Probing Interactions of Ge with Chemical and Thermal SiO₂ to Understand Selective Growth of Ge on Si during Molecular Beam Epitaxy

We have previously demonstrated that Ge selectively grows on Si over a SiO 2 mask during molecular beam epitaxy. To determine the surface phenomena responsible for the selectivity, we probed the stability of SiO 2 upon Ge exposure and Ge diffusion through thin SiO 2, 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 SiO 2 occurs only with chemical oxide upon Ge exposure via Ge + Si + 2SiO 2 → GeO(g) + 3SiO(g) at the SiO 2 /Si interface where all three reactants are present. This erosion is initiated by the Ge diffusion only through thin chemical SiO 2, and the diffusion is attributed to a larger concentration of SiOH groups (∼1 × 10 22 cm -3 ) and a greater porosity in chemical oxide than that in thermal oxide. For thermal SiO 2, 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 SiO 2 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 SiO 2, 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 SiO 2 and preferentially aggregate on the exposed Si surface.