Large- and small-angle grain boundaries in multi-crystalline silicon and implications for the evolution of grain boundaries during crystal growth

A detailed study of twin-, large-angle and small-angle grain boundaries over a small volume of multi-crystalline silicon is presented on the basis of electron backscatter diffraction and synchrotron X-ray topography (SXRT) measurements. Identical areas (ca 6 mm²) of two nearest neighbour wafers from a directional solidified Si column were analysed in order to gain information about the evolution of grain boundaries during the crystal growth process. Therefore, the emphasis was placed on the evolution of one particular grain and its neighbouring grains. In the case of `straight line' Σ3 grain boundaries, no change is observed if the twin plane corresponds to a {211} plane. Significant changes are found for non-straight line boundaries: while curved Σ3 grain boundaries are formed and eliminated very frequently, the Σ9 and Σ27a grain boundaries undergo only minor changes. By means of SXRT imaging, it is shown that the microstructure of the analysed grain exhibits numerous small-angle grain boundaries with angles in the range between 0.003 and 0.02°, corresponding to a theoretical dislocation density of the order of 10⁷–10⁸ cm⁻². The experiments give evidence that the small-angle grain boundaries with the larger angles extend, while those with smaller angles vanish during growth.