OBSERVATIONS OF AU AND AG COADSORBED SI(111) SURFACES BY USING A HIGH-RESOLUTION ULTRA-HIGH-VACUUM SCANNING ELECTRON-MICROSCOPE

Au and Ag coadsorbed Si(111) surfaces have been studied by secondary electron (SE) imaging with a high-resolution ultra-high-vacuum scanning electron microscope (UHV-SEM). Au and Ag coadsorbed onto a Si(111) surface approximately at room temperature (RT) show a regular arrangement having the 7 x 7 periodicity of the substrate. For low-coverage coadsorption onto a substrate at elevated temperature, three structures, i.e., the square-root 3 x square-root 3, 5 x 2, and 7 x 7 structures, are found to be simultaneously present. SE images of such samples show three sorts of areas with different brightness, corresponding to the three structures. The two-dimensional (2D) morphology of the areas differs between samples prepared in a different way, even if the reflection high-energy electron diffraction (RHEED) patterns are nearly the same. Ag deposited onto a 5 x 2-Au or alpha square-root 3 x square-root 3-Au layer at approximately RT forms small particles, a few nm in size. Samples prepared by Au deposition at RT onto a square-root 3 x square-root 3-Ag layer also show small particles ranging down to less than 1 nm. Au and Ag constitute a continuous series of solid solution in the bulk crystal: discussions are made on the degree of intermixing of the two elements at the Si(111) surface.