High efficiency semimetal/semiconductor nanocomposite thermoelectric materialsJ. M. O. Zide, J.-H. Bahk, R. Singh, M. Zebarjadi, G. Zeng, H. Lu, J. P. Feser, D. Xu, S. L. Singer, Z. X. Bian, A.Majumdar, J. E. Bowers, A. Shakouri, and A. C. Gossard Citation: Journal of Applied Physics 108, 123702 (2010); doi: 10.1063/1.3514145 View online: http://dx.doi.org/10.1063/1.3514145 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/108/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Carrier transfer from InAs quantum dots to ErAs metal nanoparticles Appl. Phys. Lett. 105, 103108 (2014); 10.1063/1.4895519 Erratum: “High efficiency semimetal/semiconductor nanocomposite thermoelectric materials” [J. Appl. Phys.108,123702 (2010)] J. Appl. Phys. 110, 059902 (2011); 10.1063/1.3630947 Semimetal-semiconductor rectifiers for sensitive room-temperature microwave detectors Appl. Phys. Lett. 87, 163506 (2005); 10.1063/1.2112201 Ultrafast photoresponse at 1.55 μm in InGaAs with embedded semimetallic ErAs nanoparticles Appl. Phys. Lett. 86, 051908 (2005); 10.1063/1.1852092 Carrier compensation in semiconductors with buried metallic nanoparticles J. Appl. Phys. 97, 016102 (2005); 10.1063/1.1808473

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High efficiency semimetal/semiconductor nanocomposite thermoelectricmaterials

J. M. O. Zide,1,a� J.-H. Bahk,2 R. Singh,3 M. Zebarjadi,3 G. Zeng,2 H. Lu,4 J. P. Feser,5

D. Xu,5 S. L. Singer,5 Z. X. Bian,3 A. Majumdar,5 J. E. Bowers,2 A. Shakouri,3

and A. C. Gossard4

1Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, USA2Department of Electrical and Computer Engineering, University of California, Santa Barbara,California 93106, USA3Jack Baskin School of Engineering, University of California, Santa Cruz, California 95064, USA4Department of Materials, University of California, Santa Barbara, California 93106, USA5Department of Mechanical Engineering, University of California, Berkeley, California 94720, USA

�Received 19 August 2010; accepted 9 October 2010; published online 20 December 2010�

Rare-earth impurities in III–V semiconductors are known to self-assemble into semimetallicnanoparticles which have been shown to reduce lattice thermal conductivity without harmingelectronic properties. Here, we show that adjusting the band alignment between ErAs andIn0.53Ga0.47−XAlXAs allows energy-dependent scattering of carriers that can be used to increasethermoelectric power factor. Films of various Al concentrations were grown by molecular beamepitaxy, and thermoelectric properties were characterized. We observe concurrent increases inelectrical conductivity and Seebeck coefficient with increasing temperatures, demonstratingenergy-dependent scattering. We report the first simultaneous power factor enhancement andthermal conductivity reduction in a nanoparticle-based system, resulting in a high figure of merit,ZT=1.33 at 800 K. © 2010 American Institute of Physics. �doi:10.1063/1.3514145�

I. INTRODUCTION: THERMOELECTRICS ANDELECTRON FILTERING

In recent years, thermoelectric power generation has be-come an increasingly attractive technology for waste heatrecovery and other power generation applications. In mostcases, the efficiency of the energy conversion is a key factorin determining the efficacy of the technology. The efficiencyof a thermoelectric material is directly related to the dimen-sionless figure of merit, ZT, which is given by ZT=S2�T /�, where S is the Seebeck coefficient, � is the elec-trical conductivity, T is absolute temperature, and � is thethermal conductivity. Generally, it is difficult to optimize thisfigure of merit since these parameters are interdependent;reducing thermal conductivity tends to reduce electrical con-ductivity, and there is generally a tradeoff between the See-beck coefficient and the electrical conductivity.

