Phase transition and luminescence properties from vapor etched silicon

In this work, we present a study on the structure and photoluminescence (PL) properties of a non-conventional ammonium hexafluorosilicate (NH4)2SiF6 (white powder) obtained from HNO3/HF chemical vapor etching (CVE) of silicon wafers. The CVE method leads either to the formation of luminescent Porous Silicon (PS) or SiOx/Si-containing (NH4)2SiF6 depending on the experimental conditions. At specific conditions (i.e., HNO3 / HF volume ratio > 1 / 4), the CVE technique can generate instead of PS, a (NH4)2SiF6 phase where SiOx/Si particles are embedded. The (NH4)2SiF6 marketed powder is not luminescent, while that obtained from silicon vapor-etching presents a noticeable intense and stable photoluminescence (PL), which was found to have mainly two shoulders at 1.98 and 2.1 eV. Two processes have been proposed to explain this PL property. First, the visible luminescence around 1.98 eV would come from silicon nanoparticles embedded in the powder, having a distribution size that does not allow SiOx species to influence their own PL. Second, the PL shoulder around 2.1 eV would originate from small silicon nanoparticles trapped in SiOx features, leading to oxide related states that may trap electrons or excitons, depending on the silicon nanoparticle size, wherein radiative recombination occurs. The PL shoulder could become broader at low temperatures suggesting the existence of radiative recombination in SiOx related defects.