Etching technique Hydrofluoric acid 3 Porous silicon 3 Composition ratio 2 Etching 2 Etching process 2 Porous silicon formation 2 Structural characterization 2 Atomic force microscopy 1 Catalyst selectivity 1 Chemical etching 1 Crystallite size 1 Effect of delays 1 Effective-mass theory 1 Electrochemical etching 1 Electrolytes 1 Morphology 1 Optical characteristics 1 Photo-electrochemical etching 1 Photoluminescence 1 Photoluminescence peak 1 Photoluminescence spectrum 1 Pore size 1 Porosity 1 Porous silicon structures 1 Porous structures 1 Pulse currents 1 Pulsed electrochemical etchings 1 Quantum confinement effects 1 Quantum theory 1