Typical bulk materials have ZT�1 but nanoscale mate-rials can have higher efficiency. Theoretical work has sug-gested that quantum effects in nanoscale materials such asquantum wires or wells can result in a significant increase inthe thermoelectric power factor �TPF=S2�� due to a modi-fication of the density of states.1,2 Experimental work hasresulted in large ZT in nanostructured materials; ZT�2.4has been reported in superlattices,3 and ZT�1.5 in nano-structured bulk materials4,5 have been reported in varioustemperature ranges. In these materials, the improvement inZT over conventional/bulk materials has been almost exclu-sively a result of reduced thermal conductivity. ZT�2 inPbTe quantum dot superlattices6 was also reported. However,

the recent measurements of the thermoelectric power factorand thermal conductivity did not support the earlier claims,and more careful further study of ZT is warranted.7,8 Morerecently, the TPF of PbTe has been enhanced by distortingthe electronic density of states using the resonant dopantThallium.9 TPF increases have been the result of variationson the theme of energy-dependent scattering of charge carri-ers.

Shakouri and others developed an electron filtering tech-nique to increase the thermoelectric power factor by chang-ing the distribution of carriers contributing toconduction,10–14 and the effectiveness of this technique hasrecently been demonstrated experimentally in an InGaAs/InGaAlAs superlattice containing ErAs nanoparticles.15 Inthis paper, we have generalized the physical principles be-hind this technique to a bulklike nanocomposite. For the firsttime, we present increased thermoelectric power factor anddecreased thermal conductivity in the same sample. We haveexperimentally demonstrated high ZT�1.3 at moderately-high temperatures �800 K�. This represents one of the firstreports of high ZT in this temperature range in a materialwhich is not based upon a conventional thermoelectric ma-terial such as BiTe or PbTe.

II. NONPLANAR ELECTRON FILTERING: CONCEPTAND RATIONALE

Conceptually, the electron filtering technique describedabove is simply a matter of changing the distribution of car-riers �in n-type materials: electrons� contributing to conduc-tion by designing energy-dependent scattering into the sys-tem; preventing the transport of the lower-energy, or “cold”electrons results in an increase in the moment of the differ-a�Electronic mail: zide@udel.edu.

JOURNAL OF APPLIED PHYSICS 108, 123702 �2010�

0021-8979/2010/108�12�/123702/5/$30.00 © 2010 American Institute of Physics108, 123702-1

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ential conductivity about the Fermi level, which is the math-ematical definition of the Seebeck coefficient derived fromBoltzmann transport theory. That is, “chopping” the distribu-tion by filtering the “cold” electrons increases the Seebeckcoefficient while causing only a relatively mild decrease inthe electrical conductivity, resulting in increased TPF. Al-though this previous work has focused on filtering by heter-obarriers, an increase in TPF is possible in any system withstrongly energy-dependent electron scatterers. Increasing theSeebeck coefficient by energy dependent scattering has beendemonstrated by Heremans et al.16,17 in their study of Pbprecipitates in PbTe, and this type of enhancement has beenstudied theoretically by Faleev and Leonard.18 Here, wedemonstrate increased thermoelectric efficiency using ananocomposite consisting of III–V semiconductors �morespecifically, InGaAlAs� containing nanoparticles of erbiumarsenide as energy-dependent scatterers.

Erbium arsenide is a rock-salt compound which formsself-assembled semimetallic nanoparticles. These nanopar-ticles can be epitaxially embedded within III–V semiconduc-tors during growth by molecular beam epitaxy �MBE�.19,20

The presence of nanoparticles drastically changes the elec-tronic properties of the resultant composite, allowing theiruse in wide-ranging applications.21 They can also reduce thethermal conductivity of the composite by scattering the me-dium to long wavelength phonons which are unaffected byalloy scattering.22,23 This reduction in thermal conductivity,together with the ability to effectively tune the electron trans-port properties, makes this system ideal for thermoelectricapplications.24

