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

This paper examines sources of carbon impurities in polycrystalline monoisotopic 28 Si prepared by a hydride method. Analytical data on the concentrations of carbon-containing impurities in volatile silicon compounds ( 28 SiH 4 and 28 SiH 4 ), process gases (Ar and H 2 ), and polycrystalline 28 Si are used to identify the major sources of carbon in the polycrystalline 28 Si prepared by the hydride method. These are the starting 28 SiH 4 and calcium hydride used in 28 SiH 4 conversion into 28 SiH 4 . The rate of carbon intake into polycrystalline silicon from the apparatus material during the monosilane pyrolysis process does not exceed 9 × 10 11 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 28 Si samples with concentrations of carbon impurities in the range (0.8–2.3) × 10 15 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 28 Si. Requirements are formulated for the carbon concentration in polycrystalline 28 Si which ensure a high yield of single crystals with parameters suitable for metrological applications.