Thermodynamics and phase stability in the Si–O system

A comprehensive thermodynamic assessment has been carried out on the phases SiO2(L), including SiO2(glass), the amorphous monoxide SiO(am) and on the solubility of oxygen in solid and liquid silicon, jointly with the equilibrium partition coefficient k. Contradictions in the sparse thermodynamic data of SiO(am) are pointed out. Our key experiments were performed on the decomposition and condensation of SiO(am). X-ray diffraction on samples heated in evacuated silica capsules was used. The condensation experiment was performed in local equilibrium on a silica rod in a commercial CZ-Si apparatus. With the Calphad method a complete set of Gibbs energy equations was developed for the phase SiO2(L), comprising glass, supercooled liquid and stable liquid silica, the phase SiO(am) and the solid and liquid silicon solution phases. Combining this with the data for crystalline silica and the gas phase, a consistent thermodynamic description of the entire Si–O system is constructed, covering the complete composition range. Based on that, a number of stable and metastable phase diagrams at various pressures are calculated and applied to better understand important process steps in the production of semiconductor silicon.