GB871614A - A method for the preparation of silicon - Google Patents

Abstract

<PICT:0871614/III/1> Silicon having an impurity content less than one part in a million is prepared by decomposing a silicon hydride which has first been purified by a process of gas chromatography. Silane is prepared by reacting a solution of ammonium bromide in ammonia in vessel 10 with magnesium silicide supplied from side chambers 12. The vessel is maintained at about -50 DEG C. and ammonia is supplied through tube 14. The resulting silicon and other hydrides and excess ammonia are passed through a dephlegmator 16 in a cooling jacket 17, and via refrigerating column 21 to chromatographic separating tubes 24 filled with absorbent material such as Kieselguhr wetted, if desired, with silicone oil and maintained between -60 DEG and -112 DEG C. The constituents emerge in order of their volatilities and may be varified by measuring their thermal conductivities by means of a cell comprising a wire 31 in central tube 30 surrounded by mercury 32 and maintained at a constant temperature by acetone vapour in jacket 33. Current is passed through the wire, the resistivity of which varies with its rate of cooling which depends on the thermal conductivity of the gas. The gases are then passed through a cooled quartz wool trap 35 and filter 37, and the pure monosilane component directed to a thermal decomposition chamber 42 maintained at an appropriate temperature by heater 44 and at low pressure by pump 43. Silicon of an impurity level of one in a million or less is deposited, and appropriate impurities such as boron or phosphorus to give desired electrical conductivity may then be added and the silicon crystallised into a monocrystalline body. The unwanted hydrides may be modified by chemical reaction to produce compounds of differing volatilities more distinguishable from the monosilane. For example for boron hydride the reagent may be water, acetaldehyde, acetone or methyl alcohol, and for phosphorus hydride it may be ammonium sulphate in concentrated sulphuric acid and mercuric chloride in alcohols.