Formation of luminescent (NH4)2SiF6 phase from vapour etching-based porous silicon

In this paper we describe the formation of a luminescent (NH4)2SiF6 via porous silicon (PS) obtained from HNO3/HF vapour etching (VE) silicon (Si) substrates. It was found that at specific conditions, PS transforms in a luminescent thick white powder (WP) layer. Scanning electron microscopy (SEM) revealed that the WP has a coral-like structure. It was also found that PS persists as an intermediate layer between the Si substrate and the WP, and seems to be the seed that transforms into the WP. SEM microanalysis show that the WP is essentially composed of silicon (Si), nitrogen (N) and fluorine (F). Fourier transform infrared (FTIR) spectroscopy investigations show that this WP contains SiF62− and NH4+ ions and NH chemical bonds. X-ray diffraction (XRD) patterns of the WP confirm that a (NH4)2SiF6 cubic phase is concerned. SEM microanalyses show an excess of Si in the WP matrix. FTIR spectroscopy and XRD analysis reveal the presence of crystalline Si particles and SiOx, both originating from the excess of Si. The (NH4)2SiF6 WP phase emits an intense photoluminescence (PL) band, shifted towards higher energies as compared to the starting PS layer. The possible origin and mechanism of the luminescence emission was discussed taking into account the ability of small SiOx-surrounded Si particles to emit PL at rather high energy. The wide range variation of the thickness of the (NH4)2SiF6 WP may be easily used for the grooving of Si wafers.