Critical thickness for giant thermoelectric Seebeck coefficient of 2DEG confined in SrTiO3/SrTi0.8Nb0.2O3 superlattices

Seebeck coefficient (|S|) of two-dimensional electron gas (2DEG) confined within (SrTiO3)LB/(SrTi0.8Nb0.2O3)LW superlattices were measured at room temperature to clarify the critical thicknesses of barrier SrTiO3 (LB) and well SrTi0.8Nb0.2O3 (LW) for giant |S| [H. Ohta et al., Nat. Mater. 6, 129 (2007)]. The |S| values of the superlattices increased proportionally to LW− 1/2 due to increasing of the density of states near the conduction band edge (quantum size effect), and reached 300 μV K− 1 at LW = 1 unit cell SrTi0.8Nb0.2O3 (0.39 nm), which is ∼ 5 times larger than that of the SrTi0.8Nb0.2O3 bulk (60 μV K− 1). The critical thickness of LB and LW for giant |S| was clarified to be 16 unit cells (6.25 nm). The best thermoelectric performance can be obtained at (LB, LW) = (16, 1).