The presence of ErAs nanoparticles pins the effectiveFermi level of the composite at a particular energy. In In-GaAs, the Fermi level is pinned within the conduction band,resulting in highly conductive material.22,24 In this case, im-purity scattering is minimized because the Fermi level ispinned by the nanoparticles, resulting in only a small changein potential, which is undesirable for the electron filteringconcept. By controlling the Aluminum content in InGaAlAs,the position of the conduction band edge �Ec� can be inde-pendently tuned with respect to the Fermi level �Ef�. Theenergy difference �Ec−Ef� determines the scattering mecha-nisms which electrons experience as a result of the nanopar-ticles. In this case, a scattering term is introduced because aSchottky barrier forms around the particles, the height ofwhich is linearly related to aluminum content. This Schottkybarrier is the dominant scattering mechanism �Fig. 1�. It isworth noting that in some cases, charge transfer is likelyprimarily from the particles to the matrix. Even in a codopedsample, in which charge transfer is reversed, the effectivebarrier height that carriers experience would be unchangeddespite the change in the direction of band-bending.

In Fig. 2, the effective free electron concentration ofErAs: �InAlAs�x�InGaAs�1−x composites, as measured by theHall effect, is plotted against InAlAs content. It is worthnoting that the �InAlAs�x�InGaAs�1−x matrix material is ef-fectively an InGaAlAs alloy. The curve is the predicted car-rier concentration based on a barrier height �EC−EF� whichchanges linearly with InAlAs content. The result is a tunable

energy-dependent scatterer. This electrostatic Schottky bar-rier is analogous to the heterobarrier used in previous elec-tron filtering experiments.

The ability to accomplish electron filtering using thesethree-dimensional scatterers rather than planar barriers is at-tractive for several reasons. First, the material becomes iso-tropic, greatly simplifying measurements of the thermoelec-tric properties. Second, these nanocomposites are extremelythermodynamically stable. Together with the elimination ofthe need for planar barriers, this makes synthesis by rela-tively inexpensive bulk techniques a viable option.

III. DESIGN, GROWTH, AND PROCESSING

The ErAs:InGaAlAs materials were grown by MBE on500 �m semi-insulating InP substrates. The compositionIn0.53Ga0.47−XAlXAs is chosen to be lattice matched to InPand to have a modest Schottky barrier height. The ErAs con-tent is 0.6 at. %. All ErAs:InGaAlAs nanocomposite filmsand n-InGaAlAs control films were grown by MBE in aVarian Mod Gen II MBE system. All films were grown witha cracked arsenic �As2� beam equivalent pressure of

ErAsparticleIn 0.53 Ga xAl 0.47-xAs

V0VCoulombic

VSchottky

In 0.53 Ga xAl 0.47-xAs

r

Potential EF

FIG. 1. Schematic diagram of the potential around an ErAs nanoparticles inInGaAlAs. The dominant term is the Schottky barrier, and the resultingprofile causes energy-dependent scattering of electrons.

FIG. 2. Room temperature free electron concentration vs. InAlAs contentfor ErAs: �In0.52Al0.48As�X�In0.53Ga0.47As�1−X nanocomposites. The circlesrepresent experimental data points, while the curve represents the concen-tration predicted by the Fermi–Dirac integral assuming linear changes inFermi energy and effective mass.

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10−5 torr and at a temperature of 490 °C as measured usingan infrared pyrometer. The growth rate was 2 �m /h. ErAswas codeposited,24 and the ErAs concentration was deter-mined by comparing ErAs growth rate �as determined bycalibrations to the erbium flux� to InGaAs and InGaAlAsgrowth rates. All films reported here were grown as a digitalalloy, or a short-period ��30 Å� superlattice of InGaAs andInGaAlAs, which is electronically equivalent to a conven-tional alloy.

