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Yuqiao Zhang1 Bin Feng2 Hiroyuki Hayashi3 Isao Tanaka3 Yuichi Ikuhara2 Hiromichi Ohta1

 

1, Hokkaido University, Sapporo, , Japan
2, The University of Tokyo, Tokyo, , Japan
3, Kyoto University, Kyoto, , Japan

Thermoelectric (TE) materials, which could directly convert temperature difference into electricity, are attracting increasing attentions among energy harvesting technologies. Generally, the performance of TE materials is evaluated in terms of a dimensionless figure of merit, ZT=S2σTκ−1, where Z is the figure of merit, T is the absolute temperature, S is the thermopower (≡Seebeck coefficient), σ is the electrical conductivity and κ is the sum of the electronic (κele) and lattice thermal conductivities (κlat) of a TE material. In addition to reducing κlat, enhancing S2σ, which is regarded as power factor (PF) is also a promising strategy.
Two-dimensional electron system (2DES)–carrier electrons are confined within a narrow layer (the thickness < de Broglie wavelength, λD)–is known as one of efficient strategies to achieve an enhanced PF because it could promise an enhanced S without reducing σ. [1,2] Since the degree of S-enhancement strongly depends on the two-dimensionality of 2DES, a conducting material with longer λD would be efficient to enhance PF if the carrier electrons are confined within a defined thickness layer.
Recently, we found that with increasing x in SrTi1−xNbxO3, carrier effective mass (m*) exerts a reducing tendency from 1.1me to 0.7me, when x increases across x = 0.3 point.[3] So 2DES of 1 u.c. layer thick SrTi1−xNbxO3 (x > 0.3) is hypothesized to exhibit greatly enhanced S due to its longer λD and correspondingly stronger two-dimensionality.
Here we report the TE properties of oxide 2DESs, [N unit cells SrTi1−xNbxO3|11 unit cells SrTiO3]10 superlattices (1 ≤ N ≤ 12, x=0.2−0.9), in which the λD of x > 0.3 is ~5.2 nm while that of x ≤ 0.3 is ~4.1 nm. The S-enhancement factor (Sobsd./Sbulk) of the 2DES for x=0.8 was ~1000%, while that for x=0.2 and 0.3 were 400−500%, clearly indicating that two-dimensionality can be enhanced by using a conducting material with longer λD. As a result of precise control of N and x, PF of the superlattice (N=1, x=0.6) exceeded ~5 mW m−1 K−2, which is double of the optimized bulk SrTi1−xNbxO3 (PF~2.5 mW m−1 K−2). The present results might be fruitful to design efficient TE materials with 2DES.

References
[1] L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B, 47, 12727 (1993).
[2] H. Ohta et al., Nature Mater. 6, 129 (2007); Nature Commun. 1, 118 (2010); Adv. Mater. 24, 740 (2012).
[3] Y. Zhang, H. Ohta et al., J. Appl. Phys. 121, 185102 (2017).

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