Internal Thermoelectric Effects and Scanning Probe Techniques for Inorganic and Organic Devices

Internal Thermoelectric Effects and

Scanning Probe Techniques for

Inorganic and Organic Devices

Kevin Pipe

Department of Mechanical Engineering

University of Michigan

e
-
h
+
cool
cool
heat
cool
heat
cool
n
p
E
C
E
V
E
Fn
E
Fp
cool
cool
heat
cool
heat
cool
n
p
E
C
E
V
E
Fn
E
Fp
10

m
10

m
Collaborators

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

Outline

Heating

in

Electronic

Devices


Thermoelectric

Effects

in

Devices


Thermoelectric

cooling

background


Microscale

thermoelectric

coolers


Internal

cooling

/

integrated

energy

harvesting


Scanning

Probe

Techniques

for

Energy

Transfer


Scanning

probes

with

active

organic

heterostructures


OLED

probes


Exciton

injection

probes

Heating 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/cm
2
.


Heating

in

power

electronics

and

optoelectronics

can

be

>
1000

W/cm
2
.


Traditional

thermoelectric

coolers

cool

only

~

10

W/cm
2
.

Intel Pentium
®

III Processor

Intel Itanium
®

Processor

Hot

spots

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 Gradients

GaAs/AlGaAs high
-
power

laser (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

(
D
T
≈
1000
K)

and

lattice

temperature

(
D
T
≈
100
K)

can

arise

within

active

devices

during

operation
.


Transistor

Intel 90nm MOSFET

S
.

Sinha

and

K
.

E
.

Goodson,

"Thermal

conduction

in

sub
-
100

nm

transistors,"

THERMINIC

2004

Predicted temperature distribution

5W/

m
3

heat

source over a

radius of 20nm


Semiconductor

Laser


Facet

heating

Bulk

heating

10

m
SEM
Facet temperature

cross
-
section

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

Can microscale
hot spots be
cooled
efficiently?

Device
-
Internal Temperature Gradients

Heat sink
Cooling Methods for Devices

Heat sink

Junction
-
down mounting

(better device performance and lifetime

but has practical difficulties with

electrical contacts, etc.)

Junction
-
up mounting with

device
-
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

Integrated

thermoelectric

cooler

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

p
-
i
-
n

diode

HIT cooler

Cooling 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 with

device
-
internal thermoelectric cooling

(microscale cooling source with minimal processing impact)

Electronic structure of
device optimized for
internal thermoelectric
cooling

Recent Convergence of

Thermoelectric / Device Materials


(m*)
3/2

l

(bulk thermoelectric figure
-
of
-
merit)

A. Shakouri and C. LaBounty, ICT, Baltimore, 1999.

750 W/cm
2

at 300K

BiTe/SbTe SL

R.

Venkatasubramanian et al.,
Nature

413

(2001)

680 W/cm
2

at 345K

SiGe/Si SL

A. Shakouri et al., IPRM (2002)

300 W/cm
2

cooling at 300K

InGaAs/InGaAsP SL

C. LaBounty et al.,
J. Appl. Phys.

89

(2002)

InGaAs/InGaAsP Barrier

A. Shakouri et al.,
Appl. Phys. Lett

74

(1999)

Detectors,

Mid
-
IR lasers

4x larger figure
-
of
-
merit

HgCdTe Superlattice

R. Radtke et al.,
J. Appl. Phys.

86

(1999)

12x larger figure
-
of
-
merit

GaAs/AlAs Superlattice

T. Koga et al.,
J. Comp.
-
Aid. Mat. Des.

4

(1997)

Thermoelectric Coolers

Transistors, lasers

High
-
speed

transistors, lasers

High
-
speed, high
-

power transistors

Active Devices

High
-
performance semiconductors have recently been

used to create superior thermoelectric devices

Conventional TE Cooler

T
cold

T
hot

Holes

Electrons

n

p

I

I

_

E
C

E
F

Heat

absorbed

E
V

E
F

Heat

absorbed

+

_

E
C

E
F

Heat

released

I

E
V

E
F

Heat

released

+

Z =

s P
2

l
T
2

(T
hot
-
T
cold
)
max
= ZT
2

1

2

Optimum

p
,
n

doping

•

Electrical Conductivity
s

⡭慸(浩m攠捵cr敮琩

•

Thermal Conductivity
l

⡭楮業楺攠瑨敲浡氠捯湤畣瑩潮c

•

Peltier Coefficient
P

(maximize energy difference at contacts)

Thermoelectric figure
-
of
-
merit

(sometimes written as ZT)

Internal Cooling of Devices

The

operating

current

of

a

device

causes

thermoelectric

heating/cooling

at

every

internal

device

junction
.