Although the InP substrate is semi-insulating, its intrin-sic carrier concentrations become significant ��1�1014 cm−3� at temperatures above 600 K, which results insubstrate conductance that is comparable to the conductanceof the much-thinner epitaxial films. Therefore, measurementsat higher temperatures cannot be performed unless the con-tribution of the substrate conduction can be eliminated. Thiswas accomplished by transferring the thin film onto an insu-lating sapphire substrate and completely removing the InPsubstrate. To accomplish this, the MBE-grown film wasbonded to a sapphire substrate to allow measurements of thefilm’s electric properties at high temperatures. This processshown in Fig. 3. Recently, Liang et al. have introduced arobust, low-temperature technique to create strongSiO2-to-SiO2 covalent bonding for wafer bonding III–Vsemiconductors to silicon.25 We have extended this techniqueto wafer-bonding III–V semiconductors to sapphire, whichenables this thin-film transfer. Sapphire was chosen as a sub-strate because it is a good electrical insulator even at hightemperatures and it is commercially available with extremelylow surface roughness of less than 1 nm. The sapphire sub-strate used is in c-axis orientation and has a coefficient ofthermal expansion �CTE� of 7.3�10−6 K−1, which matcheswell with that of ErAs:InGaAlAs thin film �5.7�10−6 K−1�.This CTE matching reduces the strain on the III–V thin filmafter annealing at 300 °C. The III–V sample size varies from5�5 to 8�8 mm2, and 1�1 cm2 sapphire plate is used.2 �m thick thin films were used for measurements.

Prior to the bonding, oval defects on the MBE-grownIII–V material surface are removed to achieve low surfaceroughness suitable for the bonding. These defects generally

originate from indium droplets on the surface of the substrateduring the oxide desorption, and can be as tall as 2 �m andas long as 20 �m. These defects are removed by using pho-tolithography to pattern 25 �m holes in a layer of photore-sist around the defects and wet-etching usingH3PO4:H2O2:DI=1:5 :15.

During the bonding process, gas byproducts �such as hy-drogen gas� must be removed to prevent surface bubblescaused by trapped gases. Therefore, narrow in-plane chan-nels of 0.5 �m depth are fabricated on the Sapphire sub-strate using BCl3 /Cl2 gases with 200 W rf power in an in-ductively coupled plasma �ICP� etcher as shown in step 1 ofFig. 3.

30 nm SiO2 layers are deposited using plasma-enhancedchemical vapor deposition �PECVD� at 260 °C on the sap-phire substrate and the thin film as a bonding interface layer�step 2 in Fig. 3�. The oxide interface layers are subjected toseveral activation steps where Si–O, Si–F, and Si–N bondinglegs are activated by oxygen plasma, diluted HF, andNH4OH, respectively. The interfaces are then bonded and thesample is finally annealed at 300 °C under pressure of 1.5MPa in vacuum for two hours �step 3 in Fig. 3�.

After the bonding, the InP substrate is selectively etchedoff by dilute HCl �step 4�, and only the III–V thin film re-mains on the sapphire plate. A clover-leaf van der Pauw�vdP� test pattern-structure is patterned �step 5�, and then thethin film is encapsulated by 200 nm ICP SiNx and 100 nmPECVD SiO2 layers for high temperature surface passivation�step 6�. The ICP SiNx is deposited at 250 °C with 50 W rfpower using N2 precursor instead of NH4 as the nitrogensource for silicon nitride. It is known that using N2 precursorcan reduce the hydrogen content in the silicon nitride so as toreduce surface blistering on the nitride, which is mainlycaused by accumulation of hydrogen gases at the nitride/semiconductor interface at high temperatures.26 Furthermore,the subsequently deposited PECVD SiO2 layer reduces ten-sile strain in the SiNx at high temperatures since less thermalexpansion of SiO2 can compensate some amount of expan-sion of SiNx.

For comparison, a reference sample with the same In-GaAlAs composition but which contains only silicon dopingwas grown, processed, and measured using the same tech-niques. The carrier concentration of this sample was chosento be 2�1018 cm−3 to approximate the carrier concentrationof the nanocomposite sample near room temperature.

IV. MEASUREMENTS

Electrical conductivity of the ErAs:InGaAlAs materialand reference were measured as a function of temperature�up to 850 K� using the conventional vdP method. Four metalcontacts of 150�150 �m2 in size were fabricated at thecorners of the vdP pattern to allow for four probe measure-ments of electrical conductivity. A 500 Å TiWN layer wassputtered using TiW�90/10 at.%� source in N2 ambient as ahigh temperature diffusion barrier,27 and then 5000 Å Au wasdeposited in an electron beam evaporator.

