Porous silicon by galvanostatic electrochemical anodisation of epitaxial silicon, polycrystalline silicon and silicon on insulator layers

Abstract

Porous silicon (PSi) samples were prepared by galvanostatic electrochemical anodization of epitaxial silicon, polycrystalline silicon and silicon on insulator layers. Structural and optical properties of prepared samples were investigated by Raman and photoluminescence (PL) spectroscopy, field emission scanning electron microscopy (FE-SEM) and energy dispersive x-ray spectroscopy (EDS). Epitaxial silicon layers of n-type and {111} orientation grown on n-type {111} oriented silicon substrates were anodized as a function of concentration of 48 % HF in ethanol solution and anodization time. Electrical resistivities of the epitaxial layers and of the silicon substrate were ∼2 and ∼0.015 Ω cm, successively. Within the epitaxial layer and on the substrate surface, micro- and nano-pores of different sizes in dependence on HF concentration and anodization time were obtained. For anodization times longer than 30 min epitaxially grown layer detached from the substrate. A high density of micrometer sized pores with regions of three-dimensional photonic crystal expressed in an intersecting <113> oriented macropore lattice on {111} oriented crystal was observed. After detaching the epitaxial layer, the black colored substrate consisted of fine nanometer sized cobweb-like silicon structures whose morphology and density depended on HF concentration and anodization time was found. The Raman spectra of such structures show broadened and red-shifted optical phonon band O(Γ), depending on the size of silicon nanostructures. The intensity of PL of such fine porous substrate shows the sensitivity on the level of the optical phonon confinement. Polycrystalline p-type silicon film were prepared by low pressure chemical vapor deposition (LPCVD) and anodized in aqueous hydrofluoric acid (HF)/ethanol electrolyte. The FE-SEM images showed preferential anodization and macro-PSi formation along grain boundaries. Weak PL signal was detected in all samples, while Raman measurements indicated minimal or no confinement effects. The anodization of silicon on insulator layers was performed by direct and alternating currents at 50 Hz. Raman spectra of DC samples showed no significant shift of c-Si peak while PL spectra showed intensive luminescence over the entire surface. When etched with AC, a very intensive PL was observed at the circular edge of the sample that exhibited micrometer sized island-like porous structures, while the center of the sample showed moderate PL signal similar to DC samples. The formation of such island-like structures was interpreted as stress due to difference of the piezoelectric effect of silicon and quartz layers (buried SiO2). Micro-Raman spectra of islands show strong phonon confinement in the range 1.4-3.5 nm. © 2013 Springer Science+Business Media Dordrecht.