Abstract
The surface structure of bulk and thinned single crystals have been investigated by transmission and reflection high-energy electron diffraction, low-energy electron diffraction (LEED), and x-ray photoelectron and Auger electron diffraction. The polar surface periodicity is observed both after 800 °C annealing in air and also after oxygen plasma cleaning and annealing in ultrahigh vacuum. The x-ray photoelectron spectroscopy and diffraction results were analyzed by simulations based on path-reversed LEED theory and by first-principles calculations to help distinguish between different mechanisms for the stabilization of this extremely polar oxide surface: (1) stabilization by adsorption of a hydrogen monolayer; maintaining the insulating nature of the surface and (2) stabilization of the clean O or Mg terminated surface by interlayer relaxations and two-dimensional surface metallization. The analysis favors stabilization by a single OH layer, where hydrogen sits on top of the O ions with O-H bond distance of 0.98Å. The in-plane O and Mg positions fit regular rocksalt sites, the distance between the topmost O and Mg plane is 1.04 Å, contracted by with respect to bulk MgO distance of 1.21 Å, while the interlayer separation of the deeper layers is close to that of bulk, contracted by less than 1%. The presence of a monolayer of H associated with the terminal layer of oxygen reduces significantly the surface dipole and stabilizes the surface.
1 More- Received 20 July 2004
DOI:https://doi.org/10.1103/PhysRevB.71.115434
©2005 American Physical Society