Electrical measurements were performed under vacuum�1�10−4 Torr�, and temperature control was accomplished

FIG. 3. �Color online� Process flow for transferring the ErAs:InGaAlAs filmfrom the InP growth substrate to a sapphire carrier-wafer before measuringelectrical properties. This is required to prevent spurious values caused bysubstrate conduction at temperatures �600 K. A cloverleaf vdP pattern isused for conductivity measurements.

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using a cartridge heater in a copper block. A thin type Kthermocouple was used to measure the temperature of thefilm, and electrically and thermally conductive silver pastewas applied at thermal interfaces such as heater/copperblock, copper block/sample, and thin film/thermocouple in-terfaces to increase heat transfer efficiency and achieve fastthermal response. Conductivities of the thin film on sapphirewere measured from room temperature to 850 K.

Temperature-dependent Seebeck coefficients of the ma-terials were measured using the same sample as the electricalconductivity measurements; two diagonal contacts were usedto measure the voltage with a temperature gradient appliedusing two thermally isolated heaters. By changing the tem-perature difference at the two contacts, voltage versus tem-perature difference was obtained, and the slope in their linearrelationship gives us the Seebeck coefficient.28 The Seebeckcoefficients of the thin films were measured from room tem-perature to 800 K.

Thermal conductivity was measured from 300–800 Kusing the 3� method.22,29 Briefly, an electrical current ispassed through a microfabricated heater strip �30 �m�1000 �m, Pt with 200 nm SiO2 dielectric separationlayer� at a frequency �, causing Joule heating and tempera-ture fluctuations at frequency 2�. Due to the temperaturedependence of the heater resistance, the voltage at frequency3� can be used as a sensitive measure of the surface tem-perature fluctuation. Voltage is measured in the frequencydomain using a lock-in amplifier, and thermal conductivity isextracted by fitting to an appropriate thermal model. The useof the 3� technique reduces uncertainties associated withthermal losses to the surroundings, including radiation; addi-tionally, samples are measured in vacuum with radiationshield. In this study, we used the differential method in con-junction with a data reduction scheme which accounts forminor spreading effects in the film.30 The thermal conductiv-ity results reported here have also been compared at roomtemperature to those obtained using the time-domain ther-moreflectance technique31 and show good agreement.

V. RESULTS AND DISCUSSION

The electrical conductivity and Seebeck coefficient ofthe nanocomposite and the reference sample are plotted as afunction of temperature in Figs. 4�a� and 4�b�, respectively. Itis observed that both Seebeck coefficient and the electricalconductivity of the nanocomposite increase with tempera-ture. Typically, as one increases, the other must decrease �cf.,the control sample�. This simultaneous increase is direct ex-perimental evidence of energy dependent scattering and un-ambiguous proof that electron filtering can be used to in-crease the thermoelectric power factor. In Fig. 4�b�, theSeebeck coefficient of both samples are modeled using thescattering mechanisms described above for the nanocompos-ite sample. More detailed theoretical studies of the effects ofnanoparticle scattering on the thermoelectric power factorhave been published elsewhere.32 In Fig. 5, the thermal con-ductivities are compared. As in previous work, the presenceof ErAs nanoparticles reduces the thermal conductivity be-low the “alloy limit.” Because the n-InGaAlAs sample is a

digital alloy, it is likely that interface scattering is also con-tributing to reduced thermal conductivity somewhat, explain-ing the reduction in thermal conductivity in the controlsample with respect to the conventional alloy material. Thisis consistent with similar measurements performed usingtime domain thermoreflectance.31 Combining these effects,the ZT of this material is plotted versus temperature in Fig.

