New insights into boron species in acidic digestion solutions of boron-doped silicon

Abstract

The exact and precise determination of the boron concentration in silicon is still a challenge. A systematic investigation dealing with the digestions of 60 silicon samples with HF-HNO₃ and subsequent boron determination by ICP-OES revealed that the concentration found could be up to 60% lower than the actual boron concentration depending on the composition of the sample solution. As the original boron–silicon compound that was identified was colloidally precipitated in the presence of an excess of hydrofluoric acid and then partially retained by filtration or by the sample introduction system, systematic lower boron concentrations were determined. In acidic, HF-free digestion solutions, this compound existed in a soluble form parallel to the borate in B(OH)₄⁻. In an excess of hydrofluoric acid, the compound was converted into the colloidal form and, in parallel, B(OH)₄⁻ was converted to tetrafluoroborate, BF₄⁻. For the composition of the colloidal compound, a molar ratio of boron to silicon of 1 : 4 could be determined. ¹¹B-ss-NMR analysis revealed a tetrahedral geometry compound with a central boron atom surrounded by four silicon atoms. It is assumed that a soluble form with four –Si(OH)₃ groups was present in the hydrofluoric acid-free solutions, while an insoluble form with four –SiF₃ groups was present in HF-containing solution.