Spherical crystallization of Si during free fall in drop-tubes

In the drop-tube processing for spherical crystallization of Si, the poor crystallinity caused by rapid solidification into a highly undercooled melt is the major barrier preventing widespread. To address this problem, we utilized crystalline AlP as the catalyzing agent for the melt to crystallize at low undercooling. In undoped Si used as the reference material, the surface morphologies of the as-dropped samples were classified into three types: type I was similar to a teardrop with a smooth surface, type II a polyhedral shape, and type III a rough surface with bumps and other irregularities. The photoluminescence spectra of the as-dropped samples show that both type I and II crystals are suitable for use in a solar cell. In AlP-incorporated samples, a fourth classification (type IV) having a multi-horned surface morphology was added. Although the yields of type I and II crystals are maximized in the sample with 1019/cm3 of AlP, the impurity band formed in the samples with more than 1018/cm3 of AlP degrades the quality of the crystal. According to the thermodynamic estimates of the solubility product of AlP in Si, the optimized amount of AlP is ∼1016/cm3. This corresponds to the amount of AlP to be completely dissolved into Al and P in molten Si and then precipitated in droplets during the free fall in the drop-tube.