Two‐dimensional thermoelectric Seebeck coefficient of SrTiO 3 ‐based superlattices

This review provides the origin of the unusually large thermoelectric Seebeck coefficient | S | of a two‐dimensional electron gas confined within a unit cell layer thickness (∼0.4 nm) of a SrTi 0.8 Nb 0.2 O 3 layer of artificial superlattices of SrTiO 3 /SrTi 0.8 Nb 0.2 O 3 [H. Ohta et al., Nature Mater. 6 , 129 (2007)]. The | S | 2D values of the [(SrTiO 3 ) 17 /(SrTi 0.8 Nb 0.2 O 3 ) y ] 20 superlattice increase proportional to y –0.5 , and reach 290 μV K –1 ( y = 1) at room temperature, which is ∼5 times larger than that of the SrTi 0.8 Nb 0.2 O 3 bulk (| S | 3D = 61 μV K –1 ), proving that the density of states in the ground state for SrTiO 3 increases in inverse proportion to y . The critical barrier thickness for quantum electron confinement is also clarified to be 6.25 nm (16 unit cells of SrTiO 3 ). Significant structural changes are not observed in the superlattice after annealing at 900 K in a vacuum. The value of | S | 2D of the superlattice gradually increases with temperature and reaches 450 μV K –1 at 900 K, which is ∼3 times larger than that of bulk SrTi 0.8 Nb 0.2 O 3 . These observations provide clear evidence that the [(SrTiO 3 ) 17 /(SrTi 0.8 Nb 0.2 O 3 ) 1 ] 20 superlattice is stable and exhibits a giant | S | even at high temperature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)