In the paper thin film porous silicon layers on insulator substrates were investigated. Silicon thin film (200-300 nm) was formed by ion-beam and sputter deposition on sapphire and polycorrundum. Than porous silicon was formed by chemical etching in mixture of hydrofluoric acid and sodium nitride. Some of samples were annealed in nitrogen atmosphere at 850 C before porous silicon formation. According to AFM investigation surfaces of porous silicon on polycorrundum and sapphire demonstrate different crystallographic orientation. On polycorrundum typical for (111) orientation triangles can be seen but on sapphire there are no marked crystallographic figures. So it can be assumed that initial silicon layers were approximately crystalline and replicate substrate orientation. In the other hand silicon crystalline orientation significantly affect porous silicon layer properties including optical properties. Transmission spectra of thin porous layers is shifted to shortwave region in comparison with initial silicon film spectra. In some spectra non-typical increase of transmission in UV and IR region is observed possibly due to photoluminescence. High temperature annealing prior to porous silicon formation drastically changes transmission spectra of resulting porous silicon layer despite silicon film transmission spectra did not change significantly. This can be explained by influence of initial crystal structure on velocity and direction of porous silicon formation. By this way it is possible to control optical properties of porous layers. Special interest should be taken to effect of transmission spectra inversion for samples obtained by ion-beam deposition. Photoluminescence of porous silicon thin layer was exited by ultra-violet light (285 nm) and signally differs from photoluminescence of porous silicon on bulk silicon. Beside typical for bulk porous silicon peaks at 600-700 nm also few peaks in green and blue region are observed. In the same time main peak is shifted to shortwave region (500 nm) compared to 600-700 nm. Photoluminescence spectra of porous silicon on insulator confirm most popular theories of porous silicon photoluminescence mechanism in particular correlation of photoluminescence with size of nanocrystallites. Also theory of superposition of few photoluminescence bands is proven because for stand alone porous layers these bands are separated. Ref. 13, fig. 8.
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