1. Zhixi Bian School of Engineering University of California, Santa Cruz zxbian@soe.ucsc.edu Low dimensional and nanostructured InGaAlAs materials for thermoelectric energy conversion

2. Collaborators R. Singh, M. Zebarjadi, Ali Shakouri (UC Santa Cruz) J. M.O. Zide, A. C. Gossard (UC Santa Barbara, Delaware) S. Singer, W. Kim, A. Majumdar (UC Berkeley, Yonsei University) G. Zeng, J-H. Bahk, J.E. Bowers (UC Santa Barbara)

3. Outline Thermoelectric effects Low dimensional materials: superlattices Nanoparticle materials Summary

4. Thermoelectric Effects Seebeck: a b V T1T1 T2T2 a Peltier: a b a I Q Q

5. Application: microelectronics cooling Steve Kang et al. Electrothermal analysis of VLSI Systems, Kluwer 2000  T=20C Mean-time-to-failure due to electromigration increase x5 110C 108C 90C 80C 1 cm On chip temperature contour Dependence of mean time to failure on temperature

6. Application: optoelectronics cooling Typical DFB Laser:  /  T= 0.1 nm/ o C, Heat generation kW/cm 2 Scheerer et al., Siemens AG, Elec. Lett. 35, (20, Sept. 1999) Fiber Optic Link: 3200 Gbit/s 80 Lasers, 40 Gb/s per laser 0.8nm channel spacing  ~ 0.8 nm Optoelectronic device used in high-speed, multi wavelength fiber optic communication systems generate kW/cm 2 and they need temperature stabilization.

7. Challenge: integrated optoelectronics V=2.7V V=0V Light Out Front Mirror Gain Phase Rear Mirror SG-DBR Laser Amplifier EA Modulator MQW active regions Q waveguide Sampled Grating Zhixi Bian, et al., Appl. Phys. Lett. 27, 3605 (2003) Bias (V) Temperature Change (C) Standard Thermal Design +160 C

8. Application: energy conversion Possible Applications Electric power generator with no moving part Electric Ships (Seapower 21) Waste heat recovery (cars, power plants, …) Microscale power sources

9. Thermoelectric figure-of-merit ZT Maximum Cooling: Terry M. Tritt et al., MRS Bulletin March 2006 Net Cooling: Peltier Cooling Joule Heating Heat Conduction

10. Peltier cooling: microscopic picture Density of States E EfEf  d (E) E Differential Conductivity  ST f(E) E

11. Optimal doping J. Snyder (2003) http://www.its.caltech.edu/~jsnyder/thermoelectrics/science_page.htm For almost all materials, if doping is increased, electrical conductivity increases but Seebeck coefficient is reduced.

12. Low dimensional materials (in-plane) Dresselhaus M S et al, Microscale Thermophysical Engineering 3, 89 (1999). 2D 1D Density- of-States Energy Bi [100] PbTe, QWell SL [001] PbTe, QWire SL D. A. Broido, T. L. Reinecke, Phys. Rev. B 64, 045324 (2001) (1)Full band structure (2)Inelastic scattering

13. Thermionic emission and MQW SL (cross-plane) Energy Hot electron Cold electron Cathode Barrier Anode Thermionic: If barrier is thin or nanostructured ( Can not define barrier Seebeck coef. independent of contact layers (ballistic, non-linear transport) Material 1 Material 2 Single Barrier Material 1

14. Non-planar barrier Zhixi Bian et al., Appl. Phys. Lett. 88, 012102 (2006) Barrier Emitter Collector

15. Peltier power profile Mona Zebarjadi et al., Phys. Rev. B 74, 195331 (2006)

16. Multilayers and MQW superlattices G. Chen, Phys. Rev. B 57, 14 958 (1998). M. V. Simkin and G. D. Mahan, Phys. Rev. Lett. 84, 927 (2000). R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O'Quinn, Nature 413, 597 (2001). Reduced parasitic (contact) effects Reduced thermal conductivity Optimize electronic thermal conductivity Z. Bian, et al., Phys. Rev. B 76, 205311 (2007) Material 1 Material 2 Superlattice Material 1

17. Interface scattering and coherence R. Venkatasubramanian, Phys. Rev. B 61, 3091 (2000) ~ phonon wavelength 2.5~25nm Bi 2 Te 3 /Sb 2 Te 3 superlattices  T=32.2 K, ZT ~2-2.4 R. Venkatasubramanian, Nature 413, 597 (2001). Power Factor (  W/cmK 2 ) 40 50.9 Thermal Conductivity (W/mK) 0.5 1.26 Superlattices Bulk

18. InGaAs  Mobility  Thermal conductivity mElectron effective mass  m 1.5 /  Material Optimization for Heterostructure Integrated Thermionic Coolers, Ali Shakouri, Chris Labounty, Invited Paper, International Conference on Thermoelectrics, pp. 35-39, Baltimore, MD, August 1999.

