REM studies of adsorption-induced phase transitions and faceting in the Si(111)-Au system

Initial stages of Au adsorption processes on Si(111)-(7 x 7) surfaces at similar to 780 degrees C, a temperature range where the surface structure undergoes successive phase transitions [(7 x 7)-("1 x 1")-(5 x 2)("1 x 1")], were observed in situ by reflection electron microscopy. All of the phase transitions are heterogeneous on the surface and start at surface atomic steps. During Au adsorption on Si(lll)(7 x 7), and subsequently on Si(lll)-("1 x 1":) surface with wide terraces, steps advance toward the step-down direction. At a Au coverage of similar to 0.3 ML, the 5 x 2 structure nucleates at step edges, and the nuclei expand both to the lower side terraces and to the higher side terraces. At this stage, an effect of current for heating the specimen was noted. From measurements of such movements of the steps and the domain boundaries, the density of Si atoms in the "1 x 1" phase is estimated to be 1.3-1.7 ML at a Au coverage of similar to 0.3 ML Au adsorption on Si(lll) surfaces with narrow terraces causes bunching of the steps. After nucleation of the 5 x 2 structure, the bunched [(1) over bar (1) over bar 2] steps become straight along the [1 (1) over bar 0] direction, and are transformed into the (335) facet planes at a Au coverage of 0.50 ML. It was found that the (335) facet planes are stabilized by adsorbed Au atoms. Destruction of the (335) facet is noted at a Au coverage of 0.73 ML.