Two-dimensional thermoelectric Seebeck coefficient of SrTiO3-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 (similar to 0.4 nm) of a SrTi0.8Nb0.2O3 layer of artificial superlattices of SrTiO3/SrTi0.8Nb0.2O3 [H. Ohta et al., Nature Mater 6, 129, (2007)]. The |S|(2D) values of the [(SrTiO3)(17)/SrTi0.8Nb0.2O3)(y)](20) superlattice increase proportional y(0.5), and reach 290 mu VK-1 (y = 1) at room temperature, which is similar to 5 times larger than that of the SrTi0.8Nb0.2O3 bulk (|S|(3D) = 61 mu VK-1) proving that the density of states in the ground state of SrTiO3 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 SrTiO3). 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 mu VK-1 at 900 K, which is similar to 3 times larger than that of bulk SrTi0.8Nb0.2O3. These observations provide clear evidence than the [(SrTiO3)(17)/SrTi0.8Nb0.2O3)](20) superlattice is stable and exhibits a giant |S| even at high temperature.