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_world_1812_1827.html 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 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

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, 105-114, (2008). Rowe D.M., Thermoelectrics Handbook, 2006.

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, 105-114, (2008).

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, 105-114, (2008).

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)

Architectural Enhancement Functionally graded and segmented thermoelements High-performance multisegmented thermoelectric Rowe D.M., Thermoelectrics Handbook, 2006.

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, 105-114, (2008).

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, 597-602 (2001). 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).

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, 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 Play an important role in a global sustainable energy solution Contingent on developing materials with higher thermoelectric efficiency www.solarsolutions.ca www.chinatraderonline.com Rowe D.M., Thermoelectrics Handbook, 2006.

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

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).