Enhanced Seebeck Coefficient of Amorphous Oxide Semiconductor Superlattices
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1044-U09-09
Enhanced Seebeck Coefficient of Amorphous Oxide Semiconductor Superlattices Hiromichi Ohta1, Rong Huang2, and Yuichi Ikuhara2,3 1 Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa, Nagoya, 464-8603, Japan 2 Nanostructures Research Laboratory, Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta, Nagoya, 456-8587, Japan 3 Institute of Engineering Innovation, The University of Tokyo, 2-11-16 Yayoi, Bunkyo, Tokyo, 113-8656, Japan ABSTRACT We propose herein that amorphous oxide semiconductor (AOS) superlattices, which can be deposited on various substrate including glasses or plastics without any substrate heating, are appropriate for the realization of superlattice thermoelectric devices. As an example, thermoelectric properties of AOS superlattices composed of a-In-Zn-O (well) and a-In-Ga-Zn-O (barrier) layers, fabricated on SiO2 glass substrate by pulsed laser deposition at room temperature, were measured to clarify whether enhancement of Seebeck coefficient |S| occurs or not. The |S|2D value increases drastically with decreasing a-In-Zn-O thickness (dIZO) when the dIZO is < ~5 nm, and reached 73 µV·K-1 (dIZO = 0.3 nm), which is ~4 times larger than that of bulk |S|3D (19 µV·K1 ), while it kept high electrical conductivity, clearly demonstrating that the quantum size effect can be utilized in AOS superlattices. INTRODUCTION Two-dimensionally (2D) confined electrons in extremely narrow quantum wells (thickness ≤ thermal de Broglie wavelength), the latter being composed of an electron pocket and a barrier, exhibit exotic electron transport properties as compared to the corresponding bulk materials due to the fact that the density of states (DOS) near the bottom of the conduction band and/or top of the valence band increases with decreasing thickness of the quantum well. This phenomenon is the well-known quantum size effect, which has been widely applied in optoelectronic devices such as light emitting diodes and laser diodes based on GaAs [1] and GaN [2]. Utilization of the quantum size effect should be also beneficial in obtaining high performance thermoelectric materials. In 1993, Hicks and Dresselhaus theoretically predicted that the thermoelectric figure of merit, Z2DT of thermoelectric semiconductors can be dramatically enhanced by use of superlattices, with electrons confined in the resulting quantum wells, because the only effect is that the |S| value increases with DOS, without other values decreasing [3]. Thus, the highest Z2DT can be realized if conduction electrons are confined within the narrowest possible 2D space. Very recently, we reported that two-dimensional electron gas (2DEG) confined within a unit cell layer thickness of SrTiO3 exhibit unusually large Seebeck coefficient (|S|2D/|S|3D ~5) using epitaxial superlattices of SrTiO3/SrTiO3:Nb [4, 5]. Although the 2DEGSrTiO3 concept may be applicable for the next generation of thermoelectrics, significant reduction of the production cost such as low temperature process and large area deposition is strongly required for the prac
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