NF3 induced photoluminescence enhancement and crystalline oxide growth in porous silicon

Abstract

Porous silicon formed on polished p-type (100) silicon wafers was subjected to a NF 3 /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 SiO 2 . The growth of crystalline silicon dioxide was demonstrated by AFM and SEM images and proved by X-ray diffraction. A tridymite orthorhombic configuration of SiO 2 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 NF 3 treatment temperature as well as by slight heating (75 °C) during aging. The rapid growth of the SiO 2 layer is supposed to be due to the NF 3 /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/SiO 2 ) 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.