A method of preparation of thin (1000 Angstrom) porous silicon layers by chemical etching of c-Si in HF:HNO3:H2O = 1:3:5 solution is reported wherein a thin Al film is deposited by evaporation on the silicon surface prior to etching. The method is characterized by no "incubation" period to the onset of the stain etching. The reaction between Al and HNO3 produces the required holes for the very fast start of the chemical etching of Si. Using Al patterns deposited through a mask or prepared by photolithography it is possible to achieve selective formation of porous silicon. The porous silicon layers exhibit visible photoluminescence and when prepared on p-type c-Si and a shallow p-n c-Si homojunction the resulting heterostructures also display electroluminescence and photovoltaic properties.