A 2-inch diameter Mg2Si crystal was synthesized via the vertical gradient freeze (VGF) method in an open-tube system using pyrolytic boron nitride crucibles coated with boron nitride. Using this open-tube system, Mg evaporation was significantly prevented under Ar gas flow. Many grain boundaries were observed at the crystal cross section. A 5-mm square wafer without grain boundaries was cut out from the crystal and characterized by X-ray rocking curve (XRC) analysis, exhibiting a sharp single peak with a full width at half maximum (FWHM) of 50.4 arcsec. This indicates good crystallinity of the single crystalline area of the synthesized crystal.
Multicrystalline Mg2Si crystal with a diameter of 15 mm was grown via vertical Bridgman method. To clarify the growth mechanism of the multicrystalline structure, the grain growth behavior of the crystal was analyzed. This was carried out through segmenting grains by mean shift clustering using the light intensity profile obtained from multiple optical reflection images of the wafers and stacking the segmented images through the growth direction. Further crystal orientation measurement revealed that a grain with a higher surface energy competitively expanded to the lateral direction during crystal growth. We speculated that the growth behavior occurred because the supercooling was high enough to appear the difference of each grain’s growth rate. This idea was supported by crystal growth simulation to show a tendency for the crystallization rate to increase toward the latter half growth stage, which is consistent with the assumption for crystal grew with high supercooling.
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