Overcoming the polarization catastrophe in the rocksalt oxides MgO and CaO

Interfaces between dissimilar polar materials may provide a pathway to new device functionality, including high carrier mobility layers at the interface. The development of these materials has proven challenging, in part because of the high energy cost of forming polar surfaces. Our density functional theory calculations explore the mechanisms by which a real material satisfies the electrostatic criteria for stability imposed by a polar surface. The consequences of polarity are studied by comparing the formation energies, charge distribution, and electronic structure of a number of low-index surfaces of rocksalt MgO and CaO. These surfaces are explored both in their bare, undecorated form as well as with surface reconstructions and adsorbed foreign species. Our ground-state surface energies are extended to relevant environmental conditions by use of ab initio thermodynamics. We find that the high energy of bare polar surfaces is the result of the significant charge redistribution that arises to compensate the polarity and pushes electronic states into the forbidden band gap. Other mechanisms of polarity compensation (reconstruction or foreign species adsorption) are therefore seen more frequently. We explain the experimental observations of surface roughness during growth in the [111] direction. In typical epitaxial growth conditions, there is preferential formation of an octopolar reconstruction of the {111} surface, which exposes {001}-type nanofacets. The low energy of the {001} surface likely causes these facets to grow, leading to a rough surface morphology. Our results indicate that when water vapor is present during growth, a smooth, polar surface can be stabilized by the formation of a hydroxyl layer.