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Raman scattering on porous silicon

Porous silicon (PSi) samples were prepared by electrochemical etching of silicon-on-insulator wafers, consisting of 22 μm thick p-type (111) silicon epitaxial layer on a 1 μm SiO2 layer on silicon substrates, by varying the concentration of 48% HF in ethanol solution and by varying the etching time. Within the epitaxial layer micro- and nano-pores of different sizes in dependence on HF concentration and etching time were obtained. The structural and optical properties of prepared samples were investigated by Raman spectroscopy, and scanning electron microscopy (SEM). SEM images showed high density of micrometer sized pores separated by the cobweb-like silicon structures whose morphology and density depend on the HF concentration and etching time. The Raman spectra of such structures show transversal optical TO(gama) phonon bands that were broadened and red-shifted depending on the size of cobweb wires. The diameter distributions of the cobweb wires where determined by applying the phonon confinement model. When compared to the SEM diameters, they were 3-4 times smaller indicating the sub-structure, not observable by SEM microscopy that exists in silicon wires.

Porous silicon; Raman spectroscopy; SEM

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