Zone melting

The zone melting method ( also: " float zone ") is a process for the preparation of high-purity single-crystal materials. Other names for zone melting are float-zone method, floating zone method, zone refining, zone edge fusion or floating zone.

It was 1950/51 designed by William Gardner Pfann at Bell Laboratories.

Principle of operation

The zone melting method is based on the fact that impurities in the melt an energetically more favorable chemical environment (lower chemical potential ) than in the solid state and therefore migrate from the solid into the melt. The amount that is incorporated in the crystal, is dependent inter alia on the nature of the impurity and the solidification speed. In silicon, for example, is the distribution coefficient of boron and 0.8 of iron 0.000007. Boron is therefore nearly impossible to remove with this method from the silicon, iron, however, very good. A distinction horizontal zone melting in an elongated shuttle and vertical crucible-free zone melting, which is mainly in the production of highly pure silicon single crystals of importance. Also compare Pedestalverfahren and Czochralski method.

Technique of Si - zone melting

An already -prepared, purified rod ( or a column ) with even polycrystalline crystal structure is located in a protective atmosphere. By an induction heater having a relatively narrow zone of material is melted at one end. Thus, the zone melts evenly, the rod rotates slowly. The molten zone is brought into contact with a seed crystal and grows under the assumption of its crystal structure in it. This melting zone is now moving slowly through the rod. The melt solidifies again growing cold over the entire width of material with a uniform crystal structure, so it is behind the melting zone of the desired crystal. Foreign atoms remain largely in the fusion zone and eventually attach themselves to the end of the column, which is removed after cooling. By repeated zone melting, the purity can be further increased. Doping may be achieved by addition of gaseous materials, which then penetrate into the melt. For special applications, the rod stock can be prepared with a small amount of metal through the generally very small distribution coefficient (usually remain > 99.9% in the melt) is installed in a relatively constant concentration in the crystal. In contrast to the crucible pulling, where column diameter up to 12 inches (about 30 cm) are used, can be personalized with the zone melting process industrially only column diameter to 20 cm produced. Larger diameter by this method are not expected in the foreseeable future.

Application

With this method, the production of high purity silicon, and other materials is possible, but it is associated with very high costs.

The silicon is used for example in components in high- energy technology, microsystems technology, in the computer industry for integrated circuits (see also wafer) or in photovoltaics for solar cell production. Especially for solar cells but is this purity of the silicon in the past has not been necessary so that established cost-effective method here have (see solar cell Czochralski method ). Despite all the silicon is always pure for solar cell production in order to improve the efficiency of energy production.