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
.
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