Sources of Carbon Impurities in the Preparation of High-Purity Monoisotopic 28Si by a Hydride Method

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This paper examines sources of carbon impurities in polycrystalline monoisotopic 28Si prepared by
a hydride method. Analytical data on the concentrations of carbon-containing impurities in volatile silicon
compounds (28SiF4 and 28SiH4), process gases (Ar and H2), and polycrystalline 28Si are used to identify the
major sources of carbon in the polycrystalline 28Si prepared by the hydride method. These are the starting
28SiF4 and calcium hydride used in 28SiF4 conversion into 28SiH4. The rate of carbon intake into polycrystalline
silicon from the apparatus material during the monosilane pyrolysis process does not exceed 9 × 1011 cm–2 h–1.
Polycrystalline silicon has been precipitated from monosilane with different concentrations of hydrocarbon
impurities. At hydrocarbon concentrations in the range 10–4 to 10–3 mol %, the carbon concentration in the
monosilane correlates with that in the silicon obtained from it. High-purity monosilane has been used to prepare
polycrystalline 28Si samples with concentrations of carbon impurities in the range (0.8–2.3) × 1015 cm–3.
Based on calculations of the carbon impurity distribution along the length of a zone-refined ingot, we examine
the effect of the initial carbon concentration in the starting polycrystal on the yield of single-crystal
monoisotopic 28Si. Requirements are formulated for the carbon concentration in polycrystalline 28Si which
ensure a high yield of single crystals with parameters suitable for metrological applications.

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