Float-zone silicon is very pure silicon obtained by vertical zone melting. The process was developed at Bell Labs by Henry Theuerer in 1955 as a modification of a method developed by William Gardner Pfann for germanium. In the vertical configuration molten silicon has sufficient surface tension to keep the charge from separating. Avoidance of the necessity of a containment vessel prevents contamination of the silicon.
Float-zone silicon is a high-purity alternative to crystals grown by the Czochralski process. The concentrations of light impurities, such as carbon and oxygen, are extremely low. Another light impurity, nitrogen, helps to control microdefects and also brings about an improvement in mechanical strength of the wafers, and is now being intentionally added during the growth stages.
The diameters of float-zone wafers are generally not greater than 150mm due to the surface tension limitations during growth. A polycrystalline rod of ultra-pure electronic grade silicon is passed through an RF heating coil, which creates a localized molten zone from which the crystal ingot grows. A seed crystal is used at one end in order to start the growth. The whole process is carried out in an evacuated chamber or in an inert gas purge. The molten zone carries the impurities away with it and hence reduces impurity concentration (most impurities are more soluble in the melt than the crystal). Specialized doping techniques like core doping, pill doping, gas doping and neutron transmutation doping are used to incorporate a uniform concentration of impurity.
Float-zone silicon is typically used for power devices and detector applications. It is highly transparent to terahertz radiation, and is usually used to fabricate optical components, such as lenses and windows, for terahertz applications.