emitter

base

collector

n

p

n

Heterojunction Bipolar Transistor

cool

E
C

E
V

cool

cool

heat

cool

heat

cool

n

p

E
C

E
V

E
Fn

E
Fp

P
-
N

Diode

E
C

E
V

E
Fn

E
Fp

p

n

n
+

heat

heat

cool

cool

heat

heat

Semiconductor Laser Diode

_

E
C

E
V

E
F

metal

n
-
type

cool

Thermoelectric Cooling

E
C

E
F

cool

HFET Channel

Diode Thermoelectric Effects

T
cold

T
hot

I

holes

electrons

n

p

I

I

holes

electrons

n

p

I

I

T
cold

T
hot

T
hot

The diode is the fundamental building block of most electronic and optoelectronic devices

(transistors, lasers, amplifiers, etc.)

E
C

E
F

E
V

n

p

cool

cool

heat

heat

cool

cool

heat

cool

heat

cool

n

p

E
C

E
V

E
Fn

E
Fp

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
50
10
-10
10
0
10
10
10
20
-150
-100
-50
0
50
-2
-1.5
-1
-0.5
0
0.5
1
1.5
Measurement of Bipolar Thermoelectric Effect

Unbiased GaAs diode: N
D

= 5
×
10
18

cm
-
3
, N
A

= 1
×
10
19
cm
-
3

Energy (eV)

Position (nm)

Carrier Concentration (cm
-
3
)

holes

electrons

Position (nm)

Thermoelectric Voltage (mV)

Measurement

Theory

Voltage

measured

using

SThEM,

an

STM
-
based

technique

Carrier transport calculated with self
-
consistent

drift
-
diffusion / Poisson equation software

E
F

E
C

E
V

Position (nm)

p

n

Built
-
in

potential

P
>0

for holes

P
<0

for electrons

4
x

bulk

value

10x bulk

value

•

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 Devices

Alloys with different bandgaps are added

between 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

p

E
C

E
V

electrons

holes

Semiconductor Laser

Lasers are typically

biased to “flat
-
band”

E
C

E
V

E
Fn

E
Fp

P

N

N
+

Electron

injection

QW

Electron

leakage

Hole leakage

Hole injection

radiation

(substrate)

Electron/hole injection current

Thermoelectric heating

Electron/hole leakage current

Thermoelectric cooling

Quantum well temperature

is critical to laser operation

Optimizing Thermoelectric Heat

Exchange Distribution

x

Thermoelectric

heat exchange

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 semiconductor

laser diode”,
IEEE Phot. Tech. Lett.

14
, 453 (2002).

E
C

E
V

E
Fn

E
Fp

P

N

N
+

heat

heat

cool

cool

heat

heat

E
C

E
V

E
Fn

E
Fp

P

N

N
+

cool

cool

heat

heat

less cool

less cool

Thermoelectric

heat exchange

QW

0
200
400
600
800
1000
1200
20
25
30
35
40
Current Density (A/cm
2
)
Temperature (
o
C
)
Conventional
Optimized
Optimizing Thermoelectric Heat

Exchange Distribution

K. P. Pipe, R. J. Ram, and A. Shakouri, “Internal cooling in a semiconductor

laser 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

E
C

E
V

E
Fn

E
Fp

P

N

N
+

cool

cool

heat

heat

less cool

less cool

Thermoelectric

heat exchange

Internal 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 simulation

of AlGaAs/GaAs HBT

(heatsink at emitter)

(heatsink at collector)

E
C

E
F

HFET Channel

emitter

base

collector

n

p

n

Heterojunction Bipolar Transistor

cool

E
C

E
V

Remove hot electrons

by thermionic emission

Could energy from microscale

device waste heat be harvested?

Thermoelectric Power

Generation


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, S
n

< 0

T

T+
D
T

electrons

V = S
n
D
T

+

p

T

T+
D
T

holes

V = S
p
D
T

+

p
-
type material: holes are
majority carriers, S
p

> 0

S = “Seebeck coefficient” [V/K]

P

= “Peltier coefficient” = TS [V]

Induced voltage measured from cold to hot end

T
hot
n
T
cold
+
p
V
n
R
n
+
V
p
R
p
n
p
T
hot
n
T
cold
+
p
V
n
R
n
+
V
p
R
p
n
p
T
Hot

T
Cold

R
Load

R
Load

+

_

V
tot

= a
×
(V
n
+V
p
)

R
tot

= a
×
(R
n
+R
p
)


Attaching

a

load

to

a

thermoelectric

generator

causes

current

to

flow
.

