Nanocomposites of Semimetallic ErAs Nanoparticles Epitaxially Embeddedwithin InGaAlAs-based Semiconductors for Thermoelectric Materials

J.M.O. Zide', G. Zeng2, J.H. Bahk2, W. Kim3, S. L. Singer3, D. Vashaee4, Z. X. Bian4, R. Singh4,J. E. Bowers2, A. Majumdar3, A. Shakouri4, A.C. Gossard"2

1 Materials, University ofCalifornia, Santa Barbara, California, U.S.A.2Electrical and Computer Engineering, University ofCalifornia, Santa Barbara, California, U.S.A.

3 Mechanical Engineering, University ofCalifornia, Berkeley, California, U.S.A.4Electrical Engineering, University ofCalifornia, Santa Cruz, California, U.S.A.

AbstractWe present the molecular beam epitaxial growth of

nanocomposites consisting of semimetallic ErAsnanoparticles which are epitaxially embedded withinInGaAlAs-based semiconductors. The properties ofnanocomposites can be drastically different from that of theconstituents, and in this case, the incorporation of ErAs isused to increase the thermoelectric power factor and decreasethe lattice thermal conductivity, resulting in an increase in thefigure of merit, ZT. In addition, the thermoelectric powerfactor is increased due to electron filtering (solid-statethermionic emission) by barriers within the composite. In onegeometry, barriers of InGaAlAs, a wider bandgapsemiconductor, are introduced into an ErAs:InGaAsnanocomposite. In a second geometry, ErAs particles areembedded directly into InGaAlAs. Electron filtering occursdue to the Schottky barriers which are formed surrounding theparticles. We present a 400-element array based on thesematerials for thermoelectric power generation; a powerdensity > 1 W/cm2 is demonstrated with a temperaturegradient of 120°C.

Solid-state thermionicsEfficient thermoelectric materials operating at high

temperatures ( 300-600°C) are desirable for a wide range ofpower generation applications. To be practical, thermoelectricpower generation requires efficient materials and a well-designed device with minimal parasitics and can beimpedance matched to a load. The efficiency ofthermoelectric materials is generally discussed in terms of thedimensionless figure of merit, ZT=S2GaT/p, where S is theSeebeck coefficient, a is electrical conductivity, T istemperature, and 3 is thermal conductivity. The quantity S2Gis often referred to as the thermoelectric power factor.

Researchers seeking efficient thermoelectric materials facetwo unfortunate obstacles: (1) thermal conductivity containsan electronic term which tends to increase with increasingelectrical conductivity (known as the Wiedemann-Franz law),and (2) the Seebeck coefficient and electrical conductivity aregenerally inversely related. As a result, optimizing materialsgenerally involves balancing these parameters. Forsemiconductors, the lattice thermal conductivity is frequentlylarger than the electronic thermal conductivity and this firstobstacle can be safely ignored; lattice thermal conductivity isminimized and electronic thermal conductivity is ignored. Thesecond consideration suggests that there is an optimal dopingfor a semiconductor which will result in a maximalthermoelectric power factor. Hicks and Dresselhaus [1, 2]

suggested the use of the quantum effects which accompanyreduced dimensionality in semiconductor heterostructures toincrease the performance of thermoelectric materials. Whilerecent work has demonstrated the potential of nanocompositematerials to increase ZT, this increase is primarily a result ofdecreased thermal conductivity. [3, 4] Solid-state thermionics,in which barriers are used to prevent cold (low energy)electrons from moving through a heterostructures while stillallowing hot (high energy) electron transport, offers analternative technique to traditional thermoelectrics. In thiscase, it is possible to partially decouple Seebeck coefficientfrom electrical conductivity by artificially changing thedistribution of carriers within the material. This technique hasbeen discussed in depth elsewhere. [5-10]

ErAs-based thermoelectric materialsErbium arsenide (ErAs) is a semimetal which forms self-

assembled nanoparticles in a rocksalt structure whenincorporated into Ill-V semiconductors grown by molecularbeam epitaxy (MBE). [11] These nanoparticles are epitaxialwithin the structure and do not result in large concentrationsof defects.

The presence of ErAs in Ill-As compound semiconductorsoften changes the properties of the semiconductor drastically.In Ino.53Gao.47As (lattice matched to InP and hereinafterreferred to as InGaAs), ErAs serves as a donor. [12] Growthconditions and the effects on electrical conductivity, Seebeckcoefficient, and thermoelectric power factor of incorporatingErAs into InGaAs (either as layers of particles or randomlydistributed throughout the semiconductor) have beendiscussed elsewhere. [13] We also have shown thatincorporating these nanoparticles into InGaAs increasesphonon scattering and therefore reduces thermal conductivitybelow the so-called "alloy limit" without decreasing electricalconductivity. [14]

If barriers made of InGaAlAs (also lattice matched to InP)are incorporated into this ErAs:InGaAs composite (during theMBE growth), it is possible to use the solid-state thermionictechnique (electron filtering) to increase the thermoelectricpower factor. We have recently demonstrated electronfiltering experimentally by measuring an increase in theSeebeck coefficient of a superlattice in the cross-planedirection (vs. the in-plane value) in good agreement withtheory. [15] Thermal conductivity measurements of thesesuperlattice materials show a similar reduction in thermalconductivity to that of the ErAs:InGaAs material. [16] Usingtransient measurements, we have also measured a roomtemperature ZT of 0.13-0.15, which agrees quite well with

2006 International Conference on Thermoelectrics1-4244-0811-3/06/$20.00 (02006 IEEE 280

theoretical predictions and is an improvement over themaximum room temperature ZT of optimally-doped InGaAs,which is approximately 0.06. [17] According to theory, a ZTof 1.6 is expected at T=900K.

