The float zone (FZ) method—developed by Henry Theuerer—produces the highest-purity crystalline silicon. The advantage of using this process is that the resulting silicon from this process has extremely low concentrations of common impurities such, as carbon and oxygen, with concentrations typically below 5 × 1015 cm-3. The process is performed either under vacuum or in an inert gaseous atmosphere. Typically high-purity monocrystalline rods grown using the Cz process are used with a monocrystalline seed crystal that is suspended end‑to‑end in a vertical position. A radio frequency field is applied to melt the polycrystalline rod. The monocrystalline seed makes contact with the melted form of the rod. To ensure a dislocation-free crystal, a necking process is applied. The molten zone is moved along the rod and solidifies into a single crystal while the material is purified. This process purifies the ingot by exploiting the fact that the segregation coefficient of most impurities is well below 1, i.e. they stay in the melt resulting in a purer crystal. The surface tension of the molten silicon in the vertical configuration avoids charge separation. Due to limitations imposed by maintaining the surface tension, the diameter of the FZ wafers is typically limited to 150 mm.
The float zone process is schematically shown in the animation below.