Charge imbalance at oxide interfaces: How nature deals with it

Some interfaces in semiconductors or insulators structurally cause a valence mismatch, which leads to a two-dimensional space charge that must be balanced by localised or mobile charge carriers. Screening by mobile electrons presents a lot of theoretical as well as practical interests. However it is extremely rare, so that we are aware of only one case, on which we focus here: the (0 0 1) interface between LaAlO 3 and TiO 2-terminated SrTiO 3. Theoretically, this interface between two insulators is positively charged. Electron conductivity is observed in this system, but whether it is associated with the interface screening or an extrinsic unintended doping is not yet settled. Here, we use the literature and our own numerical and practical experiments to discuss the physics of this system.