EVAPORATION OF OXYGEN-BEARING SPECIES FROM SI MELT AND INFLUENCE OF SB ADDITION

Evaporation loss from undoped and Sb-doped Si melt in both SiC-coated carbon and a silica crucible was measured in the temperature range of 1440-1560-degrees-C. For undoped Si melt in a silica crucible, the evaporation rate changed with temperature in an exponential manner and depended on the area ratio, A(c)/A(s), where A(c) is the contact area of the Si melt and silica crucible, and A(s) the free surface area of the Si melt. There were three trends of the dependence, and the evaporation rate tended to reach a certain saturation value when the area ratio approached infinity, which can be regarded as the intrinsic evaporation rate due to oxygen saturation in Si melt. It was concluded that the oxygen concentration in the Si melt was not saturated under the present experimental conditions. Evaporation from Sb-doped Si melt seemed complex. Under the assumption that the evaporating species from an Sb-containing Si melt in a SiC-coated carbon crucible was antimony vapor only and that the gb evaporation rate did not change in different crucibles, the total evaporation from the silica crucible was analyzed. From the linear time dependence and nonexponential temperature dependence, the evaporation rate excluding Sb evaporation was concluded to be that of some oxide species but not pure SiO. Such evaporation was greater than the evaporation of SiO from undoped Si melt; more than three times as large at the melting point of Si. Therefore oxygen reduction in highly Sb-doped Cz-Si crystals can be attributed mainly to the enhancement of oxygen evaporation.