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 str ucture undergoes successive phase transitions [(7 x 7)-(''1 x 1'')-(5 x 2)(''1 x 1'')], were observed in situ by reflection electron microsc opy. 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 sim ilar to 0.3 ML, the 5 x 2 structure nucleates at step edges, and the n uclei expand both to the lower side terraces and to the higher side te rraces. At this stage, an effect of current for heating the specimen w as noted. From measurements of such movements of the steps and the dom ain boundaries, the density of Si atoms in the ''1 x 1'' phase is esti mated to be 1.3-1.7 ML at a Au coverage of similar to 0.3 ML Au adsorp tion on Si(lll) surfaces with narrow terraces causes bunching of the s teps. After nucleation of the 5 x 2 structure, the bunched [(1) over b ar (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 cov erage of 0.50 ML. It was found that the (335) facet planes are stabili zed by adsorbed Au atoms. Destruction of the (335) facet is noted at a Au coverage of 0.73 ML.