##### Description

__Yuqiao Zhang__^{1} Bin Feng^{2} Hiroyuki Hayashi^{3} Isao Tanaka^{3} Yuichi Ikuhara^{2} Hiromichi Ohta^{1}

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*=*S*^{2}*σ**T**κ*^{−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*

*) of a TE material. In addition to reducing*

_{lat}*κ*

*, enhancing*

_{lat}*S*

^{2}

*σ*, 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−x}Nb

*O*

_{x}_{3}, carrier effective mass (

*m*

^{*}) exerts a reducing tendency from 1.1

*m*

_{e}to 0.7

*m*

_{e}, when

*x*increases across

*x*= 0.3 point.

^{[3]}So 2DES of 1 u.c. layer thick SrTi

_{1−x}Nb

*O*

_{x}_{3}(

*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−x}Nb

*O*

_{x}_{3}|11 unit cells SrTiO

_{3}]

_{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−x}Nb

*O*

_{x}_{3}(

*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).