Structural analysis of Si(111) surfaces during homoepitaxial growth

Atomic structures of Si(111) surfaces during silicon growth are investigated by reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). For silicon deposition on the substrate at room temperature, it is concluded that backbonds of adatoms of Takayanagi's dimer-adatom-stacking-fault (DAS) structure are broken at an initial stage of the deposition. The adsorbed atoms are nucleated randomly on the surface, and coalescence of the growth nuclei occurs with few nuclei crossing the dimer rows of the DAS structure. For further deposition, the dimers and the stacking faults remain at the interface between the substrate and the growth amorphous layer. At the substrate temperature of about 300°C, rocking curves of RHEED intensities from the surface during growth are very different from the curve before growth. The structure of the growing surface layer is determined as a pyramidal cluster-type structure by analysis with RHEED dynamical calculations. At this temperature, characteristic features of a cluster structure are found on the surface at an initial stage of the growth. Since crystal nuclei smaller than half the cluster segment are scarcely observed in the STM images, it is concluded that the cluster is the smallest unit of nucleation of homoepitaxial growth. At substrate temperatures from 400 to 600°C a mixed phase of 5 × 5 and 7 × 7 structures, which is the DAS structure, is observed by RHEED and STM. It is discussed that the formation of metastable structures, such as the pyramidal cluster-type structure and the 5 × 5 one, promotes successive epitaxial growth accompanied with stacking-fault dissolution at the dimer-stacking-fault framework.