The growing interest in electronic and optical materials, has made this field one of the fastest growing areas of research. The preparation of semiconductors starts with the purification of the material down to impurity levels below 1 ppm (parts per million) and in some cases only ppt (parts per trillion) levels of impurity are allowed.
Typically, starting materials which may purchased from commercial vendors with nominal purity of 99.9999% or synthesized from pure elements. Zone-refining and vacuum sublimation are two standard processes to achieve this goal. Zone-refining was first introduced in 1952by Pfann, and has been successfully implemented to purify elements and compounds. The main principle is difference in solubility of an impurity in the liquid and solid phases of a material. The characteristic effect of zone-refining is to accumulate impurities at the ends of an ingot, thus leaving pure material in the central section. Vacuum sublimation is a routine method to purify starting materials from impurities having higher (under dynamic vacuum) or lower (closed tube) vapor pressures
Principle of zone refining: A moving heater melts a zone of the material that can accumulate impurities. The impurities are "swept" to the right end of the ingot. The heater is next moved fast to the left (without melting the material) and the process is repeated several times.
Schematics of a zone refiner containing two heaters for improved efficiency.