350

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FIG. 4. �Color online� �a� Electrical conductivity and �b� Seebeck coeffi-cient of ErAs:InGaAlAs film �filled circles� and n-InGaAlAs control sample�open squares� vs temperature. The solid lines represent calculated Seebeckcoefficient based on multiple scattering mechanisms.

0

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ThermalConductivity(W

m-1K-1 )

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ErAs:InGaAlAs

FIG. 5. �Color online� Thermal conductivity measurements of ErAs:In-GaAlAs film �filled circles� and n-InGaAlAs control sample �open squares�vs temperature measured using the 3� technique. The error bars are prima-rily a result of uncertainty in the calibration of dT/dR for the heaters.

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6. At moderately high temperatures �800 K�, ZT reaches 1.3,which is significantly higher than conventional materials inthis temperature range. Great care was taken to reduce theerrors in these measurements, but as always, some sources oferror remain. First, the vdP method, used to measure theelectrical conductivity of the films, generally has an error of�1%. Additional errors are possible in the Seebeck measure-ment due to the possibility of nonuniform heat flow, imper-fect thermal contact and voltage fluctuations during the mea-surements and are shown as error bars in Fig. 4�b�.

Errors in the thermal conductivity measurement can pri-marily be attributed to uncertainty in the calibration of dT/dRof the heater, which leads to appreciable error in � �10% inmost cases�. At high temperature, the thermal conductivityon InP is on the same order as that for InGaAs, which causesnon-negligible lateral spreading of heat within the film.However, this was corrected by adopting a data reductionscheme which takes the spreading into account.30 All of theseerrors are represented by error bars in the presented data.

This work demonstrates the feasibility of nanoparticlesto simultaneously increase thermoelectric power factor usingenergy-dependent scattering of charge carriers and reducethermal conductivity by acting as mid/long-wavelength pho-non scattering centers. This work points toward several fur-ther improvements which can be made to achieve evenhigher efficiency. Here, the relatively modest increase in See-beck coefficient with increasing temperature is likely prima-rily the result of the rather small Schottky barrier betweenErAs and In0.53Ga0.38Al0.09As. As a result, only a modestamount of energy-dependent scattering is observed. Largerbarriers �created by changing the composition of the com-posite� are likely to result in further improvements to ther-moelectric power factor. Thus, similar materials consisting ofdifferent combinations of III–V matrix and RE-V nanopar-ticles can result in further improvements in performance.Further improvements might also be derived from largernanoparticles concentrations, which show lower thermal con-ductivities down to 1 W /m K in the similar ErAs:InGaAssystem.23 Additionally, the current approach is quite general

and should be applicable to other materials systems forwhich dense metal/semiconductor interface densities can begenerated.

ACKNOWLEDGMENTS

The authors would like to thank Herb Kroemer, SethBank, Mike Scarpulla, and Di Liang for many helpful dis-cussions. The authors also wish to acknowledge the financialsupport of the Office of Naval Research, primarily throughthe TEC-MURI program as well as the DARPA/ARMYNMP program.

1L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 16631 �1993�.2L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B 47, 12727 �1993�.3R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O’Quinn, Nature�London� 413, 597 �2001�.

4K. F. Hsu, S. Loo, F. Guo, W. Chen, J. S. Dyck, C. Uher, T. Hogan, E. K.Polychroniadis, and M. G. Kanatzidis, Science 303, 818 �2004�.

5B. Poudel, Q. Hao, Y. Ma, Y. Lan, A. Minnich, B. Yu, X. Yan, D. Wang,A. Muto, D. Vashaee, X. Chen, J. Liu, M. S. Dresselhaus, G. Chen, and Z.Ren, Science 320, 634 �2008�.

6T. C. Harman, P. J. Taylor, M. P. Walsh, and B. E. LaForge, Science 297,2229 �2002�.

7C. J. Vineis, T. C. Harman, S. D. Calawa, M. P. Walsh, R. E. Reeder, R.Singh, and A. Shakouri, Phys. Rev. B 77, 235202 �2008�.

8Y. K. Koh, C. J. Vineis, S. D. Calawa, M. P. Walsh, and D. G. Cahill,Appl. Phys. Lett. 94, 153101 �2009�.

