OXYGEN SOLUBILITY AND ITS TEMPERATURE-DEPENDENCE IN A SILICON MELT IN EQUILIBRIUM WITH SOLID SILICA

Oxygen solubility in a silicon melt is measured for the first time in an equilibrium with solid silica, the situation that occurs at the melt-crucible interface in a Czochralski silicon crystal growth. The solubility is well expressed as a function of temperature by Cs = 4.0 x 10(23) exp(-2.0 x 10(4)/T) atoms/cm3 in a range from 1698 to 1820 K (13-135 K above the melting point of silicon). This temperature dependence is considerably smaller than has been thermochemically estimated so far. Standard enthalpy change is obtained from the temperature dependence to be DELTA-H-degrees = 79 kcal/mol for the reaction of SiO2 (solid) = Si (melt) + 2O (in melt), which is consistent with the known values of other oxygen related reactions. The solubility extrapolated to the melting point is 2.8 x 10(18) atoms/cm3, which is a little larger than those measured so far of 1.8 x 10(18) and 2.2 x 10(18) atoms/cm3 for a floating melt in an oxygen atomosphere. Equilibrium with gaseous silicon monoxide or with gaseous oxygen may be a factor contributing to the small solubilities in the previous experiments.