Growth of Silicon Sheets from Metallurgical-Grade Silicon

摘要

Impure silicon is difficult to solidify in sheet form because of morphological proturberances which may result from constitutional supercooling. Sheet growth methods which require a specific crystallographic orientation or which are characterized by a narrow melt meniscus are most affected by this problem. The edge-supported pulling technique is applied to sheet growth of metallurgical grade silicon and DAR (Direct Arc Reactor) silicon. The 7mm meniscus height associated with this technique has allowed the growth of 5 cm wide sheets from both materials. In each case, the sheets were p-type. Metallurgical grade silicon resulted in .05 to .23 omega -cm resistivities while DAR silicon produced .10 to .23 omega -cm material. Constitutional supercooling occurred under some growth conditions with both materials and was sometimes severe enough to cause growth difficulties particularly with metallurgical grade silicon. The onset of morphological proturberance is sometimes triggered by beta -SiC particles. Impurity characteristics are presented for the starting materials and for both smooth and constitutionally supercooled sheets. Also, relative concentrations of impurities in grain boundaries and in adjacent grains are given. Conventional diffused solar cells fabricated directly on the sheet attained antireflection coated AM1 efficiencies up to 6.3% for DAR metallurgical silicon and 3.4% for conventional metallurgical silicon. Other DAR cells achieved 4.7% AMO, non-AR coated efficiencies. Epitaxial layer solar cells achieved AM1, coated efficiencies up to 10.5% on DAR substrates and approximately 9% on conventional metallurgical substrates. (ERA citation 06:028400)