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

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Direct measurement of thin-film thermoelectric figure of merit - eScholarship

Hot target magnetron sputtering process: Effect of infrared radiation on the deposition of titanium and titanium oxide thin films

FEM simulation on elastic parameters of porous silicon with different pore shapes

Thermoelectric properties of nanostructured porous-polysilicon thin films

In this paper, the structural, optical and thermal properties of n-type (100),
p-type (100) and (111) mesoporous silicon (MePSi) are reported. The mesoporous silicon was
prepared by an electrochemical process from bulk silicon wafer. Depending on the etching
depth, analyses show that the porosity of p-type (111) increased by 32 to 40% compared to p
(100) which, in turn, increased by 22 to 48% compared to n-type (100). The structure
morphology and the abundance of Si-Ox and Si-Hy also depended heavily on the type and
crystal orientation of MePSi. The thermal properties of the MePSi layers such as thermal
conductivity (κ), volumetric heat capacity (ρCp) and thermal contact resistance (Rth) were
determined using the pulsed photothermal method. The thermal conductivity of bulk silicon
dropped sharply after etching, decreasing by more than twenty-fold in the case of n-type (100)
and by over forty-five fold for p-type (100) and (111). According to the percolation model
depending on both porosity and phonon confinement, the drop in thermal conductivity was
mainly due to the nanostructure formation after etching. Thermal investigations showed that
the volumetric heat capacity (ρCp) followed the barycentric model which depends mainly on
the porosity. The thermal contact resistances of MePSi layers were estimated to be in the
range of 1x10-8 to 1x10-7 K⋅m2⋅W-1.

Structural, Optical, and Thermophysical Properties of Mesoporous Silicon Layers: Influence of Substrate Characteristics

Mesoporous silicon (MePSi) is a promising material for future microelectronic chip multifunctionalization systems and for microsensing devices. In addition to the electrical characteristics, the thermal properties of MePSi layers affect the heat dissipation and the thermal management in related microelectronic devices. In this study, the thermal properties of n-type MePSi, prepared by electrochemical etching, have been investigated using the pulsed photothermal method. SEM observations evidence a columnar structure of pores, whose size varies between 5 and 25 nm, for the different etching depths (0.2, 1, 10, and 50 μm). Also, Fourier transform infrared (FTIR) spectroscopy has highlighted the presence of Si–H and Si–O bonds in addition to the confirmation of the porosity values and their size distribution. By means of the pulsed photothermal method (PPT), the thermal properties of MePSi such as thermal conductivity (k), volume specific heat (ρCp), and thermal contact resistance (Rth) have been determined. ...

Structural, Optical, and Thermal Analysis of n-Type Mesoporous Silicon Prepared by Electrochemical Etching

This communication describes the development of optimized metallic contacts on Si for thermoelectric applications. Thin solid films of Ni and Pt with the same thickness, were deposited on Si substrates. Two silicides were formed in a vacuum chamber and were studied. The EDX spectroscopy and electron microscopy have supported the presence of silicides in the surface of the samples. The thermoelectric study demonstrated that silicides could play a vital role in the enhancement of the electricity generated by thermoelectric materials that are made of Si. Pt silicide was found to be better candidate than three other metallic contacts (Pt, Ni and Ni silicide), but a comparison with other silicides is needed in the future, to get the best electronic contact on thermoelectric materials.

Establishment of optimized metallic contacts on silicon for thermoelectric applications

In this paper, an original homemade system is presented in detail for the electrical and thermoelectrical characterizations of several types of materials from bulk to thin films. This setup was built using a modulated CO2 laser beam to probe the thermoelectric properties at different depths below the surface. It allows a simultaneous measurement of the electrical conductivity (σ) and the Seebeck coefficient (S), from room temperature up to 250 °C. A commercial sample of Bi2Te3 was first used to validate the Seebeck coefficient measurement. Single crystalline silicon (sc-Si) was used for the uncertainty quantification during the simultaneous measurement of the Seebeck coefficient and the electrical conductivity. At the micrometer scale, thermoelectric characterization of the mesoporous Si (50 μm thickness) was achieved and results gave very promising values (S ≈ 700 μV K-1) for micro-thermo-generator fabrication. In the case of thin film materials, metals (copper and constantan) and oxide thin films (titanium oxide) were also characterized in the in-plane configuration in order to determine the metrology limits of our thermoelectric setup. In this case, a typical sensitivity of about 2μV K-1 was achieved.

Amer Melhem

Papers

Structural, Optical, and Thermal Analysis of n-Type Mesoporous Silicon Prepared by Electrochemical Etching

Structural, Optical, and Thermophysical Properties of Mesoporous Silicon Layers: Influence of Substrate Characteristics

Establishment of optimized metallic contacts on silicon for thermoelectric applications

Laser-based setup for simultaneous measurement of the Seebeck coefficient and electrical conductivity for bulk and thin film thermoelectrics.