Investigation on the crystal growth process of spherical Si single crystals by melting

Spherical Si single crystals with a diameter of approximately I mm were grown by melting for solar cell applications. The start sources were spherical Si multicrystals fabricated by a dropping method, which had various irregular shapes. Spherical Si multicrystals were melted into droplets and recrystallized on a quartz plate sample holder that was coated with Si3N4. It was found that a surface coating Of SiO2 layer on the start sources and oxygen atmosphere during melting and recrystallization were essential to achieve almost perfect spherical shape. Defect-free single crystalline spherical Si could be obtained at recrystallization temperature ranging from 1400 to 1330 degrees C, corresponding to an undercooling ranging from 14 to 84 degrees C, with a yield of nearly 100%. At recrystallization temperatures higher than 1380 degrees C, the recrystallized spherical Si crystals were almost perfect spheres, whereas small protuberances were formed when the recrystallization temperature was lower than 1360 degrees C. It was also found that that melting at a temperature close to the melting point of Si (at similar to 1414 degrees C), a slow cooling rate of similar to 1 degrees C/min before recrystallization and relatively fast cooling rate of similar to 20 degrees C/min after recrystallization were important for achieving high carrier lifetime. The average carrier lifetime was greatly improved from lower than 2.5 mu s of start sources up to similar to 7.5 mu s by melting at optimized conditions. The influences of residual oxygen on the carrier lifetime of recrystallized spherical Si are discussed based on the measurement results with Fourier transform infrared spectrometer. (C) 2009 Elsevier B.V. All rights reserved.