Thermoelectricity of Semiconductors
Jungyun Kim
December 2, 2008
Outline
Discovery of the thermoelectricity – Seebeck
coefficient
Operation of thermoelectric devices
Architectural and materials
enhancement
Large impact of shrinking to
nanoscale
Seebeck Effect
Thermoelectricity - known in physics as the
"Seebeck Effect"
• In 1821, Thomas Seebeck, a German physicist,
twisted two wires of different metals together
and heated one end.
• Discovered a small current flow and so
demonstrated that heat could be converted to
electricity.
www.worldofenergy.com.au/07_timeline_
www.dkimages.com/discover/DKIMAGES/
chem.ch.huji.ac.il/history/seebeck.html
Seebeck Effect
Heat transfer through electrons
and phonons (lattice vibrations)
Photon
Phonon motion
Metal rod
Electron mobility
Al
Electron mobility
Al
Phonon motion
Seebeck Coefficient
dV
S
dT
Electrons in the hot region are more
energetic and therefore have great
velocities than those in the cold
region
Thermoelectric Operation
e-
h+
• Electron/hole pairs created at the hot end
absorbs heat.
• Pairs recombine and reject heat at the
cold end.
• The net voltage appears across the bottom
of the thermoelectric legs.
Rowe D.M., Thermoelectrics Handbook, 2006.
Snyder et al. Nature 7, 105-114, (2008).
Figure of Merit – Conflicting Properties
Effect of Carrier Concentration
Figure of Merit - zT
zT 
S 2T

=>
z
S 2

S - Seebeck Coefficient
8 2 k B2 *   
S
m T 
3eh 2
 3n 
2/3
σ - Electron Conductivity
1
   ne

κ - Thermal Conductivity
κ  κ e  κl
κe  LT  neLT
n – carrier concentration
m* - effective mass of carrier
μ – carrier mobility
Snyder et al. Nature 7, 105-114, (2008).
Figure of Merit – Conflicting Properties
Effect of Temperature
Figure of Merit - zT
zT 
S 2T

=>
z
S 2

S - Seebeck Coefficient
8 2 k B2 *   
S
m T 
3eh 2
 3n 
2/3
σ - Electron Conductivity
1
   ne

κ - Thermal Conductivity
κ  κ e  κl
κe  LT  neLT
n – carrier concentration
m* - effective mass of carrier
μ – carrier mobility
Snyder et al. Nature 7, 105-114, (2008).
Figure of Merit – Conflicting Properties
Figure of Merit - zT
zT 
S T
2

=>
z
S 
2

S - Seebeck Coefficient
8 2 k B2 *   
S
m T 
3eh 2
 3n 
• Best micro-scale materials operate at ZT = 1
(10% of Carnot efficiency)
• To run at 30% efficiency (home refrigeration)
need a ZT=4.
2/3
σ - Electron Conductivity
1
   ne

κ - Thermal Conductivity
κ  κ e  κl
κe  LT  neLT
n – carrier concentration
m* - effective mass of carrier
μ – carrier mobility
DiSalvo, Science, 285 (1999)
Bell. Science, 321 (2008)
Architectural Enhancement
Functionally graded and segmented
thermoelements
Rowe D.M., Thermoelectrics Handbook, 2006.
High-performance
multisegmented thermoelectric
Materials Enhancement
Void spaces in CoSb2 are filled by alloying and doping
decreasing thermal conductivity.
Fleurial, J.-P. et al. Int. Conf. Thermoelectrics, (2001).
Complex crystal structures that yield low
lattice thermal conductivity.
Snyder et al. Nature 7, 105-114, (2008).
Zn4Sb3 (left), highly disordered Zn sublattice with filled
interstitial sites, and complexity of Yb14MnSb11 (right) unit cell
Macro to Nano – Thermal conductivity
Calculated dependence of zT for Bi2Te3 structure material
Hicks, L.D. and Dresselhaus, M.S. Effect of
quantum-well structures on the thermoelectric
figure of merit. Physical Review B, 47, 1272712731 (1993).
Venkatasubramanian R. et al. Thin-film
thermoelectric devices with high roomtemperature figures of merit. Nature 413,
597-602 (2001).
Recent Developments – Si Nanowires
Thermoelectric enhancement through introduction of
nanostructures at different length scales
1. Diameter
2. Surface roughness
3. Point defects
SEM image of a Pt-bonded EE Si
nanowire. Scale bar 2um.
• Near both ends are resistive heating
and sensing coils to create a
temperature gradient.
• To measure conductivity, I-V curves
were recorded by a source meter
• Seebeck voltage (∆Vs) was measured
by multimeter with a corresponding
temperature difference ∆T
Single nanowire power factor (red) and calculated zT (blue)
for 52nm nanowire. Uncertainty in measurements 21% for
power factor and 31% for zT.
Hochbaum, A.I. et al. Enhanced thermoelectric performance of rough silicon nanowires. Nature 45, 10 163-167 (2008).
Motivation and Applications
•
•
•
Approximately 90% of world’s power is generated by heat engines that use fossil
fuels combustion
– Operates at 30-40% of the Carnot efficiency
– Serves as a heat source of potentially 15 terawatts lost to the environment
Thermoelectrics could potentially generate electricity from waste heat
Thermoelectrics could be used as solid state Peltier coolers
http://www.phys.psu.edu/nuggets/?year=2004
www.solarsolutions.ca
www.chinatraderonline.com
Rowe D.M., Thermoelectrics Handbook, 2006.
Summary
• Enhanced scattering of phonons
– Increased surface area to volume
– Greater surface roughness
– Inclusion of dopants and point defects
• Macro to Nano
– Greater decrease in thermal conductivity than
electron conductivity from decrease in diameter (3D
→ 2D → 1D)
• Current research
– Development of Si nanowire thermoelectric
properties
– Advancement in nanowire processing of well-known
thermoelectric materials
Macro to Nano - Electron Conductivity
Electron scattering from surface
imperfections and grain
boundaries and interfaces
Quantum confinement: external
conduction and valence band
move in opposite directions to
open up band-gap
Bulk
90 nm
65 nm
www.itrs.net/reports.html
Dresselhaus, M.S. Physical Review B 61, 7 (2000).