Enhanced Seebeck coefficient of quantum-confined electrons in 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=1－60, y=1－20) superlattices, which were grown on the (100) face of insulating LaAlO3 substrates to clarify the origin of the giant |S|_(2D) values of the SrTiO_3 superlattices [H. Ohta et al, Nat. Mater. 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 approximately five 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. The critical barrier thickness for quantum electron confinement was also clarified to be 6.25 nm (16 unit cells of SrTiO_3).