Wetting an oxide heterostructure: influence of surface water adsorbates on the electronic properties of LaAlO3/SrTiO3 studied by in situ photoelectron spectroscopy

Oxide heterostructures display many interesting phenomena, one example being the formation of a two-dimensional electron system (2DES) at the LaAlO3/SrTiO3 (LAO/STO) interface beyond a critical thickness of 4 monolayers (ML) of the polar LAO. An explanation for this behavior is the so-called electronic reconstruction. In this context it was recently shown that polar adsorbates can enhance the conductivity of the 2DES. Besides their electrostatic influence it was discussed that surface defects/adsorbates can also act as a charge reservoir. To examine the impact of surface adsorbates on the 2DES we performed in situ photoelectron spectroscopy on 6ML thick LAO/STO heterostructures exposed to a defined amount of water vapor. In response to the adsorbed water we observe an increase of charge carriers at titanium sites. Furthermore a comparison between bulk and interface sensitive measurements indicates that the additional charge carriers are located at the interface.