  We have measured the electronic structure of the two-dimensional electron
system (2DES) found at the Al/SrTiO3 (Al/STO) and LaAlO3/SrTiO3 (LAO/STO)
interfaces by means of laser angle resolved photoemission spectroscopy, taking
advantage of the large photoelectron escape depth at low photon energy to probe
these buried interfaces. We demonstrate the possibility of tuning the
electronic density in Al/STO by varying the Al layer thickness and show that
the electronic structure evolution is well described by self-consistent tight
binding supercell calculations, but differs qualitatively from a rigid band
shift model. We show that both 2DES are strongly coupled to longitudinal
optical phonons, in agreement with previous reports of a polaronic ground state
in similar STO based 2DESs. Tuning the electronic density in Al/STO to match
that of LAO/STO and comparing both systems, we estimate that the intrinsic
LAO/STO 2DES has a bare band width of ~ 60 meV and a carrier density of ~ 6
10^13 cm-2.
