CESIUM ON SI(100) STUDIED BY BIASSED SECONDARY-ELECTRON MICROSCOPY

An ultra-high vacuum scanning electron microscope (UHV-SEM) has been u sed to study sub-monolayers of Cs on Si(100) surface. Cs adsorption on the surface causes a considerable change in the work function. Covera ges below 1/2 monolayer (ML) have been estimated by correlating the wo rk function changes with the secondary electron (SE) signal. It has be en found that this signal is sensitive down to similar to 0.005 ML whe n the sample is biassed to a few hundred volts. Electron trajectories from a biassed sample have been simulated for electrons originating fr om different areas with different work functions across the sample. Th is indicates that variations in coverage can be determined by secondar y electron imaging provided these coverages are less than 1/2 ML. The diffusion of Cs (< 1/2 ML) above room temperature has been studied usi ng the biassed-SE imaging technique. Observed diffusion profiles have unusual features including two linear regions. These can be explained by a model which contains two competing adsorption sites, and includes blocking of the diffusion paths by other Cs atoms.