Structure and photoluminescence studies of porous silicon formed in ferric ion containing stain etchants

Stain etching of silicon in aqueous solutions composed of FeCl3 + HF + concentrated (HClO4 or H2SO4) leads to highly luminescent porous Si with a unique dual layer structure. The upper layer (similar to 3 mu m thick) exhibits bluer luminescence (peaked at 560-590 nm) and a porosity in excess of 84%. The lower layer exhibits redder luminescence (peaked at 630-645 nm) and a lower porosity. Both layers are crystalline and are composed predominantly of uniform micropores. Because of the substantially higher porosity of the upper layer, this layer is highly susceptible to cracking and exfoliation. Critical point drying greatly reduces cracking and exfoliation but does not eliminate cracking completely. Brushing easily removes the upper layer while leaving the lower layer intact and the removed material remains photoluminescent. Scanning electron micrograph and photoluminescence spectrum from a stain etched film dried in air from a methanol rinse. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim