  Using density functional theory (DFT) we demonstrate that the adsorption of
large cations such as potassium or cesium facilitates the epitaxial growth of
polar LaAlO$_3$ (LAO) on SrTiO$_3$ (STO). The low ionization potential of K
favors efficient electron transfer to the STO conduction band and results in a
2D electron gas which exactly compensates for the diverging potential with
increasing layer thickness. For large cations like K or Cs, DFT total energy
considerations show that they remain adsorbed on the LAO surface and do not
enter substitutionally into LAO. These results suggest a novel scheme for
growing clean LAO/STO interface systems, and polar systems in general, by
performing the growth process in the presence of large, low ionization
potential alkali metal ions.
