Swirls in Neutron-Transmutation Doped Float-Zoned Silicon
Today, dislocation-free float-zoned ingots are routinely grown free of swirls. However, after neutron-transmutation doping and after annealing to restore electrical properties, these ingots may show swirls after preferential etching. Statistical evidence indicates that, if both oxygen and carbon are present in concentrations less than 1 ppma (5 × 1016 atoms/cm3), then it is oxygen that causes swirls to form. The lower threshold of oxygen for swirl formation appears to be 0.15 ppma. This hypothesis has been experimentally verified by adding small amounts of oxygen during regrowth to previously swirl-free neutron-transmutation doped ingots. A second round of irradiation and annealing then produced swirls in these ingots. The paper presents evidence that neither the zone refiner nor the zone-refining process induced swirls, so that the most likely source of the oxygen is the polycrystalline silicon itself.
KeywordsOxygen Content Polycrystalline Silicon Ingot Surface Zone Refiner Preferential Etching
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