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Structural, optical and electrical characterization of porous silicon prepared on thin silicon epitaxial layer

Silicon-on insulator (SOI) wafers, consisting of 22 μ m thick p-type silicon epitaxial layer grown on 280 μ m thick n-type (1 1 1) silicon substrate, were electrochemically etched in hydrofluoric acid (HF) to produce porous silicon (PS) samples. The pores of different size and different depth were obtained by etching at different time duration, from 10 to 80 min, using the constant concentration of 48% HF in ethanol solution. The structural and optical properties of porous layers were investigated by Raman, FTIR and photoluminescence (PL) spectroscopy, and scanning electron microscopy. SEM images showed high density of micrometer-sized pores whose morphology and density depended on the etching duration. For all samples the observed PL peak is in the visible spectral range. The intensity of the PL peak was increased with the etching time. The exception was the epitaxial layer of the sample etched for 80 min. It showed the strong decrease in the PL peak intensity. For this sample the insulator layer was completely etched out and the epitaxial layer was detached from the substrate. Fine nanometer-sized pores with the strong photoluminescence were observed in the substrate layer. The fine silicon nanostructure was confirmed by the broadening and the red-shift of crystalline silicon TO(Γ ) vibrational band that indicates a strong phonon confinement.

porous Si; silicon-on insulator; IR and Raman spectroscopy; SEM

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