Pyramidal Si nanocrystals with a quasiequilibrium shape selectively grown on Si(001) windows in ultrathinSiO2films

Pyramidal Si nanocrystals selectively grown on Si(001) windows in ultrathin 0.3-nm-thick ${\mathrm{SiO}}_2$ films were studied using in situ scanning tunneling microscopy. In the initial growth stage, {1, 1, 13} facets were formed on the four equivalent sidewalls of the crystal due to the repulsion force between neighboring steps. The crystals were stable with a quasiequilibrium shape when they were surrounded by the ${\mathrm{SiO}}_2$ film, but rapidly decayed once the boundary to the ${\mathrm{SiO}}_2$ film was removed. This indicates that Si adatoms were confined within the Si window area by the surrounding ultrathin ${\mathrm{SiO}}_2$ film and the Si adatom density became stationary. The confinement was enabled by a difference in the adsorption energy of Si adatoms on ${\mathrm{SiO}}_2$ and these on Si(001).