Power generation from arraysFor power generation, single elements (or small arrays)

are impractical because their low resistance makes impedancematching impossible. Instead, we have fabricated a 400element array (200 each of p-type and n-type elements) inwhich the elements are thermally in parallel and electrically inseries. This design increases the voltage and also increases theresistance (and therefore the ideal load resistance) to tens ofOhms. The structure of the n-type elements is theErAs:InGaAs/InGaAlAs superlattice described above. Inprevious work, we have used similar structures for the p-typematerial in which the InGaAs is heavily compensated withberyllium (an acceptor) and the barrier is InAlAs rather thanInGaAlAs. InAlAs was chosen because the valence bandoffset between InGaAs and InGaAlAs is much smaller thanthe conduction band offset; InAlAs gives the same holebarrier height as the electron barrier height used in the n-typematerial. [18] In this previous work, each element was 5 ptmthick elements and had an area of 200 pm x 200 pm. Anoutput power of 0.7mW (4.4mW/cm2) was measured with a30°C temperature gradient across the packaged generator.

In the present work, the p-type material consists of Be-compensated ErAs:InGaAs without additional barriersbecause the barriers increased the resistance of the p-typematerial too drastically. Contact resistance and other parasticshave also been reduced. The element area has been reduced to120 ptm x 120 ptm, and the thickness of the elements has beendoubled to 10 ptm, which increases the thermal resistance ofthe elements and therefore allows larger temperaturegradients. Output power densities greater than 1W/cm2 havebeen achieved with a temperature gradient of 120°C acrossthe packaged device. It is estimated that the temperaturegradient across the active region is 25°C.

Figure 1: Measurement results of the output power densityfor 400 element power generator arrays. The temperaturegradient is the total across the packaged generator, and theload resistance is 27Q.

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ErAs:InGaAlAs nanoparticle compositesAs an alternative to the ErAs:InGaAs/InGaAlAs material

described previously, it is possible to incorporate ErAsdirectly into InGaAlAs, which can be described as(Ino.52AI0.48As)x(Ino.53Gao.47As) -x. At a given temperature,ErAs pins the Fermi level at a particular energy which isdetermined by the band lineup of ErAs to the particularInGaAlAs composition. Previous work measuring Schottkybarrier heights for complete films of ErAs on InGaAlAsshows a linear relationship between barrier height and InAlAscontent. [19] As a result, the incorporation of ErAs intoInGaAlAs results in Schottky barriers around the particleswith a height which can be controlled by the InAlAs content.It is possible, in principle, to use these barriers for electronfiltering. This scheme has some potential advantages over thesuperlattice described previously. One important advantage isthat this material is isotropic, which greatly simplifies themeasurement of relevant thermoelectric properties. In Figure2, the in-plane electrical and thermoelectric properties areplotted as a function of InAlAs content at room temperature.As expected, carrier concentration decreases with increasingInAlAs content; the Fermi level is pinned deeper within thebandgap. The decrease in carrier concentration is faster thanmight be expected by the decreasing Fermi level, but it isworth noting that only mobile carriers are measured. As isusually the case, the Seebeck coefficient is increasing as thecarrier concentration (and conductivity) decreases. Figure 2cdemonstrates that ErAs:InGaAs has the highest thermoelectricpower factor at room temperature, but electron concentrationof ErAs:InGaAlAs is expected to increase with temperature.

Figure 2: Room temperature measurements of thermoelectricproperties of ErAs:InGaAlAs as a function of InAlAscomposition. The properties presented are (a) electronconcentration, (b) mobility, (c) Seebeck coefficient, and (d)thermoelectric power factor.

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Careful measurements of high temperature properties ofthis material are necessary to determine its efficiency as athermoelectric material for high temperature applications.Because the temperature dependence of the relevantproperties could be rather large in this system, the potentialexists for drastic improvements at higher temperatures.

ConclusionsThe incorporation of ErAs into InGaAlAs-based materials

provides a promising route to efficient thermoelectric powergeneration at high temperatures. Electron filtering has beenexperimentally demonstrated in superlattices of ErAs:InGaAsand InGaAlAs, which offers the potential to increasethermoelectric power factors over that of optimally-dopedbulk semiconductors. Additionally, the presence of ErAsreduces thermal conductivity, resulting in a further increase inZT. While the room temperature performance of ErAsincorporated directly into InGaAlAs is somewhatdisappointing, further work and high temperaturemeasurements are necessary to determine its performance athigher temperatures.

