Semiconductors zone refining

The element is a gray-white metalloid. In its pure state, the element is crystalline and brittle, retaining its luster in air at room temperature. It is a very important semiconductor material. Zone-refining techniques have led to production of crystalline germanium for semiconductor use with an impurity of only one part in lOio. 

TJItrahigh (99.999 + %) purity tellurium is prepared by zone refining in a hydrogen or inert-gas atmosphere. Single crystals of tellurium, tellurium alloys, and metal teUurides are grown by the Bridgman and Czochralski methods (see Semiconductors). 

The primary application for floating-zone melting is crystal growth rather than purification. Semiconductor-grade siUcon is not purified by zone refining siUcon chlorides are distilled and then reduced with hydrogen. 

It is because these exU insic elecU ons can so readily be activated thermally to the conduction band, tlrat great care must be taken in producing the elemental semiconductors to a high state of purity, by such processes as zone refining. 

The semiconductor industry would have been impossible had not the process of zone refining been invented first. It is the standard way of producing ultrapure materials, both for research and for making silicon and germanium-based devices. 

Until the late H)80s the USA was the principal supplier of while arsenic (i e. AsiOi) but it now relies entirely on imports. World production has been steady for many years at about 52fX10 tonnes pa and the main producers are France (lOOOOtpa), Sweden (lOOOOtpa). Russia (SOOOtpa) and Chile (VOOOlpa). The price of refined oxide was about 480 per tonne in 1989 and commercial grade As metal (99%+) was about 2.20/kg in 1990. Fligh purity As (99.99%+) was 45 (Xl/kg and zone-refined semiconductor grade even more expensive. 

Zone refining is a technique for decreasing the level of impurities in some metals, alloys, semiconductors, and other materials this is particularly so for doped semiconductors, in which the amount of an impurity must be known and carefully controlled. The technique relies on the impurities being more soluble in a molten sample (like oxygen in water, as noted above) than in the solid state. 

High purity semiconductor-grade germanium is obtained by zone refining under controlled atmosphere. Ultrapurification of Ge can be obtained in single crystal grown in the most aseptic dust-free environment. 

For the production of superpurity aluminum on a large scale, the Hoopes cell is used. This cell involves three layers of material. Impure (99.35 to 99.9% aluminum) metal from conventional electrolytic cells is alloyed with 33% copper (cutcctic composition) which serves as the anode of the cell A middle, fused-salt layer consists of 60% barium chloride and 40% AlF 1.5NaF (chiolite), mp 72(TC. This layer floats above the aluminum-copper alloy. The top layer consists of superpurity aluminum (99.995%). The final product usually is cast in graphite equipment because iron and other container metals readily dissolve in aluminum. For extreme-purity aluminum, zone refining is used. This process is similar to that used for the production of semiconductor chemicals and yields a product that is 99.9996% aluminum and is available in commercial quantities. 

Silicon, the second most abundant element in the earth s crust, is obtained by reducing silica sand (Si02) with coke. It is purified for use in the semiconductor industry by zone refining. In the silicates, SiC>4 tetrahedra share common O atoms to give silicon oxoanions with ring, chain, layer, and extended three-dimensional structures. In aluminosilicates, such as KAlSi30g, Al3+ replaces some of the Si4+. 

Zone refining is often used when extremely pure metals are desired for such applications as solar cells and semiconductors (Section 13-17). An induction heater surrounds a bar of the impure solid and passes slowly from one end to the other (Figure 22-4). As it passes, it melts portions of the bar, which slowly recrystallize as the heating element moves away. The impurity does not fit into the crystal as easily as the element of interest, so most of it is carried along in the molten portion until it reaches the end. Repeated passes of the heating element produce a bar of high purity. 

In the case of the solids,initial experiments showed that the conventional purification techniques such as recrystallization from solutions do not offer sufficiently pure materials. For example,PMDA samples, after several recrystallizations and vacuum sublimations appear colorless,but show high levels of ionic contaminations when analyzed for inorganic ions. To eliminate this problem,the technique of zone refining as used in semiconductor materials purifications has been used to purify the starting materials. This technique has been applied for a number of organic materials in our laboratory. PMDA for example,when subjected to a simple zone refining process ( as few as 25 zones), shows removal of impurities as high as 3% even if preceded by recrystallization and sublimation. In syntheses of polyamic acids,a variation as small as 3% in stoichiometry can cause considerable variation in the final batch-to-batch synthesis. 

Zone refining was developed originally to produce very pure germanium for the semiconductor industry. It Was successful in that Bell Laboratories produced 99.99999999 % pure germanium Since then, many elements have been prepared in high purity, as well as many organic compounds. 

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