Crystallization of supercooled silicon droplets initiated through small silicon nitride particles

Small droplets of silicon melts were supercooled in polished sapphire crucibles and crystallization was observed by a microscope equipped with a CMOS camera. The pure silicon samples could be supercooled by up to ΔT≈100 K beyond the melting point Tmp=1687 K before polycrystalline solidification starts. However, by addition of amorphous silicon nitride particles with about 20–40 nm diameters we could systematically initiate the growth of single crystals in the melt at supercooling as small as 2 K<ΔT<6 K. We propose a mechanism where the amorphous nitride is first transformed into crystalline silicon nitride particles, which subsequently serve as nucleation seeds for silicon crystallization. This mechanism is supported by electron microscopic studies that revealed nitride crystallites on and closely below the surface of the solidified droplet.