New phase and surface melting of Si(111) at high temperature above the (7 x 7)-(1 x 1) phase transition

The surface structure of Si(lll) at high temperatures (950-1380 degreesC) h as been studied with reflection high-energy electron diffraction. We have f ound three different surface structures: (I) A relaxed bull;like structure with adatoms of 0.25 monolayer (ML) is formed (950-1210 degreesC); (2) ther e is a new phase where the adatom coverage decreases to 0.20 hit (1250-1270 degreesC); (3) the surface melting occurs over 1290 degreesC. The crystall ine structure below the melting layer can be explained by the vacancy model missing all adatoms and 0.45 ML of atoms in the first-double layer.