  At the interface between complex insulating oxides, novel phases with
interesting properties may occur, such as the metallic state reported in the
LaAlO3/SrTiO3 system. While this state has been predicted and reported to be
confined at the interface, some works indicate a much broader spatial
extension, thereby questioning its origin. Here we provide for the first time a
direct determination of the carrier density profile of this system through
resistance profile mappings collected in cross-section LaAlO3/SrTiO3 samples
with a conducting-tip atomic force microscope (CT-AFM). We find that, depending
upon specific growth protocols, the spatial extension of the high-mobility
electron gas can be varied from hundreds of microns into SrTiO3 to a few
nanometers next to the LaAlO3/SrTiO3 interface. Our results emphasize the
potential of CT-AFM as a novel tool to characterize complex oxide interfaces
and provide us with a definitive and conclusive way to reconcile the body of
experimental data in this system.
