Instability of a Ge_xSi_1−xO₂ film on a Ge_xSi_1−x layer

The stability of an amorphous Ge_xSi_1−xO₂ in contact with an epitaxial (100)Ge_xSi_1−x layer obtained by partially oxidizing an epitaxial Ge_xSi_1−x layer on a (100)Si substrate in a wet ambient at 700 °C is investigated for x=0.28 and 0.36 upon annealing in vacuum at 900 °C for 3 h, aging in air at room temperature for 5 months, and immersion in water. After annealing at 900 °C, the oxide remains amorphous and the amount of GeO₂ in the oxide stays constant, but some small crystalline precipitates with a lattice constant similar to that of the underlying GeSi layer emerge in the oxide very near the interface for both x. Similar precipitates are also observed after aging for both x. The appearance of these precipitates can be explained by the thermodynamic instability of Ge_xSi_1−xO₂ in contact with Ge_xSi_1−x. In water at RT, 90% of GeO₂ in the oxide is dissolved for x=0.36, while the oxide remains conserved for x=0.28.