Correlation of optical and structural properties of light emitting porous silicon

Microscopic structures of light emitting porous silicon layers have been studied. The samples prepared in an aqueous HF solution by anodizing p-type silicon substrates show a strong positional dependence of photoluminescence and Raman spectra. The photoluminescence peaks are broad around 1.8 eV, where the photoluminescence intensities are comparable to that of GaAs at 5 K. We have found from Raman studies showing two characteristic peaks at 500 and 520 cm ^-1 that microscopic structures reveal gradual changes from porous silicon to a mixture of polycrystalline and hydrogenated amorphous phases as the probing spot is moved to the edge of the sample. This is explained by the redeposition of silicon atoms on top of the porous silicon layers near the edge of the sample as a result of liquid flow caused by bubbles of hydrogen gas which was produced near the surface of the sample during the anodization process.