Homo-epitaxial growth on the Si(111)7 x 7 surface

The initial stage of silicon growth on the Si(111)7×7 surface is investigated by a field ion scanning tunneling microscope (FI-STM) as a function of substrate temperature. It is found that structures of growth nuclei depend on the substrate temperature and most of the nucleations occur randomly at a faulted half of the unit cell of the dimer-adatom-stacking-fault (DAS) structure. It appears that coalescence between growth nuclei occurs rarely at low temperatures. At the substrate temperature of 300°C, characteristic figures of a cluster structure are found on the surface at submonolayer deposition. Most of the clusters are grown symmetrically with respect to the dimer rows of the DAS structure. The half segment of the symmetric cluster is also formed on the surface. Since crystal nuclei smaller than this cluster are scarcely observed in the STM images, it is concluded that the cluster is the smallest unit of the nuclei of homo-epitaxial growth at about 300°C. From the STM image analysis it is concluded that the structure of the cluster is not made of DAS-type structure, but a structure including dimers and/or pyramidal clusters. Structure analysis of the cluster is underway. At and above 300°C, small islands of 5×5 DAS structure are frequently observed on nucleation clusters. In large islands, however, a 5×5 structure is rarely observed, and a regular 7×7 is observed. This suggests that the transition from 5×5 to 7×7 structure occurs during the island growth. At temperatures higher than 500°C, step flow growth with 7×7 structure is observed as well as a few parts of island growth including 5×5 structure.