Download Pipe, Kevin - Quantum Electronics Group

Internal Thermoelectric Effects and
Scanning Probe Techniques for
Inorganic and Organic Devices
Kevin Pipe
Department of Mechanical Engineering
University of Michigan
Collaborators
Rajeev Ram (MIT) Ali Shakouri (UCSC) Li Shi (UT) Max Shtein (UM)
cool
heat
10 m
EC
cool
EFn
p
n
cool
EV
cool
EFp
heat
h+
e-
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
 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.
Nuclear reactor
 Heating in power electronics and optoelectronics can be
>1000 W/cm2.
Hot plate
 Traditional thermoelectric coolers cool only ~ 10 W/cm2.
Intel Pentium® III Processor
M. J. Ellsworth, (IBM), ITHERM 2004
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?
Device-Internal Temperature Gradients
 Large variation in carrier temperature (DT≈1000K) and lattice temperature
(DT≈100K) can arise within active devices during operation.
MOSFET channel (carrier
temperature)
SOI MOSFET
(lattice temperature)
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?
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)
Device-Internal Temperature Gradients
Predicted temperature distribution
 Transistor
Intel 90nm MOSFET
5W/m3 heat
source over a
radius of 20nm
S. Sinha and K. E. Goodson, "Thermal conduction in
sub-100 nm transistors," THERMINIC 2004
Bulk
heating
 Semiconductor Laser
Can microscale 10 m
hot spots be
cooled
efficiently?
SEM
Facet temperature
cross-section
Facet
heating
P. K. L. Chan et al., Appl. Phys. Lett. 89, 011110 (2006)
Cooling Methods for Devices
Large heat sinks inefficient at cooling microscale hot spots
Integrated
thermoelectric
cooler
Device
Device
Substrate
Substrate
Heat sink
Heat sink
Heat sink
Junction-up mounting
Junction-down mounting
Monolithic integration with TE cooler
(difficult to remove heat)
(better device performance and lifetime
but has practical difficulties with
electrical contacts, etc.)
(complicated processing)
p-i-n diode
Electronic structure of
device optimized for
internal thermoelectric
cooling
HIT cooler
Heat sink
Junction-up mounting with
device-internal thermoelectric cooling
(microscale cooling source with minimal processing impact)
C. LaBounty, Ph.D. thesis, UC Santa Barbara (2001).
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
Electronic structure of
device optimized for
internal thermoelectric
cooling
Heat sink
Junction-up mounting with
device-internal thermoelectric cooling
(microscale cooling source with minimal processing impact)
Recent Convergence of
Thermoelectric / Device Materials
Thermoelectric Coolers
12x larger figure-of-merit
GaAs/AlAs Superlattice
(m*)3/2
l
(bulk thermoelectric figure-of-merit)
