Pyramidal Si nanocrystals with a quasiequilibrium shape selectively grown on Si(001) windows in ultrathin SiO2 films

Pyramidal Si nanocrystals selectively grown on Si(001) windows in ultrathin 0.3-nm-thick SiO2 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 SiO2 film, but rapidly decayed once the boundary to the SiO2 film was removed. This indicates that Si adatoms were confined within the Si window area by the surrounding ultrathin SiO2 film and the Si adatom density became stationary. The confinement was; enabled by a difference in the adsorption energy of Si adatoms on SiO2 and these on Si(001).