Stain etchants made from (HF+V2O5) or (HF+FeCl3 center dot 6H(2)O+HCl or H2SO4) exhibited virtually no initiation time before the formation of porous silicon. Etching with Fe(III) solutions for extended periods resulted in a unique dual layer structure that can reach a thickness >10 mu m and exhibited not only red-orange but also green photoluminescence (PL). Etching with (CeF4+H2SO4) produced extremely uniform films. Visible PL was observed immediately after etching except for those films produced with (CeF4+H2SO4), which required several days of air exposure before bright and robust PL developed.