19. ErAs/InGaAs-InGaAlAs for energy conversion The barrier height can be adjusted by Al composition ErAs particles reduce the thermal conductivity (InGaAs) 0.6(InAlAs) 0.4 digital alloy (n-InGaAs) embedded with 0.3% Er nanoparticles randomly distributed (2 × 10 18 cm -3 ) +Si dopants [(0-2-4-8) × 10 18 cm -3 ] × 70 n-InP substrate 50nm 5E18 n-InGaAs 20nm n-InGaAs 10nm InGaAlAs 20nm n-InGaAs Cap layer

20. Seebeck coefficients and modeling Zhixi Bian, et al., Phys. Rev. B 76, 205311 (2007)

21. Oscillation with doping The thermoelectric power factor and electronic thermal conductivity can be optimized with doping and SL thickness

22. Phonon scattering by particles Bulk Alloy Bulk Alloy + Nanoparticles After W. Kim

23. Reduced thermal conductivity Thermal conductivity is reduced by interface and nanoparticle scattering of phonons W. Kim et al., Appl. Phys. Lett. 88, 242107 (2006)

24. Energy conversion module AlN InP Flip chip bonding AlN Substrate remove InP AlN Contact metal deposition on top of superlattice G. Zeng et al, Appl. Phys. Lett. 88, 113502 (2006)

25. Nanostructured materials PbTe/PbTeSe Quantum Dot Superlattices 0D confinement ???? Particle scattering of phonons/ electrons Ternary: ZT=1.3-1.6 Quaternary: ZT=2  T=43.7 K, Bulk  T=30.8 K T.C. Harman et al., Science 297, 2229(2002) Power Factor (  W/cmK 2 ) 25.5 28 Thermal Conductivity (W/mK) 0.5 2.0 QD Bulk PbTe

26. ErAs/InGaAlAs -- thermal 0.4 ML 40 nm 0.1 ML 10 nm In 0.53 Ga 0.47 As W. Kim et al., Phys. Rev. Lett. 30, 045901 (2006) In 0.53 Ga 0.47 As 0.3 % ErAs 3.0 % ErAs 3.0 % ErAs:In 0.53 Ga 0.28 Al 0.19 As

27. ErAs/InGaAlAs -- electrical ErAs nanoparticles might dope the holding materials more efficiently Free carrier concentration can be adjusted by particle size, and the holding material composition D. Driscoll (UCSB), PhD Thesis Conduction band edge ErAs/InGaAs

28. (InGaAs) 1-x (InAlAs) x —electrical conductivity By changing the composition of Al, the carrier concentration can be adjusted The carrier concentration also increases with temperature. This self-adaptability might offer an optimal material for a large temperature range Measured at JPL with help from T. CaillatSubstrate contribution is negligible <600K 20% Al

29. (InGaAs) 1-x (InAlAs) x - Seebeck coefficient The Seebeck coefficient still increases with temperature, even the carrier concentration becomes larger 20% Al Measured at UCSC

30. (InGaAs) 1-x (InAlAs) x —thermal conductivity Measured at UC Berkeley 20% Al I(ω) V(ω), V(3ω)

31. Power factor and ZT Thermoelectric power factor increases and thermal conductivity decreases with the increase of temperature A usually poor thermoelectric material achieves ZT ~1 at 600 K, when ErAs nanoparticles are embedded 20% Al

32. Where we are ErAs:InGaAlAs

33. Power generation module Made by flip-chip bonding and wafer transfer at UCSB 2.5 W/cm 2 power output is demonstrated with the most recent module

34. Some modeling---scattering Three major electron scattering mechanisms Electron energy By UCSB and UCSC

35. Some modeling---fitting Fitting with experimental results with two parameters in nanoparticle scattering 20% Al

36. Some modeling---prediction To improve the performance at ~400 K, smaller particle size might help Current sample After W. Kim, UC Berkeley

37. Improved thermoelectric power factor Poudel, B., et al. (2008). "High-Thermoelectric Performance of Nanostructured Bismuth Antimony Telluride Bulk Alloys." Science: 1156446

38. 38 Power Factor (S 2 σT): 0.3% Er / control 0.3% Er:InGaAlAs Control (2E18 Si, no Er)

39. Summary Thermoelectric materials have applications in thermal management and thermal-to-electrical energy conversion. Low dimensional and nano structures may improve the thermoelectric performance. We have made superlattices and nanoparticle materials using conventional semiconductor materials. A power generation density (2.5 W/cm2) have been achieved. Similar material systems and optimal potential barrier, particle size and concentration may offer increased thermoelectric power factor besides reduced thermal conductivity, in turn, higher thermal to electrical energy conversion efficiency. More accurate modeling of the thermoelectric effects of nanoparticles is ongoing.