  LaAlO3/SrTiO3 structures showing high mobility conduction have recently
aroused large expectations as they might represent a major step towards the
conception of all-oxide electronics devices. For the development of these
technological applications a full understanding of the dimensionality and
origin of the conducting electronic system is crucial. To shed light on this
issue, we have investigated the magnetotransport properties of a LaAlO3 layer
epitaxially grown at low oxygen pressure on a TiO2-terminated (001)-SrTiO3
substrate. In agreement with recent reports, a low-temperature mobility of
about 10^4 cm2/Vs has been found. We conclusively show that the electronic
system is three-dimensional, excluding any interfacial confinement of carriers.
We argue that the high-mobility conduction originates from the doping of SrTiO3
with oxygen vacancies and that it extends over hundreds of microns into the
SrTiO3 substrate. Such high mobility SrTiO3-based heterostructures have a
unique potential for electronic and spintronics devices.
