NF₃ induced photoluminescence enhancement and crystalline oxide growth in porous silicon

Porous silicon formed on polished p-type (100) silicon wafers was subjected to a NF₃/UV photo-chemical treatment at 300°C and 400°C at 60 Torr. When the treated porous silicon surface contacted with the ambient air at room temperature two amazing effects were observed: (i) a drastic increase of the s-band photoluminescence by 1-2 orders of magnitude; (ii) spontaneous room temperature growth of crystalline silicon dioxide SiO₂. The growth of crystalline silicon dioxide was demonstrated by AFM and SEM images and proved by X-ray diffraction. A tridymite orthorhombic configuration of SiO₂ was found. The size of the crystallites reached several micrometer after a few days aging at room temperature. The size and density of the crystals can be affected by the NF₃ treatment temperature as well as by slight heating (75°C) during aging. The rapid growth of the SiO₂ layer is supposed to be due to the NF₃/UV photothermal etching of the as-formed native oxide, as well as to the cleaning and enrichment of the porous silicon surface with fluorine. The latter can induce a catalytic action on the crystallization process. The formation of a better interface between porous silicon and the oxide (PS/SiO₂) as compared to the not treated porous silicon covered by the non-stoichiometric SiO_x (x ≈ 1) layer is claimed to cause a strong photoluminescence enhancement.