ATOMIC DEPTH DISTRIBUTION ANALYSIS OF AG AND AU ON SI(111) DURING EPITAXIAL-GROWTH BY TOTAL-REFLECTION ANGLE X-RAY SPECTROSCOPY

When an electron beam is incident upon a surface, the electron trajectories near the surface in the sample change drastically depending upon the change in glancing angle theta(g). The X-ray emission from surface atoms changes markedly depending on their atomic depth distribution. Thus, by measuring the theta(g) dependence of the X-ray emission, the composition in depth direction near the surface can be estimated. This method is very sensitive and a depth resolution better than 1 ML can be attained. By applying this method, it was found that when Ag was deposited on a Si(111)-alpha-square-root 3 x square-root 3-Au structure at room temperature, Ag grew on Au in the normal fashion; on the other hand, when 1 ML of Au was deposited on a Si(111)- square-root 3 x square-root 3-Ag structure at room temperature, the sites of Ag and Au atoms exchanged with each other and Ag atoms moved to the topmost layer of the surface. At further Au deposition, Au atoms moved continuously below the Ag layer, resulting in a condition as if Ag atoms were floating. These are new epitaxial growth modes, wherein the former should be called the ''substitution atom growth mode'' and the latter the ''floating atom growth mode''. These phenomena were confirmed also by UHV-SEM observation. The results are further explained by Monte Carlo electron trajectory simulation.