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 (κ_ele) and lattice thermal conductivities (κ_lat) of a TE material. In addition to reducing κ_lat, 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, λ_D)–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 λ_D 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 SrTi_1−xNb_xO₃, carrier effective mass (m^*) exerts a reducing tendency from 1.1m_e to 0.7m_e, when x increases across x = 0.3 point.^[3] So 2DES of 1 u.c. layer thick SrTi_1−xNb_xO₃ (x > 0.3) is hypothesized to exhibit greatly enhanced S due to its longer λ_D and correspondingly stronger two-dimensionality.
Here we report the TE properties of oxide 2DESs, [N unit cells SrTi_1−xNb_xO₃|11 unit cells SrTiO₃]₁₀ superlattices (1 ≤ N ≤ 12, x=0.2−0.9), in which the λ_D of x > 0.3 is ~5.2 nm while that of x ≤ 0.3 is ~4.1 nm. The S-enhancement factor (S_obsd./S_bulk) 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 λ_D. 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 SrTi_1−xNb_xO₃ (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).