The initial process of molecular beam epitaxial growth of Si on Si(111)7 × 7: a model for the destruction of the 7 × 7 reconstruction

Images of scanning tunneling microscopy (STM) for molecular beam epitaxial growth of silicon on Si(111)7 × 7 published by Köhler, Demuth and Hamers [J. Vac. Sci. Technol. A 7 (1989) 2860] have been analyzed in detail within the framework of the dimer-adatom-stacking fault (DAS) model of SK(111)7 × 7. We propose a model for the mechanism of the initial process of the epitaxial growth on terraces by using the experimental facts which were not noticed previously. The STM images at the initial stages of the epitaxial growth at 330°C provide useful information concerning its mechanism. We noticed that not only large crystallized clusters but also small amorphous clusters are formed on Si(111)7 × 7. The model is based on a simultaneous occurrence of three processes; coalescence of three amorphous clusters sitting side-by-side on each triangular subunit of the 7 × 7 unit cell in the substrate surface; conversion of the DAS structure underneath the clusters into the normal structure of the Si crystal and crystallization of the coalesced cluster. These complicated processes correspond to nucleation and this peculiar nucleation is needed to cancel the DAS reconstruction which has a stable and large unit cell. During growth of the nucleated crystallized clusters, cycles of the simultaneous occurrences of coalescence, conversion and crystallization are necessary. This kind of complicated nucleation-growth process has not been considered previously in the field of crystal growth. By using this model, STM images for the growing surface are understood. It is clearly explained why the crystallized clusters have equilateral triangular shape.