Patent GB-774270-A (GB)
Title: Method of producing bodies of metals or matalloids
IPC: B01J19/08
Filed: 1953-12-16
Abstract: In a method of producing a body of controlled purity of material consisting of a metal or a metalloid or a mixture thereof, a molten portion of such dimension that it is sustained solely by surface tension is formed on a solid matrix of the material and frozen progressively at its interface with the matrix whilst maintaining the <PICT:0774270/III/1> <PICT:0774270/III/2> size of the molten portion substantially constant by the addition of further material, which may be in the form of molten droplets or obtained by decomposing a gaseous compound or, as shown in Fig. 1 obtained by melting more solid from a rod 11 of the material by moving a molten zone 30 progressively upwards through the rod. Helium or other inert or reducing gas is passed through the chamber 16 by tubes 29 and 291, and the molten zone 30 set up at the bottom of the rod 11 and moved slowly upwards by raising the graphite heater 25 in the field of induction coil 24 by means of a rack and pinion 35, the impurities being carried to the top of the rod.  Alternative apparatus effects the freezing from the molten zone by moving the rod of material slowly downwards through a fixed induction heater, while fresh material in the form of molten droplets is added to the liquid zone. Small amounts of acceptor or donor impurities may be added with silicon or germanium to form a rod of semiconductor having successive N and P type zones, and by starting with a single crystal as matrix a long single crystal may be obtained.  As shown in Fig. 5, a reduction of a gaseous compound, e.g. silicon or germanium tetrachloride, by hydrogen in the reaction chamber 110 causes the semiconductor to fall on to the matrix 112 and form a molten zone there on.  Hydrogen from source 126 is purified and dried at 136 and 137 and is supplied direct to the reaction chamber via valve 133 and also through retorts 114, one of which contains the semiconductor compound and the other or others compounds such as hydrides or halides of significant impurity elements e.g. members of groups 3 and 4 in the case of germanium and silicon, where it is saturated with these compounds, the retorts 114 being each associated with condensers 121 maintained at the required temperatures by heaters 118 and thermostats 120.  The reduction is effected by raising the temperature in the chamber 110 to 1000-1100 DEG C. for germanium or 1100-1600 DEG C. for silicon. Alternatively the gas mixtures may be decomposed by passing them through hollow bores in electrodes to an electric arc, a transverse jet of hydrogen being used to control the arc position in order to produce a molten zone of the required dimensions on the matrix.  In addition to the production, refining and growing of single crystals of silicon and germanium, the invention may be applied to other metalloids such as boron, gray tin. selenium and tellurium, and to the redistribution of impurities and growth of single crystals of copper and iron, and other metals.  Redistribution of impurities may be effected by the multiple-pass processes and reverse-pass processes described in Specification 769,673.  The growth of single crystals from a seed crystal is preferably effected at growth rates of 1/32 "-\ba1/4 " per minute, and in the case of growth by adding molten droplets to the molten zone the crystal may be rotated to obtain more uniform growth.  Specification 723,179 also is referred to.