

+-

Dec

ay r

ate

(a.u

.)

Cathode: 18nm AgETL: 60nm Alq3

HTL: 50nm -NPDAnode: 100nm Al / 13nm NiSubstrate: Silicon

1 2 3 4 5

x 10-3

1.8

2.2

2.6

x 10-3

kx / 2

/(

2c)

Leakymode

Waveguided

Surface plasmon-polariton

kx

520nm spectrum

• The amount of dipole energy that goes to a specificmode can be tailored by changing layer materials and thicknesses

• By placing an active device on a scanning probe, we can couple this energy to a sample.

Energy Outcoupling from ActiveEnergy Outcoupling from ActiveOrganic DevicesOrganic Devices

520nm

Si Cantilever

+-

Cathode

Active Layers

Insulator

Anode

Tipless Cantilever

OLED on an AFM CantileverOLED on an AFM Cantilever

35 m

6 m

Light Emission from the OLED

Light emissionfrom the OLED edge

K. H. An et al., Appl. Phys. Lett. 89, 111117 (2006)

SummarySummary

• Recent advances in thermoelectrics have produced large cooling powers over micron-scale regions.

• Every junction in a device has thermoelectric heating or cooling.

• The bipolar nature of active devices can lead to enhanced thermoelectric effects.

• The optimization of internal thermoelectric effects can lead to targeted cooling inside a device.

• Large temperature gradients in devices can potentially be used for thermoelectric conversion of waste heat into electricity.

• Active devices placed on cantilevers can be used to couple energy to a sample.