a = # of
n
/
p

pairs

Thermoelectric Power Generator

Efficiency

For

an

optimized

TE

device

with

a

matched

load

(R
load

=

R
TE
),

T
H

T
C

R
Load

Q
H

I

h
=

I
2
R
Load

Q
H

h
opt

=

T
H
-

T
C

T
H

M
-

1

T
C

T
H

M +

M
=

1 +
Z


T
H

+ T
C

2

where

Z
=

S
2
s

k

Thermoelectric figure

of merit
ZT

averaged

over the operating

temperature range

Carnot efficiency

Efficiency Curves

T
Cold

(K)

T
Cold

(K)

T
Cold

(K)

T
Cold

(K)

T
Hot

-

T
Cold

(K)

T
Hot

-

T
Cold

(K)

T
Hot

-

T
Cold

(K)

T
Hot

-

T
Cold

(K)

Efficiency

(%)

ZT = 1

ZT = 2

ZT = 3

ZT = 4

Increasing

Carnot

efficiency

In

order

to

generate

significant

power

density,

device

must

maintain

a

large

D
T

⡨楧h

h
)

潲

h慶e

a

h楧h

h敡e

晬ux
.
†
Th敳e

two

敦晥捴e

慲a

汩n步k
.

Efficiency Increase with Increasing Heat Flux

10
-
8
10
-
7
10
-
6
10
-
5
10
-
4
10
-
3
10
-
2
10
-
1
Q/A (W/cm
2
)
(cm
2
K/W)
Thickness
Thermal Conductivity
Efficiency
h
10
-
8
10
-
7
10
-
6
10
-
5
10
-
4
10
-
3
10
-
2
10
-
1
Q/A (W/cm
2
)
(cm
2
K/W)
Thickness
Thermal Conductivity
Thickness
Thermal Conductivity
Efficiency
h
ZT = 2


As

heat

flux

Q/A

increases,

D
T

=

T
hot

-
T
cold

increases,

and

therefore

the

efficiency

increases
.


Assuming

1
D

heat

flow,

D
T
=

LQ

歁

L
:

Thickness

of

TE

generator

Q
:

Heat

source

k
:

Thermal

conductivity

A
:

Cross
-
sectional

area

≈

L

k

10
-
5

to 10
-
2

cm

㄰
-
2

to 10
-
1

W/cmK


For

most

devices

made

from

(nanostructured)

TE

materials

with

high

ZT,

Increasing heat flux

Increased Efficiency for Energy Conversion

from Small Hot Spots Using Small TE Generators

R
L2

Small one
-
leg generator

for each heat source

T
Cold

Q
H

3

(each)

Net area reduced to A
2

T
Cold

R
L1

One
-
leg generator

Q
H

Area A
1


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

Q
H
/A
.

Same Q
H

Wasted

heat

Wasted

heat

Larger

T
H
-
T
C

What systems have micro/nanoscale

heat sources with high heat flux?

I
2
R
L1

I
2
R
L2

Intel

Itanium
®

Processor

Device
-
Level Thermoelectric

Generation 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

R
Load

V
Device

+

-

Substrate

Thermoelectric Generator

Device

V
Device

+

-

R
Load

Q
H

Heat sink

C. LaBounty, Ph.D. thesis,

UC Santa Barbara (2001)

0
25
50
75
100
0
5
10
15
20
Temperature (

C)
Threshold Current (
mA
)
Slope Efficiency (W/A)
0.4
0.3
0.2
0.1
0
25
50
75
100
0
5
10
15
20
Temperature (

C)
Threshold Current (
mA
)
Slope Efficiency (W/A)
0.4
0.3
0.2
0.1
flat
QD lasers can

have small

temperature

dependence

(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
.


Radiation

Wave

guided

Surface Plasmon




Leaky mode

(
Radiation
)

Waveguided

SPP

Si Substrate

Anode

HTL

ETL

Cathode

V

+

-

Decay rate (a.u.)

Cathode:


18nm Ag

ETL:


60nm Alq
3

HTL:


50nm
a
-
NPD

Anode:


100nm Al / 13nm Ni

Substrate: Silicon

1
2
3
4
5
x 10
-3
1.8
2.2
2.6
x 10
-3
k
x

/ 2
p

w
/⠲
p
挩

Leaky

mode

Waveguided

Surface
plasmon
-
polariton

k
x

520nm spectrum

•

The amount of dipole energy that goes to a specific


mode 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 Active

Organic Devices

520nm

Si Cantilever

+

-

Cathode

Active

Layers

Insulator

Anode

Tipless Cantilever

OLED on an AFM Cantilever

35

m
6

m
Light Emission from the OLED
Light emission
from the OLED edge
K. H. An et al.,
Appl. Phys. Lett.

89
, 111117 (2006)

Summary

•

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
.