Impurity effects on the formation of grain boundaries during solidification of solar cell grade silicon in situ HREM observation

In situ HREM (high resolution transmission electron microscope) experiment was applied to directly observe the solidification of polycrystalline high-purity FZ (floating zone) silicon and two solar-cell grade polycrystalline silicon doped with boron and phosphor respectively from the liquid state, and probabilities of the formation of grain boundaries during solidification were compared for these three types of specimen. Although the number of times for the present in situ experiment is statistically small, the results suggest that the addition of impurities such as boron and phosphor increase the probability of the formation of grain boundaries during solidification. The result of the formation of a twist boundary in boron-doped silicon especially suggests the impurity effects on lowering the grain boundary energy.