Origin of Giant Seebeck Coefficient for High Density 2DEGs Confined in the SrTiO_3/SrTi_(0.8)Nb_(0.2)O_3 Superlattices

摘要

We report two-dimensional Seebeck coefficients (|S|_(2D)) of [(SrTiO_3)_x/(SrTi_(0.8)Nb_(0.2)O_3)_y]_(20) (x = 1-50, y = 1-16) superlattices, which were grown on the (100)-face of insulating LaAlO_3 substrates, to clarify the origin of the giant |S|_(2D) values of the SrTiO_3 superlattices [H. Ohta et al., Nature Materials 6, 129 (2007)]. The |S|_(2D) values of the [(SrTiO_3)_(17)/(SrTi_(0.8)Nb_(0.2)O_3)_y]_(20) superlattices increased proportionally to y~(0.5), and reached 320 μV K~(-1) (y = 1), which is ～5 times larger than that of the SrTi_(0.8)Nb_(0.2)O_3 bulk (|S|_(3D) = 61 μV K~(-1)). The slope of the log |S|_(2D)-log y plots was -0.5, proving that the density of states in the ground state for SrTiO_3 increases inversely proportionally to y. Further, the |S|_(2D) value monotonically increases with x-value and is saturated when x-value > 16 (6.25 nm). We clarified that the critical barrier thickness for electron tunneling in [(SrTiO_3)_x)/(SrTi_(0.8)Nb_(0.2)O_3)_y]_z superlattice is 6.25 nm (16 unit cell layers of SrTiO_3).