Strain effects on the electronic properties in δ-doped oxide superlattices

Strain effects on the electronic properties of (LaTiO₃)1/(SrTiO₃)N superlattices were investigated using density functional theory. Under biaxial in-plane strain within the range of-5%≤ ε≤ 5%, the d_xy orbital electrons are highly localized at the interfaces whereas the d_yz and d_xz orbital electrons are more distributed in the SrTiO₃ (STO) spacer layers. For STO thickness N≥ 3 unit cells (u.c.), the d_xy orbital electrons form two-dimensional (2D) electron gases (2DEGs). The quantized energy levels of the 2DEG are insensitive to the STO spacer thickness, but are strongly dependent on the applied biaxial in-plane strain. As the in-plane strain changes from compressive to tensile, the quantized energy levels of the d_xy orbitals decrease thereby creating more states with 2D character. In contrast to the d_xy orbital, the d_yz and d_xz orbitals always have three-dimensional (3D) transport characteristics and their energy levels increase as the strain changes from compressive to tensile. Since the charge densities in the d_xy orbital and the d_yz and d_xz orbitals respond to biaxial in-plane strain in an opposite way, the transport dimensionality of the majority carriers can be controlled between 2D and 3D by applying biaxial in-plane strain.