Float-zone silicon
In contrast to the Czochralski process the polysilicon is not entirely molten, but, as in the zone cleaning, only a small area (a few millimeters).
Again, a seed crystal, which will be introduced to the end of the polycrystalline silicon rod, sets the crystal structure. The polycrystal is molten and assumes the structure of the seedling. The heated region is slowly guided along the rod, the polycrystalline silicon rod slowly transforms into a single crystal.
Since only a small portion of the polycrystalline silicon is molten, it can hardly be polluted (impurities accumulate at the bottom since their higher solubility). The doping is done by additions of dopants into the inert gas (eg with diborane or phosphine) which flows around the apparatus.
Illustration of the float-zone process
