1. Thermoelectricity of Semiconductors
Jungyun Kim
December 2, 2008

2. Outline Discovery of the thermoelectricity – Seebeck coefficient
Operation of thermoelectric devices
Architectural and materials enhancement
Large impact of shrinking to nanoscale

3. 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.
chem.ch.huji.ac.il/history/seebeck.html

4. Seebeck Effect Seebeck Coefficient
Heat transfer through electrons and phonons (lattice vibrations)
Phonon motion
Photon
Metal rod
Electron mobility Al
Electron mobility
Phonon motion
Seebeck Coefficient
Electrons in the hot region are more
energetic and therefore have greater
velocities than those in the cold region

5. Thermoelectric Operation
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.
Snyder et al. Nature 7, , (2008).
Rowe D.M., Thermoelectrics Handbook,

6. Figure of Merit – Conflicting Properties
Figure of Merit - zT
Effect of Carrier Concentration
=>
S - Seebeck Coefficient
σ - Electron Conductivity
κ - Thermal Conductivity
n – carrier concentration
m* - effective mass of carrier
μ – carrier mobility
Snyder et al. Nature 7, , (2008).

7. Figure of Merit – Conflicting Properties
Figure of Merit - zT
Effect of Temperature
=>
S - Seebeck Coefficient
σ - Electron Conductivity
κ - Thermal Conductivity
n – carrier concentration
m* - effective mass of carrier
μ – carrier mobility
Snyder et al. Nature 7, , (2008).

8. Figure of Merit – Conflicting Properties
Figure of Merit - zT
Best micro-scale materials operate at ZT = 1
(10% of Carnot efficiency)
To run at 30% efficiency (home refrigeration) need a ZT=4.
=>
S - Seebeck Coefficient
σ - Electron Conductivity
κ - Thermal Conductivity
n – carrier concentration
m* - effective mass of carrier
μ – carrier mobility
DiSalvo, Science, 285 (1999)
Bell. Science, 321 (2008)

9. Architectural Enhancement
Functionally graded and segmented thermoelements
High-performance
multisegmented thermoelectric
Rowe D.M., Thermoelectrics Handbook,

10. 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.
Zn4Sb3 (left), highly disordered Zn sublattice with filled
interstitial sites, and complexity of Yb14MnSb11 (right) unit cell
Snyder et al. Nature 7, , (2008).

11. Macro to Nano – Thermal conductivity
Calculated dependence of zT for Bi2Te3 structure material
Venkatasubramanian R. et al. Thin-film thermoelectric devices with high room-temperature figures of merit. Nature 413, (2001).
Hicks, L.D. and Dresselhaus, M.S. Effect of quantum-well structures on the thermoelectric figure of merit. Physical Review B, 47, (1993).

12. Recent Developments – Si Nanowires
Thermoelectric enhancement through introduction of
nanostructures at different length scales
Diameter
Surface roughness
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, (2008).

13. 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
Play an important role in a global sustainable energy solution
Contingent on developing materials with higher thermoelectric efficiency
Rowe D.M., Thermoelectrics Handbook,

14. Summary Enhanced scattering of phonons Macro to Nano Current research
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

15. 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
Dresselhaus, M.S. Physical Review B 61, 7 (2000).