  Oxide interfaces feature unique two-dimensional (2D) electronic systems with
diverse electronic properties such as tunable spin-orbit interaction and
superconductivity. Conductivity emerges in these interfaces when the thickness
of an epitaxial polar layer surpasses a critical value, leading to charge
transfer to the interface. Here, we show that depositing amorphous alumina on
top of the polar oxide can reduce the critical thickness and enhance the
superconducting properties for the (111) and the (100) SrTiO$_3$-based
interfaces. A detailed transmission electron microscopy analysis reveals that
the enhancement of the superconducting properties is linked to the expansion of
the LaAlO$_3$ lattice in a direction perpendicular to the interface. We propose
that the increase in the superconducting critical temperature, Tc, is a result
of epitaxial strain
