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. Intel Pentium® III Processor Intel Itanium® Processor
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 >1000 W/cm2.
Traditional thermoelectric coolers cool only ~ 10 W/cm2.
Nuclear reactor
Hot plate
Intel Pentium® III Processor
M. J. Ellsworth, (IBM), ITHERM 2004
Intel Itanium® Processor
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 (DT≈1000K) and lattice temperature (DT≈100K) 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)

5. 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
Bulk
heating
Facet temperature
cross-section
Semiconductor Laser
Facet
heating
Can microscale hot spots be cooled efficiently?
P. K. L. Chan et al., Appl. Phys. Lett. 89, (2006)

6. Large heat sinks inefficient at cooling microscale hot spots
Cooling Methods for Devices
Integrated
thermoelectric
cooler
Large heat sinks inefficient at cooling microscale hot spots
Device
Device
Heat sink
Junction-down mounting
(better device performance and lifetime
but has practical difficulties with
electrical contacts, etc.)
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)

7. 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)

8. Thermoelectric / Device Materials
Recent Convergence of
Thermoelectric / Device Materials
Thermoelectric Coolers
m(m*)3/2 l
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. 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

9. Thermoelectric figure-of-merit (sometimes written as ZT)
Conventional TE Cooler _ EC EF
Heat
absorbed EV EF
Heat
absorbed +
Tcold I
Holes
Electrons p n _ EC EF
Heat
released
Thot EV EF
Heat
released + I I
Thermoelectric figure-of-merit
(sometimes written as ZT)
Z =
s P2
lT2
(Thot-Tcold)max= ZT2 1 2
Electrical Conductivity s (maximize current)
Thermal Conductivity l (minimize thermal conduction)
Peltier Coefficient P (maximize energy difference at contacts)
Optimum
p,n doping

10. Thermoelectric Cooling Heterojunction Bipolar Transistor
Internal Cooling of Devices
cool
heat n p EC EV
EFn
EFp
P-N Diode _ EC EV EF
metal
n-type
cool
Thermoelectric Cooling
emitter
base
collector n p
Heterojunction Bipolar Transistor
cool EC EV
heat EC
heat
cool EC EF
cool
HFET Channel p n
EFn
EFp n+
heat EV
cool
heat
Semiconductor Laser Diode
The operating current of a device causes thermoelectric heating/cooling at every internal device junction.

11. (transistors, lasers, amplifiers, etc.)
Diode Thermoelectric Effects
Conventional TE Cooler
P-N Diode
Thot
Tcold
Thot
Tcold
electrons
holes p n
electrons p n I I I
holes I I
Thot
cool EC
heat EC
cool p p
cool
heat
EFn EF n
EFp 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, (2002).

12. p n Measurement of Bipolar Thermoelectric Effect
Unbiased GaAs diode: ND = 5×1018 cm-3, NA = 1×1019 cm-3 p n
Measurement EF
4x bulk
value
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
P>0
for holes
P<0
for electrons
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)

13. Quantum well temperature is critical to laser operation
Alloys in Devices n p EC EV
electrons
holes
Semiconductor Laser
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.
Lasers are typically
biased to “flat-band”
Quantum well temperature
is critical to laser operation
Electron
leakage
Electron
injection EC P N
EFn QW
radiation
EFp N+
(substrate) EV
Hole injection
Hole leakage
Electron/hole injection current
Thermoelectric heating
Electron/hole leakage current
Thermoelectric cooling

14. Injection Current Internally Cooled Light Emitter
Optimizing Thermoelectric Heat
Exchange Distribution
Conventional Design
Injection Current Internally Cooled Light Emitter EC EV
EFn
EFp P N N+
heat
cool EC EV
EFn
EFp P N N+
cool
heat
less cool x
Thermoelectric
heat exchange
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).

15. Optimizing Thermoelectric Heat Exchange Distribution
Injection Current Internally Cooled Light Emitter EC EV
EFn
EFp P N N+
cool
heat
less cool
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).

16. by thermionic emission Boltzmann transport simulation
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
emitter
base
collector n p
Heterojunction Bipolar Transistor
cool EC EV
cooling
(heatsink at collector)
(heatsink at emitter)
Could energy from microscale
device waste heat be harvested?
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

17. Thermoelectric Power Generation
Induced voltage measured from cold to hot end
T+DT
T+DT + n p
A temperature difference applied across a material causes a net motion of charge and hence an open-circuit voltage to develop.
electrons
V = SnDT
holes
V = SpDT + T T
S = “Seebeck coefficient” [V/K]
n-type material: electrons are majority carriers, Sn < 0
p-type material: holes are majority carriers, Sp > 0
P = “Peltier coefficient” = TS [V]
Attaching a load to a thermoelectric generator causes current to flow.
THot
TCold
a = # of n / p pairs
Vtot = a×(Vn+Vp) _ +
Rtot = a×(Rn+Rp)
RLoad
RLoad

18. Thermoelectric figure
Thermoelectric Power Generator Efficiency
For an optimized TE device with a matched load (Rload = RTE),
TH - TC TH
M - 1
hopt = QH TH TC TH
M +
Carnot efficiency I
where TC
M =
1 + Z
TH + TC 2
h =
I2RLoad QH
RLoad
Thermoelectric figure
of merit ZT averaged
over the operating
temperature range
Z =
S2s k

19. Efficiency Curves ZT = 1 ZT = 2 ZT = 3 ZT = 4 Efficiency (%)
TCold (K)
THot - TCold (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 DT (high h) or have a high heat flux. These two effects are linked.

20. Efficiency Increase with Increasing Heat Flux
As heat flux Q/A increases, DT = Thot -Tcold increases, and therefore the efficiency increases.
ZT = 2
Assuming 1D heat flow,
DT = LQ kA
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, ≈ L k
10-5 to 10-2 cm
10-2 to 10-1 W/cmK

21. Increased Efficiency for Energy Conversion from Small Hot Spots Using Small TE Generators
TCold
RL1
One-leg generator QH
Area A1
Net area reduced to A2
Same QH
I2RL1
I2RL2 QH 3
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?

22. Thermoelectric Generator
Device-Level Thermoelectric Generation Methods
Substrate
Thermoelectric Generator
Device
VDevice + -
RLoad QH
Heat sink
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

23. 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.

24. Energy Outcoupling from Active Organic Devices
520nm spectrum
Radiation
Wave
guided
Surface Plasmon
Leaky mode
(Radiation)
Decay rate (a.u.) kx V
Cathode
ETL
SPP + -
Waveguided
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.

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

26. 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.