  We report on the fabrication of conducting interfaces between LaAlO$_3$ and
SrTiO$_3$ by 90$^o$ off-axis sputtering in an Ar atmosphere. At a growth
pressure of 0.04 mbar the interface is metallic, with a carrier density of the
order of $10^{13}$ cm$^{-2}$ at 3 K. By increasing the growth pressure, we
observe an increase of the out-of-plane lattice constants of the LaAlO$_3$
films while the in-plane lattice constants do not change. Also, the
low-temperature sheet resistance increases with increasing growth pressure,
leading to an insulating interface when the growth pressure reaches 0.10 mbar.
We attribute the structural variations to an increase of the La/Al ratio, which
also explains the transition from metallic behavior to insulating behavior of
the interfaces. Our research emphasizes the key role of the cation
stoichiometry of LaAlO$_3$ in the formation of the conducting interface, and
also the control which is furnished by the Ar pressure in the growth process.
