Competition between Polar and Nonpolar Growth of MgO Thin Films on Au(111)

We report a growth study of MgO thin films on an Au(111) support, performed with scanning tunneling microscopy, X-ray photoelectron spectroscopy, and low-energy-electron and X-ray-diffraction techniques. Depending on the deposition temperature, the O2 partial pressure, and the availability of water during oxide formation, two growth regimes can be distinguished. At high oxygen pressure, the MgO mainly adopts a square-lattice configuration and exposes the nonpolar (001) surface, whereas at low O2 pressure a hexagonal lattice develops that resembles the (111) surface of rocksalt MgO. For films beyond the monolayer limit, the emerging electrostatic dipole along the MgO(111) direction becomes important for the film morphology. Depending on the preparation conditions, the system takes either structural or adsorption-mediated routes to remove the polarity. Whereas surface roughening is identified as main polarity-compensation mechanism at perfect vacuum conditions, hydroxylation becomes important if water is present during oxide growth.