Porous Silicon (PS) exhibits unique properties due to its nanoscale structure and very active surface with enormously, large area. In this work, for the first time, we report a spontaneous room temperature growth of crystalline silicon dioxide on PS after a specific photo-chemical treatment. PS formed on polished silicon W wafer Was subjected to NF3/UV surface treatment at 300degreesC and 60Torr for native oxide removal and PS passivation. When the treated PS surface came into contact with the ambient air at room temperature. a layer of hillocks with regular crystalline shape was observed at the PS surface. The size of the crystallites was about 2 mum as was measured with atomic force microscope. These crystallites were composed of silicon dioxide as was shown by Auger analysis together with X-ray diffraction. No crystallite growth was observed on the reference PS surface (not treated by NF3) which was covered by the native oxide SiOx (x approximate to 1). The growth of the crystallites was not observed at the adjacent polished silicon wafer surface treated by the same NF3/UV treatment. Thus, the combination of PS with the NF3/UV photo-chemical surface treatment is needed for the rapid room temperature growth of crystalline SiO2 in air. This rapid growth of the SiO2 layer is supposed to be due to the NF3/UV photo-thermal etching of the as-formed native oxide, as well as to the cleaning and enrichment of the PS surface with fluorine. The latter exhibits a catalytic action on the crystallization process. The growth of the silicon dioxide crystallites was found to enhance the photoluminescence (PL) of PS by 1-2 orders of magnitude. The luminescence enhancement correlated with the coverage of the PS surface with the crystallites as well as with the aging time in air. The PL enhancement can be explained by the formation of a better interface between PS and the oxide (PS/ SiO2) as compared to the non-treated PS covered by the non-stoichiometric SiOx (x approximate to 1) layer. (C) 2002 Elsevier Science B.V. All rights reserved.