The photoluminescence of porous silicon prepared by etching in an aqueous solution of hydrofluoric acid mixed with phosphoric and that with nitric acid were red and orange, respectively. The optical characteristics were stable under laser illumination or when stored in air for the aging test. For etching in an aqueous hydrofluoric acid/phosphoric acid electrolyte, the stability is attributed to the existence of Si-P and P-O-Si bonds formed on the porous silicon surface. For etching in an aqueous hydrofluoric acid/nitric acid electrolyte, it is attributed to more Si-O bonds formed on the porous silicon surface.

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