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Probing Interactions of Ge with Chemical and Thermal SiO 2 to Understand Selective Growth of Ge on Si during Molecular Beam Epitaxy

2006; American Chemical Society; Volume: 111; Issue: 2 Linguagem: Inglês

10.1021/jp062966o

1932-7455

Qiming Li, Joshua L. Krauss, S. D. Hersee, Sang M. Han,

Semiconductor materials and interfaces

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 (Edes) of Ge adspecies from the thermal SiO2 surface. The experimentally measured Edes is 42 ± 3 kJ/mol on the order of Van der Waals force. The low Edes 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.