Direct measurement of thin-film thermoelectric figure of merit - eScholarship (2009)

APPLIED PHYSICS LETTERS 94, 212508 2009 Direct measurement of thin-ﬁlm thermoelectric ﬁgure of merit Rajeev Singh, 1,a Zhixi Bian, 1 Ali Shakouri, 1 Gehong Zeng, 2 Je-Hyeong Bahk, 2 John E. Bowers, 2 Joshua M. O. Zide, 3 and Arthur C. Gossard 3 Baskin School of Engineering, University of California, Santa Cruz, California 95064, USA Department of Electrical and Computer Engineering, University of California, Santa Barbara, California 93106, USA Department Materials, University of Santa Barbara, California 93106, USA Received 20 October 2008; accepted 13 February 2009; published online 28 May 2009 We utilize the transient Harman technique to measure the thermoelectric ﬁgure of merit of thin ﬁlms. A device structure is designed and fabricated to extract the thermoelectric properties of 20 ␮ m thick ﬁlm composed of InGaAlAs semiconductor with embedded ErAs nanoparticles. High-speed voltage measurements with 63 dB of dynamic range and 200 ns resolution are achieved. Surface temperature measurements of the devices are used to extract the cross-plane Seebeck coefﬁcient and thermal conductivity of the thermoelectric material. Self-consistent ﬁnite-element simulations of the three-dimensional temperature distributions in the active devices are in close agreement with the experimental thermal maps. © 2009 American Institute of Physics. DOI: 10.1063/1.3094880 Thermoelectric materials are playing an increasingly im- portant role in a technologically advanced and resource- limited world. An important application of thermoelectric materials is in direct thermal-to-electrical energy conversion. Because a substantial amount of energy in our world is in thermal form, there are tremendous applications for thermo- electric devices that efﬁciently convert thermal energy into electricity. The energy conversion efﬁciency of a thermoelec- tric material is a function of its dimensionless ﬁgure of merit that is deﬁned as ZT ϵ S 2 ␴ T / ␬ , where S, ␴ , ␬ , and T are the material’s Seebeck coefﬁcient, electrical conductivity, ther- mal conductivity, and ambient temperature, respectively. The numerator of ZT is known as the thermoelectric power factor P. Recent work to improve the ZT of materials designed for high-temperature power generation has focused on thin ﬁlms due to the freedom to engineer the materials at the nanoscale. In bulk thermoelectric materials, there is a dra- matic tradeoff between S and free carrier concentration n. Therefore, n is chosen to optimize P at a particular T. Efforts have been made to decouple S and n to increase the P of thin ﬁlms beyond bulk values using low dimensional structures or with electronic energy ﬁltering for large n in tall barrier heterostructures. 1 However, the majority of improvement in ZT has been achieved through the reduction in ␬ due to interfacial phonon scattering. An extension of this concept was recently demonstrated in isotropic nanoparticle thin ﬁlms that exhibit a ␬ that is a factor of 2 below the bulk alloy limit due to enhanced scattering of longer wavelength phonons. 2 This reduction in ␬ is achieved while enhancing ␴ due to an increase in n by the semimetallic nanoparticles. Nanostructured thin-ﬁlm research offers great potential to in- crease the ZT of thermoelectric materials. Accurate characterization of thin ﬁlm thermoelectrics is an important step in the development of these materials. In order to evaluate the ZT of thin ﬁlms, we utilize direct cross- plane ZT measurement using the Harman method. 3 The Har- man method utilizes measurements of transient voltages in a thermoelectric device to extract the resistive voltage drop a and the Seebeck-induced voltage drop slower thermal re- sponse. Further study of the Seebeck voltage under opposite polarities permits the separation of the Joule effect-induced and the Peltier effect-induced Seebeck voltages. However, the Harman method is quite difﬁcult to apply to thin ﬁlms. Due to the fast thermal response of thin ﬁlms, high-speed detection of the small transient Seebeck voltage is necessary. 4 In addition, because this method relies on closed- circuit electrical measurements, electrical parasitics such as resistances in series with the material under investigation will result in a reduction in the measured ZT relative to the intrinsic material value. Additionally, the side of the mea- surement device that is not heat-sinked must be free of any parasitic heat load, making electrical probing difﬁcult for the requirement of uniform device current injection. Speciﬁc device geometries are used to obtain accurate measurement of the cross-plane ZT of InGaAlAs thin ﬁlms with embedded ErAs nanoparticles Fig. 1. The devices are Electronic mail: rsingh@soe.ucsc.edu. FIG. 1. Color online a Diagram of device designed for direct measure- ment of the cross-plane ZT of ErAs:InGaAlAs thin-ﬁlm thermoelectric ma- terial. b Magniﬁed optical image of patterned devices of different mesa areas. © 2009 American Institute of Physics