Micro and nano structure of electrochemically etched silicon epitaxial wafers

Silicon epitaxial wafers, consisting of 280 μm thick n-type substrate layer and 4-5 μm thick epitaxial layer, were electrochemically etched in hydrofluoric acid ethanol solution, to produce porous silicon samples. The resistivity of epitaxial layer was 1 Ω cm, while the substrate was much better conductor with resistivity 0.015 Ωcm. By varying the etching time, the different structures were obtained within the epitaxial layer, and on the substrate surface. Due to the lateral etching the epitaxial layer was partially detached from the substrate and could be peeled off. The influence of etching time duration on the structural properties of porous samples was investigated by Raman, spectroscopy. The samples were analysed immediately after the etching and six months later, while being stored in ambient air. The Raman spectra showed the shift in positions of transversal optical (TO) phonon bands, between freshly etched samples and the one stored in ambient air.