Patent GB-795191-A (GB)
Title: Improvements in or relating to processes for the production of very pure crystallinesubstances
IPC: C01B19/00
Filed: 1954-10-26
Abstract: Very pure crystalline substances, such as for use in transistors, are produced by reacting a volatile compound of the substance in an electric discharge which is operated under such conditions of e.g. discharge current strength, electrode dimensions and arrangement, and gas pressure, that the temperature of the discharge is high enough to ensure that the compound decomposes, whereas the temperature of the electrodes is low enough to ensure that the <PICT:0795191/III/1> substance is deposited on one or more electrodes between which the discharge takes place, in the form of a molten mass which cools to form a coarsely crystalline or monocrystalline body of the substance, and as the body grows, the electrodes are drawn apart.  One of the compounds SiH4, SiH3Cl, SiH2Cl2, SiHCl3 and SiCl4 may be used to produce silicon, and triborine triamine B3N3H6 to produce boron nitride BN (B : N = 1 : 1).  The production of SiC is also mentioned. The compound may be reacted with a reducing agent such as hydrogen or silane.  The discharge may be operated with D.C. or with A.C. which may be polyphase and the voltage is preferably between 1000 and 10,000 volts.  The electrodes may be moved apart by hand or automatically and may be moved continuously out of the reaction vessel through an oil seal. The electrodes may be at an angle to one another, or coaxial, and one or both electrodes may rotate in the same or opposite directions.  Suitable electrode materials are tungsten, tantalum and carbon or a purified piece of the material it is desired to produce.  Tungsten rods may have carbon caps.  The reaction vessel may consist of fused glass, quartz, ceramic or steel, or of an organic synthetic material.  In an apparatus for producing very pure silicon from silico-chloroform with very pure hydrogen as reducing agent, Fig. 1, the hydrogen passes in turn through a tube 3 filled with palladium asbestos and heated at 180 DEG  C. for burning the oxygen in the hydrogen, a tube 3 filled with silica gel and cooled at -196 DEG  C. to absorb impurities such as helium and neon, a trap 6 filled with the silicon compound which is to be employed and cooled or heated to produce a partial pressure of e.g. 500-600 mm. Hg. and into the reaction vessel 8.  The vessel is water cooled.  The electrode rods 8e, 8f are sealed into glass tubes which slide in greased ground glass tubes 8c, 8d.  The unconsumed and reaction products leave the vessel at 8b and pass through cooling traps 10, 11 for condensing silico-chloroform and 12 for liberating hydrogen chloride formed in the reaction.  The unconsumed starting materials may be recirculated.  The yield may be increased by preheating the substances in the gas phase, e.g. to 600 DEG  or 800 DEG  C.  In the production of semi-conductor materials donors and/or acceptors may be present in the gas phase and may be included in the starting compound or in the reducing agent, e.g. antimony hydride, indium hydride, arsenic hydride or hydrogen phosphide instead of pure hydrogen. Solid additions may be introduced so as to fall directly into the electric discharge or into an electrode crater.  Gaseous additions of different type may be introduced alternately into the reaction vessel in a particular rhythm.  The electric discharge is preferably such that the current density is a minimum and the melting zone and the discharge extend over the maximum surface of the electrode or electrodes.  The gas pressure may be atmospheric or above atmospheric or subatmospheric.  The substances produced may be subjected later to a zone melting process (see Group II).  A donor or an acceptor may be added to the melting zone.