  Realization of a fully metallic two-dimensional electron gas at the interface
between artificially-grown LaAlO$_3$ and SrTiO$_3$ thin films has been an
exciting challenge. Here we present for the first time the successful
realization of a superconducting 2DEG at interfaces between artificially-grown
LaAlO$_3$ and SrTiO$_3$ thin films. Our results highlight the importance of two
factors-the growth temperature and the SrTiO$_3$ termination. We use local
friction force microscopy and transport measurements to determine that in
normal growth conditions the absence of a robust metallic state at low
temperature in the artificially-grown LaAlO$_3$/SrTiO$_3$ interface is due to
the nanoscale SrO segregation occurring on the SrTiO$_3$ film surface during
the growth and the associated defects in the SrTiO$_3$ film. By adopting an
extremely high SrTiO$_3$ growth temperature, we demonstrate a way to realize
metallic, down to the lowest temperature, and superconducting 2DEG at
interfaces between LaAlO$_3$ layers and artificially-grown SrTiO$_3$ thin
films. This study paves the way to the realization of functional
LaAlO$_3$/SrTiO$_3$ superlattices and/or artificial LaAlO$_3$/SrTiO$_3$
interfaces on other substrates.
