Study of the Si(111) 7 × 7 surface structure by alkali- metal adsorption

When alkali-metals such as Li, Na, K, Rb and Cs were adsorbed on clean Si(111) 7 × 7 surface at room temperature, the intensity distribution of the original 7 × 7 RHEED pattern changes gradually with the increase of alkali-metal adsorption, and at last a new superstructure with 7 × 7 periodicity, here named δ-7 × 7 structure, has been observed. This change at room temperature can be explained if the 7 × 7 structure is mostly made of displacement. The structure model estimated from the intensity distribution of the δ-7 × 7 pattern is the one that has one vacancy at the corners of the 7 × 7 unit mesh and relaxed surrounding atoms. The change of the 7 × 7 structure by alkali-metal adsorption to this model is naturally understood with our new model (1984). For all alkali metals, by adsorption at high temperature 3 × 1 superstructure has also been observed for the first time.