Internal Thermoelectric Effects and

1
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)
2
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
3
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.
• Heating in power electronics and optoelectronics
  can be gt1000 W/cm2.
• Traditional thermoelectric coolers cool only 10
  W/cm2.

Nuclear reactor
Hot plate
M. J. Ellsworth, (IBM), ITHERM 2004
Hot spots
Is it possible to generate targeted cooling or
harvest waste heat energy?
C.-P. Chiu (Intel), Cooling challenges for
silicon integrated circuits, SRC/SEMATECH Top.
Res. Conf. on Reliability, Oct. 2004
4
Device-Internal Temperature Gradients

• Large variation in carrier temperature (DT1000K)
  and lattice temperature (DT100K) can arise
  within active devices during operation.

SOI MOSFET (lattice temperature)
MOSFET channel (carrier 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)
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Device-Internal Temperature Gradients
Predicted temperature distribution

• Transistor

Intel 90nm MOSFET
5W/mm3 heat source over a radius of 20nm
S. Sinha and K. E. Goodson, "Thermal conduction
in sub-100 nm transistors," THERMINIC 2004
Facet temperature cross-section
Bulk heating

• Semiconductor Laser

Facet heating
Can microscale hot spots be cooled efficiently?
P. K. L. Chan et al., Appl. Phys. Lett. 89,
011110 (2006)
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Cooling Methods for Devices
Integrated thermoelectric cooler
Device
Large heat sinks inefficient at cooling
microscale hot spots
Device
Substrate
Substrate
Heat sink
Heat sink
Junction-up mounting (difficult to remove heat)
Monolithic integration with TE cooler (complicated
processing)
p-i-n diode
Electronic structure of device optimized for
internal thermoelectric cooling
HIT cooler
C. LaBounty, Ph.D. thesis, UC Santa Barbara
(2001).
Junction-up mounting with device-internal
thermoelectric cooling (microscale cooling source
with minimal processing impact)
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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
Junction-up mounting with device-internal
thermoelectric cooling (microscale cooling source
with minimal processing impact)
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Recent Convergence of Thermoelectric / Device
Materials
Thermoelectric Coolers
Active Devices
(bulk thermoelectric figure-of-merit)
12x larger figure-of-merit GaAs/AlAs
Superlattice T. Koga et al., J. Comp.-Aid. Mat.
Des. 4 (1997)
Transistors, lasers
4x larger figure-of-merit HgCdTe Superlattice R.
Radtke et al., J. Appl. Phys. 86 (1999)
Detectors, Mid-IR lasers
300 W/cm2 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)
High-speed transistors, lasers
High-speed, high- power transistors
680 W/cm2 at 345K SiGe/Si SL A. Shakouri et al.,
IPRM (2002)
750 W/cm2 at 300K BiTe/SbTe SL R. Venkatasubramani
an 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
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Conventional TE Cooler
Tcold
I
Holes
Electrons
n
p
Thot
I
I
Thermoelectric figure-of-merit (sometimes written
as ZT)

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

Optimum p,n doping
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Internal Cooling of Devices
heat
EC
cool
heat
p
n
EFn
EFp
n
heat
EV
heat
cool
Semiconductor Laser Diode
The operating current of a device causes
thermoelectric heating/cooling at every internal
device junction.
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Diode Thermoelectric Effects
Conventional TE Cooler
P-N Diode
Tcold
Thot
Thot
Tcold
electrons
n
p
n
p
holes
electrons
I
I
I
holes
I
I
Thot
cool
EC
heat
EC
cool
p
p
cool
heat
EFn
EF
EFp
n
n
heat
cool
cool
EV
cool
heat
EV
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).
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Measurement of Bipolar Thermoelectric Effect
Unbiased GaAs diode ND 51018 cm-3, NA
11019 cm-3
p
n
Measurement
4x bulk value
EF
Theory
Energy (eV)
EC
EV
Built-in potential
Thermoelectric Voltage (mV)
10x bulk value
Position (nm)
holes
Voltage measured using SThEM, an STM-based
technique
Plt0 for electrons
Pgt0 for holes
Carrier Concentration (cm-3)
Position (nm)
electrons

