High speed growth of square-like Si single bulk crystals with a size of 23 × 23 cm² for solar cells using the noncontact crucible method

A noncontact crucible method was proposed to obtain a crystal-diameter as large as a crucible-diameter. In this method, a Si melt used has a large low-temperature region in its central upper part to ensure Si crystal growth inside it. Therefore, the present method has several merits such as the convex shape of the growing interface in the growth direction, the possibility of growing large ingots even using a small crucible, and the growth of square-like single bulk crystals. In these ingots, dislocations in the ingot moved to the periphery of the ingot from its center during crystal growth, and the dislocation density was on the order of 10²-10³/cm². The effective minority carrier lifetime was measured to be as high as 750 μs by the Quasi-Steady-State Photoconductance (QSSPC) method after phosphorus diffusion gettering and Al₂O₃ thin-film passivation. Especially, this method has a possibility to attain a high growth rate using a high cooling rate because the growth rate was determined by the expansion rate of the low-temperature region in Si melts. The growth rate increases as the cooling rate increases. At the cooling rate of 0.4 K/min, the horizontal growth rate became higher to 1.5 mm/min in the <110> direction. The vertical growth rate was determined as 0.3-0.6 mm/min, and it had a tendency to increase as the depth of Si melts increased. The diameter of ingots can be kept constant during crystal growth using a high cooling rate because the horizontal growth rate increases as the cooling rate increases. An ingot with a diagonal length of 24.5 cm was obtained using the high cooling rate of 0.4 K/min. The diagonal length was as large as 82% of the crucible diameter.