Seebeck coefficient (vertical bar S vertical bar) 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 (L-B) and well SrTi0.8Nb0.2O3 (L-W) for giant vertical bar S vertical bar [H. Ohta et al., Nat. Mater. 6, 129 (2007)]. The vertical bar S vertical bar values of the superlattices increased proportionally to L-W(-1/2) due to increasing of the density of states near the conduction band edge (quantum size effect), and reached 300 mu V K-1 at L-W = 1 unit cell SrTi0.8Nb0.2O3 (0.39 nm), which is similar to 5 times larger than that of the SrTi0.8Nb0.2O3 bulk (60 mu V K-1). The critical thickness of L-B and L-W for giant vertical bar S vertical bar was clarified to be 16 unit cells (6.25 nm). The best thermoelectric performance can be obtained at (L-B, L-W)=(16, 1). (C) 2007 Elsevier B.V. All rights reserved.