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

Position (nm)
Carrier transport calculated with
self-consistent drift-diffusion / Poisson
equation software
H.-K. Lyeo, A.A. Khajetoorians, L. Shi, K.P.
Pipe, R.J. Ram, A. Shakouri, and C.K. Shih.
Science 303, 816 (2004)
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Alloys in Devices
Quantum well temperature is critical to laser
operation
Electron injection
Electron leakage
EC
EFn
P
N
QW
radiation
EFp
N
(substrate)
EV
Hole leakage
Hole injection
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Optimizing Thermoelectric Heat Exchange
Distribution
Conventional Design
Injection Current Internally Cooled Light Emitter
Thermoelectric heat exchange
QW
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).
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Optimizing Thermoelectric Heat Exchange
Distribution
Injection Current Internally Cooled Light Emitter
Thermoelectric heat exchange
GaInAsSb-based laser simulation
x
Active region cooling
QW
18 reduction in operating temperature
K. P. Pipe, R. J. Ram, and A. Shakouri, Internal
cooling in a semiconductor laser diode, IEEE
Phot. Tech. Lett. 14, 453 (2002).
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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
HFET Channel
cooling
(heatsink at collector)
(heatsink at emitter)
Could energy from microscale device waste heat be
harvested?
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Thermoelectric PowerGeneration
Induced voltage measured from cold to hot end
TDT
TDT

n
p

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

V SnDT
V SpDT
electrons
holes

T
T
S Seebeck coefficient V/K
p-type material holes are majority carriers, Sp
gt 0
n-type material electrons are majority carriers,
Sn lt 0
P Peltier coefficient TS V

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

a of n / p pairs
Vtot a(VnVp)
_

Rtot a(RnRp)
RLoad
RLoad
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Thermoelectric Power GeneratorEfficiency
For an optimized TE device with a matched load
(Rload RTE),
TH - TC TH
M - 1
hopt
TH
QH
M
Carnot efficiency
I
where
TC
RLoad
Thermoelectric figure of merit ZT averaged over
the operating temperature range
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Efficiency Curves
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.
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Efficiency Increase with Increasing Heat Flux

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

• Assuming 1D heat flow,

Increasing heat flux
L Thickness of TE generator Q Heat source k
Thermal conductivity A Cross-sectional area

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

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Increased Efficiency for Energy Conversionfrom
Small Hot Spots Using Small TE Generators
Net area reduced to A2
Same QH
I2RL2
I2RL1
RL2
Wasted heat
Wasted heat
Larger TH-TC
(each)
TCold
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.

Intel Itanium Processor
What systems have micro/nanoscale heat sources
with high heat flux?
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Device-Level ThermoelectricGeneration Methods
Device-External

• 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
VDevice
(data from P. Bhattacharya)
-

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

• 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
RLoad
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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.
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Energy Outcoupling from ActiveOrganic Devices
520nm spectrum
Radiation
Wave guided
Surface Plasmon
Leaky mode (Radiation)
Decay rate (a.u.)
kx
V
Cathode
ETL
-
Waveguided
SPP
HTL
Waveguided
Anode
Leaky mode
Si Substrate
w/(2pc)
520nm
Cathode 18nm Ag ETL 60nm Alq3 HTL 50nm
a-NPD Anode 100nm Al / 13nm Ni Substrate
Silicon
Surface plasmon-polariton
kx / 2p

• 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.

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OLED on an AFM Cantilever
Cathode
Tipless Cantilever
Active Layers
Insulator
Si Cantilever
Anode
K. H. An et al., Appl. Phys. Lett. 89, 111117
(2006)
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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.