Modulation of electron carrier density at the n-type LaAlO3/SrTiO3 interface by water adsorption

We investigate energetic stability and dissociation dynamics of water adsorption at the LaAlO3 surface of the n-type LaAlO3/SrTiO3 (LAO/STO) interface and its effect on electronic properties of the interface by carrying out first-principles electronic structure calculations. In an ambient atmosphere at room temperature the configuration of 1 monolayer (ML) of water molecules including 3/4 ML of dissociated water molecules adsorbed at the surface is found to be most stable, the configuration of 1 ML of dissociated water molecules is metastable. Water molecule dissociation induces a shift-up of the valence band maximum (VBM) of the LAO surface, reducing the gap between the VBM of the LAO surface and the conduction band minimum of the STO. For the LAO/STO interface with three LAO unit-cell layers, once the coverage of dissociated water molecules reaches 1/2 ML the gap is closed, the interface becomes metallic and the carrier density at the LAO/STO interface increases with increasing the coverage of dissociated water molecules. Our findings suggest two ways to control the conductivity at the LAO/STO interface: (I) insulator-metal transition by adsorbing a mount of water at the bare surface; (II) carrier density change by the transition between the most stable and the metastable adsorption configurations for 1 ML coverage in an ambient atmosphere at room temperature.