Transient Harman Measurement of the Cross-plane ZT of InGaAs/InGaAlAs Superlattices with Embedded ErAs Nanoparticles
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Transient Harman Measurement of the Cross-plane ZT of InGaAs/InGaAlAs Superlattices with Embedded ErAs Nanoparticles Rajeev Singh, Zhixi Bian, Gehong Zeng1, Joshua Zide1, James Christofferson, Hsu-Feng Chou1, Art Gossard1, John Bowers1, and Ali Shakouri Electrical Engineering Department, University of California Santa Cruz, CA 95064, U.S.A. 1 Department of Electrical and Computer Engineering, University of California Santa Barbara, CA 93106, U.S.A. ABSTRACT The transient Harman technique is used to characterize the cross-plane ZT of InGaAs/InGaAlAs superlattice structures with embedded ErAs nanoparticles in the well layers. ErAs nanoparticles have proven to substantially reduce the thermal conductivity while slightly increasing the electrical conductivity of bulk InGaAs. The InGaAs/InGaAlAs superlattice structure was designed to have a barrier height of approximately 200meV. Although ErAs nanoparticles provide free carriers inside the semiconductor matrix, additional doping with Si increased the Fermi energy to just below the barrier height. The bipolar transient Harman technique was used to measure device ZT of samples with different superlattice thicknesses in order to extract the intrinsic cross-plane ZT of the superlattice by eliminating the effects of device Joule heating and parasitics. High-speed packaging is used to reduce signal ringing due to electrical impedance mismatch and achieve a short time resolution of roughly 100ns in transient Seebeck voltage measurement. The measured intrinsic cross-plane ZT of the superlattice structure is 0.13 at room temperature. This value agrees with calculations based on the Boltzmann transport equation and direct measurements of specific film properties. Theoretical calculations predict cross-plane ZT of the superlattice to be greater than 1 at temperatures greater than 700K. INTRODUCTION Thin-film semiconducting materials are receiving great interest for use in thermoelectric devices due to the ability to enhance the Seebeck coefficient (S) and reduce the thermal conductivity (κ) of the films by utilizing nanostructures. In particular, superlattice structures are being studied to achieve larger material S and smaller κ without significantly reducing the electrical conductivity (σ) of the material. These factors impact the dimensionless thermoelectric material figure-of-merit (ZT) given by ZT =
S 2σ
κ
T,
(1)
where T is the ambient temperature. By utilizing tall barriers and large well dopant concentrations that place the Fermi level (Ef) within a thermal energy (kT) below the barrier, material σ can be maintained while increasing material S through the enhancement of differential conductivity [σ(E)] via electron filtering [1-3]. In addition, material κ in superlattice structures can be reduced by interfacial phonon scattering [4,5]. Embedded metallic nanoparticles in the
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superlattice well regions can further increase phonon scattering in the material while providing additional free carriers [6-8]. Experimental measurement of cross-plan
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