Active Devices
T. Koga et al., J. Comp.-Aid. Mat. Des. 4 (1997)
Transistors, lasers
4x larger figure-of-merit
HgCdTe Superlattice
Detectors,
Mid-IR lasers
R. Radtke et al., J. Appl. Phys. 86 (1999)
300 W/cm2 cooling at 300K
InGaAs/InGaAsP SL
High-speed
transistors, lasers
C. LaBounty et al., J. Appl. Phys. 89 (2002)
InGaAs/InGaAsP Barrier
A. Shakouri et al., Appl. Phys. Lett 74 (1999)
High-speed, highpower transistors
680 W/cm2 at 345K
SiGe/Si SL
A. Shakouri et al., IPRM (2002)
750 W/cm2 at 300K
BiTe/SbTe SL
R. Venkatasubramanian et al., Nature 413 (2001)
A. Shakouri and C. LaBounty, ICT, Baltimore, 1999.
High-performance semiconductors have recently been
used to create superior thermoelectric devices
Conventional TE Cooler
Tcold
EF
EV
Heat
absorbed
p
+
Heat
released
Optimum
p,n doping
EF
EV
I
Thot
Electrons
_
Holes
+
Heat
absorbed
n
I
I
Heat
released
EC
EF
_
Thermoelectric figure-of-merit
(sometimes written as ZT)
s P2
Z = lT2
1
(Thot-Tcold)max= ZT2
2
• Electrical Conductivity s (maximize current)
• Thermal Conductivity l (minimize thermal conduction)
• Peltier Coefficient P (maximize energy difference at contacts)
EC
EF
Internal Cooling of Devices
cool
heat
EC
cool
cool
_
EFn
EC
EF
EFp
n
EV
metal
p
cool
EV
n-type
cool
Thermoelectric Cooling
heat
P-N Diode
cool
EC
p
heat
heat
EFp
EFn
n
heat EV
cool heat
cool
n+
Semiconductor Laser Diode
EC
n
EV
collector
p
cool
n
EC
base
emitter
Heterojunction Bipolar Transistor
EF
HFET Channel
The operating current of a device causes thermoelectric heating/cooling
at every internal device junction.
Diode Thermoelectric Effects
Conventional TE Cooler
P-N Diode
Thot
p
I
I
Thot
n
electrons
holes
Tcold
EF
heat
electrons
p
I
I
cool
heat
n
EV
n
I
cool
heat
cool
Thot
holes
EC
p
Tcold
EC
EFn
cool
p
n
cool
cool
EFp
EV
heat
The diode is the fundamental building block of most electronic and optoelectronic devices
(transistors, lasers, amplifiers, etc.)
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).
Measurement of Bipolar Thermoelectric Effect
Unbiased GaAs diode: ND = 5×1018 cm-3, NA = 1×1019 cm-3
p
Energy (eV)
1
0.5
EF
0
EC
-0.5
-1
EV
Built-in
potential
-1.5
-2
-150
Carrier Concentration (cm-3)
n
-100
-50
0
50
Position (nm)
20
10
holes
10
10
P<0
for electrons
P>0
for holes
0
20
Measurement
Theory
4x bulk
value
10
0
10x bulk
value
-10
Voltage measured using SThEM,
an STM-based technique
-20
-150
-100
-50
0
50
Position (nm)
10
electrons
-10
10
Thermoelectric Voltage (mV)
1.5
-150
-100
-50
0
50
Position (nm)
Carrier transport calculated with self-consistent
drift-diffusion / Poisson equation software
• 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
Semiconductor Laser
EC
p
electrons
EV
holes
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
Quantum well temperature
is critical to laser operation
Electron
leakage
PE
QW
Fp
Electron
injection
radiation
Lasers are typically
biased to “flat-band”
EC
N
EFn
N+
(substrate)
EV
Hole injection
Electron/hole injection current
Electron/hole leakage current
Hole leakage
Thermoelectric heating
Thermoelectric cooling
Optimizing Thermoelectric Heat
Exchange Distribution
Injection Current Internally Cooled Light Emitter
Conventional Design
PE
heat
Fp
EFn
heat
Thermoelectric
heat exchange
cool
heat
QW
N
EC
cool
P
N+
EFp
heat cool
cool heat
less cool
EV
x
Thermoelectric
heat exchange
EC
heat
less cool
EFn
N
N+
EV
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).
Optimizing Thermoelectric Heat
Exchange Distribution
Injection Current Internally Cooled Light Emitter
EC
P
EFp
heat cool
QW
Temperature o(C)
40 GaInAsSb-based laser simulation
35
Conventional
Thermoelectric
heat exchange
cool heat
less cool
less cool
EFn
N
N+
EV
x
Active region cooling
30
Optimized
25
18% reduction in operating temperature
20
0
200
400
600
Current Density
800
1000 1200
(A/cm2)
K. P. Pipe, R. J. Ram, and A. Shakouri, “Internal cooling in a semiconductor
laser diode”, IEEE Phot. Tech. Lett. 14, 453 (2002).
Internal Cooling of Transistors
Remove hot electrons
by thermionic emission
Optimizing for thermoelectric/thermionic
cooling could reduce device heating.
EC
Boltzmann transport simulation
of AlGaAs/GaAs HBT
EF
cool
cooling
(heatsink at emitter)
(heatsink at collector)
HFET Channel
EC
n
EV
collector
p
n
base
emitter
Heterojunction Bipolar Transistor
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
Could energy from microscale
device waste heat be harvested?
