Precipitation in High-Purity Silicon Single Crystals

Precipitation in high-purity Lopex (low-oxygen dislocation-free) silicon has been examined by x-ray diffraction topography and transmission electron microscopy. It has been found that the impurity concentration in the as-grown silicon crystals is sufficient to cause precipitation of a second phase if the specimens are annealed in the temperature range 700–1000°C, followed by a critical cooling rate. The precipitating phase has been tentatively identified as α Fe3Si. The precipitates appear in colonies. Each colony has a three-dimensional starlike configuration, where the different arms are planar arrangements of precipitates enveloped by a dislocation loop. The precipitates have a blocky shape and the dislocation loops are of interstitial edge type and both 12−a 〈110〉 and a 〈100〉 Burgers vectors are involved. The nucleation of colonies is heterogeneous and the presence of small silica particles as nucleation centers is suggested.