  At the (001) interface between the two band-insulators LaAlO3 and SrTiO3, a
high-mobility electron gas may appear, which has been the object of numerous
works over the last four years. Its origin is a subject of debate between the
interface polarity and unintended doping. Here we use electron energy loss
'spectrum images', recorded in cross-section in a scanning transmission
electron microscope, to analyse the Ti3+ ratio, characteristic of extra
electrons. We find an interface concentration of Ti3+ that depends on growth
conditions.
