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Optical properties of porous silicon on an insulator layer

Silicon wafers, consisting of 280 microm tick n-type (Sb doped) upper layer and 20 microm n-type (P doped) lower layer, were electrochemically etched in hydrofluoric acid (HF) ethanol solution. The resistivity of upper layer was 0.015 Ohmcm, while the lower layer was much worse conductor with resistivity 2 Ohmcm. Porous silicon (PS) samples were produced by etching the non polished side of wafers at the constant current density. The etching process was performed with different concentration of HF in ethanol solution. It was found that such n-type silicon on insulator can be selectively etched without illumination. The possible mechanism of the etching phenomenon is discussed. Samples were investigated by Raman spectroscopy, photoluminescence (PL) and scanning electron microscopy (SEM). The most intensive photoluminescence peak showed samples which were etched with the lowest HF concentration. The Raman spectra of all samples showed broadened and red-shifted transversal optical (TO) phonon bands. SEM images showed high density of less than 100 nm sized pores whose density and morphology depended on HF concentration. The etching of the unpolished side of wafers caused interesting topography, presented in electron micrograph Fig. 1 b), and inhomogeneous luminescence presented in Fig. 1 a). Figure 1 c) shows the high magnification FE-SEM micrograph of strong luminescence region with nanometers pores in silicon.

Porous silicon; Raman spectroscopy; SEM

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