DIFFRACTION PROFILE ANALYSIS FOR EPITAXIAL-GROWTH ON FCC(100) SUBSTRATES - DIFFUSIONLESS MODELS

We develop a kinematic theory to determine surface-sensitive diffracted intensity profiles for epitaxial growth on fcc(100) substrates. On such surfaces, atoms adsorb in fourfold hollow sites (thereby continuing the fcc lattice) rather than in on-top sites (which would create a simple cubic lattice). In a kinematic theory, the "scattering units" are most appropriately taken as vertical columns of atoms, and multiple-scattering between these is ignored. For fcc growth, the tops of these columns are exposed to various degrees, so differing scattering factors are naturally assigned. For systems with weak short-range order, this produces significant differences from behavior analyzed previously for simple cubic geometries. For example, layer-by-layer growth with random partially filled layers yields diffracted intensities with vanishing Bragg intensities at non-half-monolayer coverages and non-uniform background profiles. Our focus is on the determination of the diffuse background profile for two models of epitaxial growth at low temperatures where thermal diffusion is inoperative. Our calculations agree well with experimental LEED data for Pd/Pd(100) and Pt/Pd(100), though some unresolved questions remain.