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

摘要

Spherical Si single crystals with a diameter of approximately 1 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 Si_3N_4. It was found that a surface coating of SiO_2 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 ℃, corresponding to an undercooling ranging from 14 to 84 ℃, with a yield of nearly 100%. At recrystallization temperatures higher than 1380 ℃, the recrystallized spherical Si crystals were almost perfect spheres, whereas small protuberances were formed when the recrystallization temperature was lower than 1360 ℃. It was also found that that melting at a temperature close to the melting point of Si (at ～1414 ℃), a slow cooling rate of ～1 ℃/min before recrystallization and relatively fast cooling rate of ～20℃/min after recrystallization were important for achieving high carrier lifetime. The average carrier lifetime was greatly improved from lower than 2.5 μs of start sources up to ～7.5 μs 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.