AcknowledgmentsThe authors acknowledge financial support from the

Office of Naval Research through the Thermionic EnergyConversion Center MURI.

References1. L. D. Hicks and M. S. Dresselhaus, "Thermoelectric Figure

of Merit of a One-Dimensional Conductor" Phys. Rev. B.47, 12727 (1993).

2. L. D. Hicks, T. C. Harman, and M. S. Dresselhaus, "Useof Quantum-Well Superlattices to Obtain a High Figure ofMerit from Nonconventional Thermoelectric-Materials,"Appl. Phys. Lett. 63, 3230 (1993).

3. R. Venkatasubramanian, E. Siivola, T. Colpitts, and B.O'Quinn, "Thin-film thermoelectric devices with highroom-temperature figures of merit," Nature 413, 597(2001).

4. T. C. Harman, P. J. Taylor, M. P. Walsh, and B. E.LaForge, "Quantum dot superlattice thermoelectricmaterials and devices," Science 297, 2229 (2002).

5. A. Shakouri and J. E. Bowers, "Heterostructure integratedthermionic coolers," Appl. Phys. Lett. 71, 1234 (1997).

6. G. D. Mahan and L. M. Woods, "Multilayer thermionicrefrigeration," Phys. Rev. Lett. 80, 4016 (1998).

7. C. B. Vining and G. D. Mahan, "The B factor inmultilayer thermionic refrigeration," J. Appl. Phys. 86,6852 (1999).

8. A. Shakouri, C. Labounty, P. Abraham, J. Piprek, and J.E. Bowers, Prof Materials Research Society, 545, 449(1999).

9. D. Vashaee and A. Shakouri, "Improved thermoelectricpower factor in metal-based Superlattices," Phys. Rev.Lett. 92, 106103/1 (2004).

10. D. Vashaee and A. Shakouri, "Electronic andthermoelectric transport in semiconductor and metallic

I1. D. 0. Klenov, D. C. Driscoll, A. C. Gossard, and S.Stemmer, "Scanning transmission electron microscopy ofErAs nanoparticles embedded in epitaxialInO.53GaO.47As layers," Appl. Phys. Lett. 86, 111912(2005).

12. D. C. Driscoll, M. Hanson, C. Kadow, and A. C. Gossard,"Electronic structure and conduction in a metal-semiconductor digital composite: ErAs:InGaAs," Appl.Phys. Lett. 78, 1703 (2001).

13. J. M. Zide, D. 0. Klenov, S. Stemmer, A. C. Gossard, G.Zeng, J. E. Bowers, D. Vashaee, and A. Shakouri,"Thermoelectric power factor in semiconductors withburied epitaxial semimetallic nanoparticles," Appl. Phys.Lett. 87, 112102 (2005).

14. W. Kim, J. Zide, A. Gossard, D. Klenov, S. Stemmer, A.Shakouri, and A. Majumdar, "Thermal conductivityreduction and thermoelectric figure of merit increase byembedding nanoparticles in crystalline semiconductors,"Phys. Rev. Lett. 96 (2006).

15. J. M. 0. Zide, D. Vashaee, Z. X. Bian, G. Zeng, J. E.Bowers, A. Shakouri, and A. C. Gossard. "Demonstrationof electron filtering to increase the Seebeck coefficient inErAs:InGaAs/InGaAlAs superlattices," Phys. Rev. B.submitted (2006).

16. W. Kim, S. L. Singer, A. Majumdar, D. Vashaee, Z. X.Bian, A. Shakouri, G. Zeng, J. E. Bowers, J. M. 0. Zide,and A. C. Gossard, "Cross-plane lattice and electronicthermal conductivities of ErAs:InGaAs/InGaAlAssuperlattices," Appl. Phys. Lett. 88, 242107 (2006).

17. Rajeev Singh, Zhixi Bian, Gehong Zeng, Joshua Zide,James Christofferson, Hsu-Feng Chou, Art Gossard, JohnBowers, and Ali Shakouri, "Transient HarmanMeasurement of the Cross-plane ZT of InGaAs/InGaAlAsSuperlattices with Embedded ErAs Nanoparticles," Proc.ofMRS Fall Meeting, Boston, November 2005.

18. G. Zeng, J. E. Bowers, J. M. 0. Zide, A. C. Gossard, W.Kim, S. Singer, A. Majumdar, R. Singh, Z.X. Bian, Y.Zhang, and A. Shakouri. "ErAs:InGaAs/InGaAlAssuperlattice thin-film power generator array." Appl. Phys.Lett. 88, 113502 (2006).

19. J. D. Zimmerman, E. R. Brown, and A. C. Gossard."Tunable all epitaxial semimetal-semiconductor Schottkydiode system: ErAs on InAlGaAs," J. Vac. Sci. Tech. B,Vol. 23, No. 5. (2005), pp. 1929-35.

superlattices," J. Appl. Phys. 95, 1233 (2004).