  In polar-oxide interfaces, a certain number of monolayers (ML) is needed for
conductivity to appear. This threshold for conductivity is explained by the
accumulation of sufficient electric potential to initiate charge transfer to
the interface. Here we study the (111) SrTiO3/LaAlO3 interface where a critical
thickness of nine epitaxial LaAlO3 ML is required to turn the interface from
insulating to conducting and even superconducting. We show that this critical
thickness decreases to 3ML when depositing a cobalt over-layer (capping) and
6ML for platinum capping. The latter result contrasts with the (100) interface
where platinum capping increases the critical thickness beyond that of the bare
interface. These results suggest that the work function of the metallic capping
plays an important role in both interfaces. Interestingly, for (111)
SrTiO3/LaAlO3/Metal interfaces conductivity appears concomitantly with
superconductivity in contrast with the SrTiO3/LaAlO3/Metal interface with
LaAlO3 layer smaller than four ML (unit-cells), which are conducting but not
superconducting. We suggest that this difference is related to the different
sub-bands involved in conductivity for the (111) interfaces, comparing to the
(100) interfaces. Our findings can be useful for superconducting devices made
of such interfaces.
