Strain-induced indirect-to-direct bandgap transition in an np-type LaAlO₃/SrTiO₃(110) superlattice

Using first-principles calculations, we studied the influence of in-plane strain on the electronic properties of the polar LaAlO₃/SrTiO₃ (LAO/STO)(110) superlattice. We propose that alternate positively (n-type) and negatively (p-type) charged interfaces can be introduced along the [110] direction, without changing the stoichiometry of the system, if the np-type LAO/STO(001) superlattice is constructed in a 45° stepped pattern. We find that when the LAO-STO layer thickness is larger than the critical thickness of the insulator-metal transition, a quasi-two dimensional hole gas and an anisotropic quasi-two dimensional electron gas are formed at alternate interfaces. By applying uniaxial in-plane strains, an unexpected indirect-to-direct bandgap transition occurs in the polar LAO/STO(110) superlattices. The strain-induced changes in the O-2p orbitals near the p-type interface modify the dispersion of valence-band edges, leading to the transition.