  The discovery of a two-dimensional electron system (2DES) at the interfaces
of perovskite oxides such as LaAlO3 and SrTiO3 has motivated enormous efforts
in engineering interfacial functionalities with this type of oxide
heterostructures. However, its fundamental origins are still not understood,
e.g. the microscopic mechanisms of coexisting interface conductivity and
magnetism. Here we report a comprehensive spectroscopic investigation of the
depth profile of 2DES-relevant Ti 3d interface carriers using depth- and
element-specific techniques, standing-wave excited photoemission and resonant
inelastic scattering. We found that one type of Ti 3d interface carriers, which
give rise to the 2DES are located within 3 unit cells from the n-type interface
in the SrTiO3 layer. Unexpectedly, another type of interface carriers, which
are polarity-induced Ti-on-Al antisite defects, reside in the first 3 unit
cells of the opposing LaAlO3 layer (~10 {\AA}). Our findings provide a
microscopic picture of how the localized and mobile Ti 3d interface carriers
distribute across the interface and suggest that the 2DES and 2D magnetism at
the LaAlO3/SrTiO3 interface have disparate explanations as originating from
different types of interface carriers.
