(512b) Formation of Epitaxial Ge Nanorings on Si by Self-Assembled Sio2 Particles and Touchdown of Ge through a Thin Layer of Sio2

We demonstrate that hexagonally packed single-crystalline Ge rings can be grown around the contact region between self-assembled SiO₂ spheres and 1.2-nm-thick chemical SiO₂ on Si. When the oxide-covered Si substrate is pulled from a colloidal suspension of SiO₂ spheres, the SiO₂ spheres self-assemble into a hexagonally packed monolayer on the substrate. These SiO₂ spheres provide a surface diffusion path to guide the Ge adspecies to reach the substrate. We have previously determined that the Ge adspecies readily desorb from the bulk SiO₂ surface with a desorption activation energy of 42±3 kJ/mol. This low desorption activation energy gives rise to a low surface diffusion barrier, which in turn leads to a high diffusion length on the order of several micrometers, exceeding the dimension of the SiO₂ spheres. With a flux of Ge impinging at 45º from the surface normal, the Ge beam cannot directly impinge on the underlying substrate through the openings between SiO₂ spheres. The Ge adspecies diffuse around the SiO₂ spheres and ?touchdown?[Li et al., APL, 85(11), 1928 (2004)] through the chemical SiO₂, forming epitaxial ring structures. The touchdown process anchors nanoscale Ge seed pads to the underlying Si substrate. The ring formation uniquely takes advantage of the SiO₂ sphere self-assembly; the weak interaction between Ge adspecies and SiO₂; and the touchdown where Ge densely nucleate on Si surface through the 1.2-nm-thick chemical oxide.