Enhanced Seebeck coefficient of quantum-confined electrons in SrTiO3/SrTi0.8Nb0.2O3 superlattices

We report two-dimensional Seebeck coefficients (parallel to S parallel to(2D)) of [(SrTiO3)(x)/(SrTi0.8Nb0.2O3)(y)](20) (x=1-60, y=1-20) superlattices, which were grown on the (100) face of insulating LaAlO3 substrates to clarify the origin of the giant parallel to S parallel to(2D) values of the SrTiO3 superlattices [H. Ohta , Nat. Mater. 6, 129 (2007)]. The parallel to S parallel to(2D) values of the [(SrTiO3)(17)/(SrTi0.8Nb0.2O3)(y)](20) superlattices increased proportionally to y(-0.5) and reached 320 mu V K-1 (y=1), which is approximately five times larger than that of the SrTi0.8Nb0.2O3 bulk (parallel to S parallel to(3D)=61 mu V K-1). The slope of the log parallel to S parallel to(2D)-log y plots was -0.5, proving that the density of states in the ground state for SrTiO3 increases inversely proportionally to y. The critical barrier thickness for quantum electron confinement was also clarified to be 6.25 nm (16 unit cells of SrTiO3).