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The stability and growth of three-dimensional (3D) nanostructures in the Ge on Si system is controlled in part by the strain- and overlayer-thickness-dependent surface energies of the crystal facets involved. Here, we use density functional theory (DFT) with local-density approximation calculations to calculate the strain- and thickness-dependent energy of various Ge(113) and Si(113) surface reconstructions. Results of DFT calculations are compared to Tersoff potential calculations to assess the relative importance of stress-strain effects compared to electronic effects not captured by empirical atomistic potentials. We find that the self-interstitial-based 3 × 2 adatom-dimer-interstitial and 3×2 adatom-interstitial surface reconstructions are stable for Ge overlayer thicknesses from 0 to 4 monolayers and at applied biaxial strains from ∼−4% to 0%. We leverage calculated surface energies to determine net effective surface energies of various experimentally observed 3D Ge on Si nanostructures. [ABSTRACT FROM AUTHOR]

Using both density functional theory-local density approximation and Tersoff potentials, we calculate (1 1 10),(105),and(001)surfaceenergiesrelevanttotheself-assemblyofGe(105)ripplesonSi(l 1 10) substrates. Surface energies are calculated as a function of Ge overlayer thickness and applied strain. Comparison of density functional theory (DFT) and Tersoff potential results reveals qualitative differences in the predicted dependence of surface energies on Ge overlayer thickness and the stability of the Ge on Si (1 I 10) surface relative to the Ge on Si (001) surface. DFT calculations show that γ(1110)Ge/Si strongly influenced by the presence of tilted dimers, and provide an explanation for the differing stability predictions. Finally, a multiscale model including strain- and thickness-dependent γ Ge/si is used to show that surface energy is a driving force for formation of Ge {105} ripples on Si (1 1 10), supporting recent experimental results of Ge-deposition-induced (105} faceting on Si (1 1 10). [ABSTRACT FROM AUTHOR]

Oxide ionic conductors typically operate at high temperatures, which limits their usefulness. Colossal room-temperature ionic conductivity was recently discovered in multilayers of yttria-stabilized zirconia (YSZ) and SrTiO3. Here we report density-functional calculations that trace the origin of the effect to a combination of lattice-mismatch strain and O-sublattice incompatibility. Strain alone in bulk YSZ enhances O mobility at high temperatures by inducing extreme O disorder. In multilayer structures, O-sublattice incompatibility causes the same extreme disorder at room temperature.