Czochralski, Float-Zone and Ribbon Silicon

Czochralski Silicon
In the Czochralski process, a seed crystal is dipped into a crucible of molten silicon and withdrawn slowly, pulling a cylindrical single crystal as the silicon crystallizes on the seed.

Float-Zone Silicon
The float-zone process produces purer crystals than the Czochralski method, because they are not contaminated by the crucible used in growing Czochralski crystals. In the float-zone process, a silicon rod is set atop a seed crystal and then lowered through an electromagnetic coil. The coil's magnetic field induces an electric field in the rod, heating and melting the interface between the rod and the seed. Single-crystal silicon forms at the interface, growing upward as the coils are slowly raised.

Once the single-crystal rods are produced, by either the Cz or FZ method, they must be sliced or sawn to form thin wafers. Such sawing, however, wastes as much as 20% of the valuable silicon as sawdust, known as "kerf." The resulting thin wafers are then doped to produce the necessary electric field. They are then treated with a coating to reduce reflection, and coated with electrical contacts to form functioning PV cells.

Ribbon Silicon
Although single-crystal silicon technology is well developed, the Czochralski and float-zone processes are complex and expensive (as are the ingot-casting processes discussed under multicrystalline silicon). Another group of crystal-producing processes, however, goes by the general name of "ribbon growth." These single crystals may cost less than other processes, because they form the silicon directly into thin, usable wafers of single-crystal silicon. These methods involve forming thin crystalline sheets directly, thus avoiding the slicing step required of cylindrical rods.

One "ribbon growth" technique—edge-defined film-fed growth—starts with two crystal seeds that grow and capture a sheet of material between them as they are pulled from a source of molten silicon. A frame entrains a thin sheet of material when drawn from a melt. This technique does not waste much material, but the quality of the material is not as high as Cz and FZ silicon.