Quantum Size Effect of 2DEG Confined Within BaTiO 3 /SrTiO 3 :Nb Superlattices
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1044-U09-06
Quantum Size Effect of 2DEG Confined Within BaTiO3/SrTiO3:Nb Superlattices Yuki Nakanishi1, Hiromichi Ohta1,2, Teruyasu Mizoguchi3, Yuichi Ikuhara3, and Kunihito Koumoto1,2 1 Graduate School of Engineering, Nagoya University, Furo-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 Carrier transport properties such as carrier concentration (ne), Hall mobility (µHall) and Seebeck coefficient (S) for the superlattices of [(BaTiO3)x/(SrTi0.7Nb0.3O3)1]z (x = 1, 4, 6, 9 and 15) were measured to clarify the critical barrier thickness for the electron confinement in a unit cell layer thickness of SrTi0.7Nb0.3O3. The µHall values drastically decreased with the x-value due to the Ba-alloying occurred. We observed enhancement of |S| value in BaTiO3/SrTi1-xNbxO3 superlattices due to the quantum size effect, which is similar to SrTiO3/SrTi0.8Nb0.2O3 superlattices. Critical BaTiO3 layer thickness for fully confinement of the conduction electron significantly decreased (6 unit cells) as compared to that of SrTiO3/SrTi0.8Nb0.2O3 superlattices. INTRODUCTION Utilization of the quantum size effect [1] should be beneficial in obtaining high performance thermoelectric materials as demonstrated by Hicks and Dresselhaus [2, 3]. They theoretically predicted that the thermoelectric figure of merit, Z2DT (= S2⋅σ⋅T⋅κ-1, where Z2D, T, S, σ, and κ are figure of merit, absolute temperature, Seebeck coefficient, electrical conductivity, and thermal conductivity, respectively) of thermoelectric semiconductors can be dramatically enhanced by use of superlattices. Recently, we have demonstrated a new class of thermoelectric material: SrTiO3 with two-dimensional electron gas (2DEG) [4, 5]. High-density (~1021 cm−3) 2DEG, which has been realized in SrTiO3/Nb-doped SrTiO3 superlattices or TiO2/SrTiO3 heterointerfaces, is confined to the thickness of a SrTiO3 unit cell layer (a = 0.3905 nm). 2DEG demonstrates that the 2D-thermoelectric Seebeck coefficient (|S|2D) is enhanced by a factor of ~5 compared to that of the bulk at room temperature. We have also clarified that the origin of the giant |S|2D of SrTiO3/SrTi0.8Nb0.2O3 superlattices is the quantum size effect [6, 7]: the density of states at the ground state for the SrTi0.8Nb0.2O3 layer is inversely proportional to the SrTi0.8Nb0.2O3 layer thickness. Although, extremely high ZT2DEG of ~2.4 can be obtained at room temperature, the effective ZTeff is still small because 16 unit cells of electrically insulating SrTiO3 layers is required to fully confine the conduction electrons in a unit cell layer thickness of SrTi0.8Nb0.2O3 [7]. When the barrier SrTiO3 thickness is less than the critical thickness (16 unit cells), a part of the conduction electrons may transmit
through the barrier layer (tunnel effect). Generally transmissivity of conduction electrons in the quantum well tend to decrease with bar
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