(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 (Edes) of Ge adspecies from the thermal SiO2 surface. The experimentally measured Edes is 42±3 kJ/mol. The low Edes 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.