Origin of Giant Seebeck Coefficient for High Density 2DEGs Confined in the SrTiO 3 /SrTi 0.8 Nb 0.2 O 3 Superlattices
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1044-U09-05
Origin of Giant Seebeck Coefficient for High Density 2DEGs Confined in the SrTiO3/SrTi0.8Nb0.2O3 Superlattices Yoriko Mune1, Hiromichi Ohta1,2, Teruyasu Mizoguchi3, Yuichi Ikuhara3, and Kunihito Koumoto1,2 1 Graduated School of Engineering, Nagoya University, Fro-cho, Chikusa, Nagoya, 464-8603, Japan 2 CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, 332-0012, Japan 3 Institute of Engineering Innovation, The University of Tokyo, 2-11-16 Yayoi, Bunkyo, Tokyo, 113-8656, Japan ABSTRACT We report two-dimensional Seebeck coefficients (|S|2D) of [(SrTiO3)x/(SrTi0.8Nb0.2O3)y]20 (x = 1-50, y = 1-16) superlattices, which were grown on the (100)-face of insulating LaAlO3 substrates, to clarify the origin of the giant |S|2D values of the SrTiO3 superlattices [H. Ohta et al., Nature Materials 6, 129 (2007)]. The |S|2D values of the [(SrTiO3)17/(SrTi0.8Nb0.2O3)y]20 superlattices increased proportionally to y−0.5, and reached 320 µV K−1 (y = 1), which is ~5 times larger than that of the SrTi0.8Nb0.2O3 bulk (|S|3D = 61 µV K−1). The slope of the log |S|2D-log y plots was -0.5, proving that the density of states in the ground state for SrTiO3 increases inversely proportionally to y. Further, the |S|2D value monotonically increases with x-value and is saturated when x-value > 16 (6.25 nm). We clarified that the critical barrier thickness for electron tunneling in [(SrTiO3)x/(SrTi0.8Nb0.2O3)y]z superlattice is 6.25 nm (16 unit cell layers of SrTiO3). INTRODUCTION Recently, metal oxides attract much attention for thermoelectric power generation at high temperatures on the basis of their potential advantages over heavy metallic alloys in chemical and thermal robustness [1-5]. Among metal oxides, SrTiO3 crystal is a promising candidate as thermoelectric material applicable at high temperatures. SrTiO3 is a popular metal oxide with cubic perovskite structure (lattice parameter, a = 0.3905 nm). The melting point of SrTiO3 is 2080°C, applicable at high temperatures. All the constituents of SrTiO3 are rich in natural resources. Further, the electrical conductivity of SrTiO3 can be easily controlled from insulator to metal by the substitutional doping of La3+ or Nb5+. Since the ZT value of Nb-doped SrTiO3 (SrTi0.8Nb0.2O3) is ~0.37 at 1000 K [4, 5] smaller than that of heavy-metal-based materials, significant improvement of ZT is required for the practical thermoelectric application of SrTiO3. In order to improve the ZT of SrTiO3, reduction of κ value without reducing the S2·σ is essentially required. Muta et al. [6, 7] have reported that Sr2+-site substitution of SrTiO3 with Ca2+ or Ba2+ may be a good way to reduce κ value of SrTiO3, most likely due to that introduction of defects such as site substitution and/or layered structure are effective to reduce the κ value. However, Yamamoto et al. [8] clarified that the S2·σ drastically decreases when Ca2+ and/or Ba2+ are substituted for Sr2+ in SrTi0.8Nb0.2O3 (Fig. 3), indicating that Sr2+-site substitution negatively affects the thermoelectric performance of SrTi0
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