The reduction of silicon tetraiodide by hydrogen to form pure silicon and hydrogen iodide has been investigated. The reaction is heterogeneous, taking place on a hot surface. Dense silicon layers as well as crystals can be deposited. The reaction gives a chemical yield as high as 96%, but the over-all best conditions found resulted in a rate of deposition of 4.4 g Si/dm~/hr at a chemical yield of 55 %. Data on the purity of the silicon obtained are presented.The reaction of silicon and iodine to form silicon tetraiodide and the fractional distillation of this compound are discussed. An apparatus is described which permits good control over the composition of a mixture consisting of two gaseous reactants, one of which is generated from the liquid state by vaporization at a uniform rate of flow.At the inception of this work two processes for preparing pure Si were prominent. In one, Si crystals are deposited in a quartz tube, kept at 950~ by passing Zn and SiCI~ vapors through it at atmospheric pressure (1). This process is currently used to fill almost the entire demand for Si needed for semiconductor devices in this country. The second (2) permits deposition of solid Si layers on a hot Ta wire (980~ by thermally decomposing SiL vapor at a greatly reduced pressure.The aim of this work was to find a means of combining the attractive features of both of these processes, name]y convenient deposition of Si in a furnace tube at atmospheric pressure and high volatility of reactants and by-products to insure highest obtainable purity. For this purpose, the hitherto unreported reaction 2H~(g) -t-SiL(g) -----Si(s) -1-4HI(g) was investigated.
Small-Scale ExperimentsIn a preliminary set of experiments H~, purified with respect to O~, was passed over liquid SiI, (mp about 120~ bp about 300~ and the vapor mixture then passed through graphite tubes maintained at temperatures between 900 ~ and 1000~ Small needles as well as more compact crystals were deposited, which by means of x-ray diffraction were unambiguously identified as elemental Si. 1 In these preliminary experiments a relatively low Hr flow rate was used resulting in a correspondingly low deposition rate, but in one experiment the chemical yield was found to be 96%. This approaches the theoretical yield based on a calculated equilibrium constant. The SiL was prepared by passing vapors of reagent grade I~ over an inexpensive grade of Si at about 800~ 1 Tammann (3) showed that Si and graphite do not react until temperatures several hundred degrees above the temperatures of these experiments are reached. The absence of SiC formation at these temperatures has been confirmed by means of a chemical test, by means of x-ray diffraction, and by means of metallographic examinations.
ApparatusApparatus was built to study the reaction further and, incidentally, increase the rate of Si deposition. During operation, H~ was passed through a catalytic oxygen removal unit, through a drying tower containing silica gel, through a fiowmeter, and through a vessel containing liquid SiL. Th...
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