Porous silicon prepared by electrochemical etching of silicon eptaxial layer

Porous silicon (PSI) samples were prepared by electrochemical etching of n-type (111) epitaxially grown silicon layer on n-type silicon (111) substrates, by varying the concentration of 48% HF in ethanol solution, and by varying the etching time. Electrical resistivity of epitaxial layer was ~2 Ω cm and of silicon substrate was ~ 0.015 Ω cm. Within the epitaxial layer, and on the substrate surface, the micro- and nano-pores of different sizes in dependence on HF concentration and etching time were obtained. With the etching time longer than 30 minutes, the epitaxially layer was detached from the substrate. The structural and optical properties of prepared samples were investigated by Raman and photoluminescence spectroscopy, field emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) spectroscopy. The FE-SEM images showed high density of micrometer sized pores on epitaxial layers. After detaching the epitaxial layer, the substrate showed fine nanometer sized cobweb-like silicon structures whose morphology and density depend on HF concentration and etching time. The Raman spectra of such structures show transversal optical (TO) phonon band that broadens and red-shifts depending on the size of silicon nanostructures. The size distributions of the silicon basic structural units where determined by applying the phonon confinement model and were compared with those determined by FE-SEM. The intensity of photoluminescence peak of such fine porous substrate shows the sensitivity on a degree of optical phonon confinement.