  We report on surface effects on the electronic properties of interfaces in
epitaxial LaAlO$_3$/SrTiO$_3$ heterostructures. Our results are based on
first-principles electronic structure calculations for well-relaxed multilayer
configurations, terminated by an ultrathin LaAlO$_3$ surface layer. On varying
the thickness of this layer, we find that the interface conduction states are
subject to almost rigid band shifts due to a modified Fermi energy. Confirming
experimental data, the electronic properties of heterointerfaces therefore can
be tuned systematically by alterating the surface-interface distance. We expect
that this mechanism is very general and applies to most oxide heterostructures.