Thermoelectric Power
Generation
Induced voltage measured from cold to hot end
S = “Seebeck coefficient” [V/K]
T+DT
+
n
V = SnDT
holes
 A temperature difference applied across a
material causes a net motion of charge and
hence an open-circuit voltage to develop.
electrons
T+DT
V = SpDT
+
T
T
p-type material: holes are
majority carriers, Sp > 0
n-type material: electrons
are majority carriers, Sn < 0
P = “Peltier coefficient” = TS [V]
p
 Attaching a load to a Tthermoelectric
generator causes current to flow.
hot
THot
a = # of n / p pairs
Vn
+
n
p
+
Rn
Vp
Rp
Tcold
R
Load
n
Vtot = a×(Vn+Vp)
p
_ +
TCold
Rtot = a×(Rn+Rp)
RLoad
Thermoelectric Power Generator
Efficiency
For an optimized TE device with a matched
load (Rload = RTE),
QH
hopt =
TH
I
TC
TH
M-1
M+
TC
TH
Carnot efficiency
where
M=
I2RLoad
h= Q
H
TH - TC
TH + TC
1+Z
2
RLoad
S2s
Z=
k
Thermoelectric figure
of merit ZT averaged
over the operating
temperature range
Increasing
Carnot
efficiency
ZT = 1
THot - TCold (K)
THot - TCold (K)
Efficiency Curves
ZT = 3
TCold (K)
ZT = 2
TCold (K)
THot - TCold (K)
THot - TCold (K)
TCold (K)
Efficiency
(%)
ZT = 4
TCold (K)
In order to generate significant power density, device must maintain a
large DT (high h) or have a high heat flux. These two effects are linked.
Efficiency Increase with Increasing Heat Flux
L: Thickness of TE generator
Q: Heat source
k: Thermal conductivity
A: Cross-sectional area
10-1
10-2
Increasing heat flux
10-3
10-4
10-5
10-6
ZT = 2
10-7
10-8
 For most devices made from
(nanostructured) TE materials
with high ZT,
L
10-5 to 10-2 cm
≈ 10-2 to 10-1 W/cmK
k
Q/A (W/cm2)
Efficiency h
LQ
DT =
kA
Thickness
Thermal Conductivity
 Assuming 1D heat flow,
(cm2K/W)
 As heat flux Q/A increases, DT = Thot -Tcold increases, and therefore
the efficiency increases.
Increased Efficiency for Energy Conversion
from Small Hot Spots Using Small TE Generators
Area A1
Same QH
Net area reduced to A2
I2RL2
I2RL1
QH
Wasted Wasted
heat
heat
RL1
Larger
TH-TC
QH
3
RL2
(each)
TCold
TCold
One-leg generator
Small one-leg generator
for each heat source
 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.
What systems have micro/nanoscale
heat sources with high heat flux?
Intel
Itanium®
Processor
VDevice
Device-Level Thermoelectric
Generation Methods
Device-Internal
C. LaBounty, Ph.D. thesis,
UC Santa Barbara (2001)
RLoad
Device
QH
Thermoelectric Generator
Device-External
Substrate
0.4
20
15
10
0.3
flat
0.2
5
0.1
(data from P. Bhattacharya)
0
0
25
50
75
Temperature ( C)
 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?
Slope Efficiency (W/A)
Heat sink
Threshold Current (mA)
 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.
+-
QD lasers can
have small
temperature
dependence
100
VDevice
+-
Device
RLoad
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.
Leaky mode
(Radiation)
SPP
Cathode
ETL
+
HTL
V
Waveguided
Decay rate (a.u.)
Energy Outcoupling from Active
520nm spectrum
Organic Devices
2.6
Si Substrate
Cathode: 18nm Ag
ETL:
60nm Alq3
HTL:
50nm a-NPD
Anode:
100nm Al / 13nm Ni
Substrate: Silicon
w/(2pc)
Anode
Wave
Surface Plasmon
guided
Radiation
kx
-3
x 10
Waveguided
Leaky
mode
2.2
520nm
1.8
Surface
plasmonpolariton
1
2
3
4
5
-3
kx / 2p
x 10
• 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.
OLED on an AFM Cantilever
Cathode
Tipless Cantilever
Active
Layers
+
-
Insulator
Si Cantilever
Anode
Light Emission from the OLED
6 m
35 m
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.