We have previously demonstrated that Ge selectively grows on Si over a SiO2 mask during molecular beam epitaxy. To determine the surface phenomena responsible for the selectivity, we probed the stability of SiO2 upon Ge exposure and Ge diffusion through thin SiO2, using X-ray photoelectron spectroscopy, ellipsometry, atomic force microscopy, transmission electron microscopy, and multiple internal reflection Fourier transform infrared spectroscopy. We observe that the consumption of SiO2 occurs only with chemical oxide upon Ge exposure via Ge + Si + 2SiO2 → GeO(g) + 3SiO(g) at the SiO2/Si interface where all three reactants are present. This erosion is initiated by the Ge diffusion only through thin chemical SiO2, and the diffusion is attributed to a larger concentration of SiOH groups (∼1 × 1022 cm-3) and a greater porosity in chemical oxide than that in thermal oxide. For thermal SiO2, where Ge diffusion and subsequent oxide degradation are not observed, we have determined that the selectivity stems from the low desorption activation energy (E des) of Ge adspecies from the thermal SiO2 surface. The experimentally measured E des is 42 ± 3 kJ/mol on the order of Van der Waals force. The low E des entails a low activation barrier (∼13 kJ/mol) for the surface diffusion of Ge adspecies on thermal SiO2, leading to a characteristic diffusion length greater than 1 μm. Under typical Ge growth conditions where the interdistance between exposed Si areas is much less than 1 μm, the large diffusion length would cause Ge adspecies to migrate over SiO2 and preferentially aggregate on the exposed Si surface.

No abstract

We demonstrate that hexagonally packed single-crystalline Ge rings can be grown around the contact region between self-assembled SiO 2 spheres and 1.2-nm-thick chemical SiO 2 on Si. When the oxide-covered Si substrate is pulled from a colloidal suspension of SiO 2 spheres, the SiO 2 spheres self-assemble into a hexagonally packed monolayer on the substrate. These SiO 2 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 SiO 2 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 SiO 2 spheres. With a flux of Ge impinging at 45° from the surface normal, the Ge beam cannot directly impinge on the underlying substrate through the openings between SiO 2 spheres. The Ge adspecies diffuse around the SiO 2 spheres and "touchdown"[1] through the chemical SiO 2 , 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 SiO 2 sphere self-assembly; the weak interaction between Ge adspecies and SiO 2 ; and the touchdown where Ge densely nucleates on Si surface through the 1.2-nm-thick chemical oxide.

No abstract