9J. P. Heremans, V Jovovic, E. S. Toberer, A. Saramat, K. Kurosaki, A.Charoenphakdee, S. Yamanaka, and G. J. Snyder, Science 321, 554�2008�.

10G. D. Mahan and L. M. Woods, Phys. Rev. Lett. 80, 4016 �1998�.11A. Shakouri and J. E. Bowers, Appl. Phys. Lett. 71, 1234 �1997�.12D. Vashaee and A. Shakouri, Phys. Rev. Lett. 92, 106103 �2004�.13C. B. Vining and G. D. Mahan, J. Appl. Phys. 86, 6852 �1999�.14D. Vashaee and A. Shakouri, J. Appl. Phys. 95, 1233 �2004�.15J. M. O. Zide, D. Vashaee, Z. X. Bian, G. Zeng, J. E. Bowers, A. Shakouri,

and A. C. Gossard, Phys. Rev. B 74, 205335 �2006�.16J. P. Heremans, C. M. Thrush, and D. T. Morelli, J. Appl. Phys. 98, 063703

�2005�.17J. P. Heremans, C. M. Thrush, and D. T. Morelli, Phys. Rev. B 70, 115334

�2004�.18S. V. Faleev and F. Léonard, Phys. Rev. B 77, 214304 �2008�.19D. O. Klenov, D. C. Driscoll, A. C. Gossard, and S. Stemmer, Appl. Phys.

Lett. 86, 111912 �2005�.20I. Poole, K. E. Singer, A. R. Peaker, and A. C. Wright, J. Cryst. Growth

121, 121 �1992�.21M. Hanson, S. Bank, J. Zide, J. Zimmerman, and A. Gossard, J. Cryst.

Growth 301–302, 4 �2007�.22W. Kim, J. Zide, A. Gossard, D. Klenov, S. Stemmer, A. Shakouri, and A.

Majumdar, Phys. Rev. Lett. 96, 045901 �2006�.23W. Kim, S. L. Singer, A. Majumdar, J. M. O. Zide, D. Klenov, A. C.

Gossard, and S. Stemmer, Nano Lett. 8, 2097 �2008�.24J. M. Zide, D. O. Klenov, S. Stemmer, A. C. Gossard, G. Zeng, J. E.

Bowers, D. Vashaee, and A. Shakouri, Appl. Phys. Lett. 87, 112102�2005�.

25D. Liang A. W. Fang, H. Park, T. E. Reynolds, K. Warner, D. E. Oakley,and J. E. Bowers, J. Electron. Mater. 37, 1552 �2008�.

26J. Mort and F. Jansen, Plasma Deposited Thin Films �CRC, Boca Raton,1986�, p. 135.

27A. V. Kuchuk, V. P. Kladko, V. F. Machulin, A. Piotrowska, E. Kaminska,K. Golaszewska, R. Ratajczak, and R. Minikayev, Mater. Sci. 8, 22�2004�.

28CRC Handbook of Thermoelectrics �CRC, Boca Raton, 1995�.29D. G. Cahill, Rev. Sci. Instrum. 61, 802 �1990�.30T. Tong and A. Majumdar, Rev. Sci. Instrum. 77, 104902 �2006�.31Y. K. Koh, S. L. Singer, W. Kim, J. M. O. Zide, H. Lu, D. G. Cahill, A.

Majumdar, and A. C. Gossard, J. Appl. Phys. 105, 054303 �2009�.32M. Zebarjadi, K. Esfarjani, A. Shakouri, J.-H. Bahk, Z. Bian, G. Zeng, J.

Bowers, H. Lu, J. Zide, and A. Gossard, Appl. Phys. Lett. 94, 202105�2009�.

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n-InGaAlAs (control)

FIG. 6. �Color online� ZT of ErAs:InGaAlAs film �filled circles� andn-InGaAlAs control sample �open squares� vs temperature.

123702-5 Zide et al. J. Appl. Phys. 108, 123702 �2010�

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