(646b) Probing Interactions of Ge with Chemical and Thermal Si02 to Understand Selective Growth of Ge on Si during Molecular Beam Epitaxy

We have previously demonstrated that Ge selectively grows on Si over a SiO₂ mask during molecular beam epitaxy. We have also demonstrated that epitaxial Ge rings selectively form at the contact region between chemical-oxide-covered Si and self-assembled SiO₂ sphere. In order to determine the surface phenomena responsible for the selectivity, we probed the interactions of Ge with 1.2-nm-thick chemical SiO₂ films and 6 to 200-nm-thick dry thermal SiO₂ films grown on Si(100). The change in Si2p x-ray photoelectron intensity and position, intermittently measured during the Ge beam exposure, reveals that the chemical oxide degrades at substrate temperatures near 600°C. In contrast, the change in thickness and surface roughness of the thermal oxide, measured by ellipsometry and atomic force microscopy, is below the detection limit after a prolonged Ge exposure at temperatures below 700°C. For thermal SiO₂, where oxide degradation is 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. The low E_des entails a low activation barrier (≈13 kJ/mol) for surface diffusion. We expect the large diffusion length on the order of 1 μm to cause Ge adspecies to migrate over SiO₂ and preferentially aggregate on exposed Si surface. In order to determine the identity of stable Ge adspecies on SiO₂, the nucleation of Ge on thermal SiO₂ surface is also studied by plan-view, high-resolution scanning electron microscopy at substrate temperatures ranging from 300 to 500°C. We have found that the saturation Ge island density is a strong function of substrate temperature, but a weak function of absolute Ge flux. This result suggests that Ge monomers can exist as a stable nucleus on thermal SiO₂.