A standard technique for purifying semiconductors is the zone refining process. A horizontal semi-cylinder of material is held in a boat made from an unreactive material such as quartz and a narrow molten zone is passed from one end to the other of the charge. At the end of the pass the heater is rapidly moved to the beginning of the charge and the process is repeated. Impurities are dissolved in the molten zone and preferentially segregated to the first to freeze or last to freeze ends of the charge. In some, cases several hundred passes are used to maximize the purification. The picture to the left shows a TlBr crystal being purified by this technique. The solid crystal is yellow and the molten zone is brown.
A TlBr crystal being purified by the Molten Zone technique. The solid crystal is yellow and the molten zone is brown.