Tuning the two-dimensional electron gas at the LaAlO3/SrTiO3(001) interface by metallic contacts

Density functional theory (DFT) calculations reveal that adding a metallic overlayer on LaAlO3/SrTiO3(001) alters significantly the electric field within the polar LaAlO3 film. For Al or Ti metal contacts the electric field is eliminated, leading to a suppression of the thickness-dependent insulator-to-metal transition observed in uncovered films. Independent of the LaAlO3 thickness, both the surface and the interface are metallic, with an enhanced carrier density at the interface relative to LaAlO3/SrTiO3(001) after the metallization transition. Monolayer thick contacts of Ti develop a finite magnetic moment and for a thin SrTiO3 substrate induce a spin-polarized two-dimensional electron gas at the n-type interface, due to confinement effects in the SrTiO3 slab. For transition (Fe, Co, Pt) and noble metal contacts (Cu, Ag, Au) a finite and even enhanced (Au) internal electric field develops within LaAlO3. Results for a representative series of metallic overlayers on LaAlO3/SrTiO3(001) (Na, Al; Ti, Fe, Co, Pt; Cu, Ag, Au) reveal broad variation of band alignment, size of Schottky barrier, carrier concentration and lattice polarization at the LaAlO3/SrTiO3(001) interface. The identified relationship to the size of work function of the metal on LaAlO3 provides guidelines on how the carrier density at the LaAlO3/SrTiO3 interface can be controlled by the choice of the metal contact.