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Porous silicon (PS) is usually prepared by the electrochemical anodization or the stain etching techniques. Recently, a new method consisting of exposing silicon substrates to acid vapours issued from a mixture of HNO 3 /HF was employed to generate highly luminescent PS layers. The so-called HNO 3 /HF vapour etching (VE) technique can be easily applied in various large area of silicon-based devices. Depending on the HNO 3 /HF volume ratio, we found that VE silicon results in the formation of PS and/or a luminescent (NH 4 ) 2 SiF 6 powder-like phase. FTIR spectra of VE-based PS layers formed at HNO 3 /HF volume ratio ranging between 1/100-1/3 contain N-H and Si-F bonds related to NH 4 + and SiF 6 2-ions in addition to the conventional SiH x species. These nitride and fluoride groups were identified to be associated to the (NH 4 ) 2 SiF 6 powder-like phase which in turn contains small Si nanoparticles embedded in a SiO x matrix. The presence of such structures was explained as being the product of the VE technique itself. For vapours rich in HNO 3 (HNO 3 /HF volume ratio > 1/4), the VE method can produce almost only the luminescent (NH 4 ) 2 SiF 6 compound. The VE-based PS is essentially composed of dot-like Si particles with sizes not exceeding 5 nm and emitting a photoluminescence (PL) band around 1.93 eV. The PL band of the VE-based PS presents a shoulder at 2.09 eV, which becomes more significant after oxidation in air. This shoulder at 2.09 eV was attributed to an excitonic emission from the energy levels of the SiO x surrounding the smallest Si nanocrystallites. The PL band emission of the (NH 4 ) 2 SiF 6 powder presents two peaks. The first one was attributed to Si nanocrystallites emitting at 1.98 eV. The second peak could be associated to the smallest nanocrystallites (≤ 1.5 nm). For these crystallites, excitons are trapped on the SiO x energy levels, leading to a maximum PL band emission around 2.1 eV. This PL band seems to have the same origin than the small shoulder observed in the PL emission of PS.

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