Thermoelectricity of Semiconductors

Jungyun KimDecember 2, 2008

Outline

Discovery of the thermoelectricity – Seebeck coefficient

Operation of thermoelectric devices

Architectural and materials enhancement

Large impact of shrinking to nanoscale

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_world_1812_1827.html

Seebeck Effect

www.dkimages.com/discover/DKIMAGES/Discover/Home/Science/Physics-and-Chemistry/Electricity-and-Magnetism/General/General-18.html

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

Seebeck Effect

Metal rod

Electron mobility Phonon motion

Photon

Phonon motion

Electron mobility

Electrons in the hot region are more energetic and therefore have greater velocities than those in the cold region

dT

dVS

Seebeck Coefficient

Heat transfer through electrons and phonons (lattice vibrations)

Al Al

Thermoelectric Operation

Rowe D.M., Thermoelectrics Handbook, 2006. Snyder et al. Nature 7, 105-114, (2008).

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.

TS

zT2

Figure of Merit – Conflicting Properties

S - Seebeck Coefficient3/2

*2

22

33

8

n

Tmeh

kS B

n – carrier concentration m* - effective mass of carrier μ – carrier mobility

σ - Electron Conductivity

ne1

Figure of Merit - zT

κ - Thermal Conductivity

LTneTLκ

κκκ

e

le

=>2S

z

Effect of Carrier Concentration

Snyder et al. Nature 7, 105-114, (2008).

TS

zT2

Figure of Merit – Conflicting Properties

S - Seebeck Coefficient3/2

*2

22

33

8

n

Tmeh

kS B

n – carrier concentration m* - effective mass of carrier μ – carrier mobility

σ - Electron Conductivity

ne1

Figure of Merit - zT

κ - Thermal Conductivity

LTneTLκ

κκκ

e

le

=>2S

z

Effect of Temperature

Snyder et al. Nature 7, 105-114, (2008).

TS

zT2

Figure of Merit – Conflicting Properties

S - Seebeck Coefficient3/2

*2

22

33

8

n

Tmeh

kS B

n – carrier concentration m* - effective mass of carrier μ – carrier mobility

σ - Electron Conductivity

ne1

Figure of Merit - zT

κ - Thermal Conductivity

LTneTLκ

κκκ

e

le

=>2S

z

Bell. Science, 321 (2008)

DiSalvo, Science, 285 (1999)

• Best micro-scale materials operate at ZT = 1

(10% of Carnot efficiency)

• To run at 30% efficiency (home refrigeration) need a ZT=4.

Architectural Enhancement

Functionally graded and segmented thermoelements

High-performance multisegmented thermoelectric

Rowe D.M., Thermoelectrics Handbook, 2006.

Materials Enhancement

Fleurial, J.-P. et al. Int. Conf. Thermoelectrics, (2001).

Snyder et al. Nature 7, 105-114, (2008).

Void spaces in CoSb2 are filled by alloying and doping decreasing thermal conductivity.

Complex crystal structures that yield lowlattice thermal conductivity.

Zn4Sb3 (left), highly disordered Zn sublattice with filled interstitial sites, and complexity of Yb14MnSb11 (right) unit cell

Calculated dependence of zT for Bi2Te3 structure material

Macro to Nano – Thermal conductivity

Hicks, L.D. and Dresselhaus, M.S. Effect of quantum-well structures on the thermoelectric figure of merit. Physical Review B, 47, 12727-12731 (1993).

Venkatasubramanian R. et al. Thin-film thermoelectric devices with high room-temperature figures of merit. Nature 413, 597-602 (2001).

Recent Developments – Si Nanowires

Hochbaum, A.I. et al. Enhanced thermoelectric performance of rough silicon nanowires. Nature 45, 10 163-167 (2008).

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.

Thermoelectric enhancement through introduction ofnanostructures at different length scales1. Diameter2. Surface roughness3. Point defects

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

www.chinatraderonline.com

www.solarsolutions.ca

Rowe D.M., Thermoelectrics Handbook, 2006.

http://www.phys.psu.edu/nuggets/?year=2004

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 ConductivityElectron 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).