Behaviors and Interactions of Adatoms on Tungsten Surfaces.

Random walk diffusion of single W atoms on clean and hydrogen-saturated W{123} planes was investigated. The activation energy on a hydrogen-saturated surface was found to be only (TURN)0.05 eV. higher than on a clean surface. No interaction was detected between a W diatomic cluster along a channel of the W{123} and a single W atom in an adjacent channel of the same plane. From 1265 FIM observations, the W diatomic cluster was found to migrate mainly through three elementary displacement steps. Pair interactions of some transition metal adatoms on the W{110} were studied by measuring two-dimensional pair distribution functions with two adatoms on a plane. Each distribution was obtained from approximately 1000 FIM observations. Pair energies over a large distance range were derived. The nonmonotonic behaviors of adatom-adatom interaction on the smooth W{110} surface were clearly established. This study represents the first time statistically reliable amounts of data have been obtained for two-dimensional pair distribution with only two adatoms on a plane. Behaviors of single silicon adatoms on several surfaces of tungsten were successfully studied for the first time. Single Si atom diffusion parameters and field desorption behavior on the W{110} were found to be similar to that of some metal atoms. Si-Si adatom -adatom interaction on the W{110} showed an oscillatory behavior but with features different from those of metal adatoms. A direct observation of the formation of a superstructure in a silicon adlayer on the W{110} was made for the first time. A dramatically different behavior was found for Si atoms on the W{112} plane in comparison to that exhibited by metallic adatoms on the same plane.